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galvanic isolation of sensors

Discussion in 'Embedded' started by Simon, Jan 12, 2012.

  1. Simon

    Simon Guest

    Hi all,

    So, I want to put 10 pH/ORP probes onto a microcontroller, with each
    probe being galvanically isolated from its peers so that the
    measurements of different probes aren't interfering with each other.
    Being more of a digital than an analogue guy, I'm not really sure how
    to proceed...

    Another requirement is that it's a small solution - there's not much
    space to play with here, about 0.5" x 1.5" per probe PCB... And of
    course, the cheaper the better, given that I want 10 of them!

    One chip I found to do the isolation is the adum5241 (http://
    www.analog.com/en/interface/digital-isolators/adum5241/products/product.html)
    from analog devices, I could get an isolated GND, +5v, dIn and dOut
    from that, which is what I need, but I also want -5v for the probe,
    and I'm not sure how to go about getting it. If I try to tack on a +5v-
    >-5v conversion after the adum5241, I think I'll over-run its maximum

    current delivery (10mA).

    So, questions:

    - Do I need to isolate the -5v if the +5v and ground are isolated ?
    I'd think so, but better to ask :)

    - Is there a better way to do it ? Bound to be - this is just the
    first one I've come across :)

    Cheers
    Simon.
     
    Simon, Jan 12, 2012
    #1
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  2. Simon

    Tim Wescott Guest

    On Thu, 12 Jan 2012 08:25:57 -0800, Simon wrote:

    > Hi all,
    >
    > So, I want to put 10 pH/ORP probes onto a microcontroller, with each
    > probe being galvanically isolated from its peers so that the
    > measurements of different probes aren't interfering with each other.
    > Being more of a digital than an analogue guy, I'm not really sure how to
    > proceed...
    >
    > Another requirement is that it's a small solution - there's not much
    > space to play with here, about 0.5" x 1.5" per probe PCB... And of
    > course, the cheaper the better, given that I want 10 of them!
    >
    > One chip I found to do the isolation is the adum5241 (http://
    > www.analog.com/en/interface/digital-isolators/adum5241/products/

    product.html)
    > from analog devices, I could get an isolated GND, +5v, dIn and dOut from
    > that, which is what I need, but I also want -5v for the probe, and I'm
    > not sure how to go about getting it. If I try to tack on a +5v-
    >>-5v conversion after the adum5241, I think I'll over-run its maximum

    > current delivery (10mA).
    >
    > So, questions:
    >
    > - Do I need to isolate the -5v if the +5v and ground are isolated ?
    > I'd think so, but better to ask :)
    >
    > - Is there a better way to do it ? Bound to be - this is just the
    > first one I've come across :)


    I haven't had to select something like this in anger, so I can't tell you
    the Very Best Thing.

    Alternatives that I can think of are:

    * Go looking for a higher power version of the same chip.
    * Use two, one for +5V and one for -5V (assuming that 10mA
    from each is enough)
    * Use one of the data-only galvanic isolation chips,
    (they're out there -- look) and do power separately.
    * If you can find a teeny tiny transformer, rolling your
    own power supply might be small enough, particularly
    if you don't care too much about regulation.

    --
    My liberal friends think I'm a conservative kook.
    My conservative friends think I'm a liberal kook.
    Why am I not happy that they have found common ground?

    Tim Wescott, Communications, Control, Circuits & Software
    http://www.wescottdesign.com
     
    Tim Wescott, Jan 12, 2012
    #2
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  3. Simon

    cassiope Guest

    On Jan 12, 8:25 am, Simon <> wrote:
    > Hi all,
    >
    > So, I want to put 10 pH/ORP probes onto a microcontroller, with each
    > probe being galvanically isolated from its peers so that the
    > measurements of different probes aren't interfering with each other.
    > Being more of a digital than an analogue guy, I'm not really sure how
    > to proceed...
    >
    > Another requirement is that it's a small solution - there's not much
    > space to play with here, about 0.5" x 1.5" per probe PCB... And of
    > course, the cheaper the better, given that I want 10 of them!
    >
    > One chip I found to do the isolation is the adum5241 (http://www.analog.com/en/interface/digital-isolators/adum5241/products/produ...)
    > from analog devices, I could get an isolated GND, +5v, dIn and dOut
    > from that, which is what I need, but I also want -5v for the probe,
    > and I'm not sure how to go about getting it. If I try to tack on a +5v->-5v conversion after the adum5241, I think I'll over-run its maximum
    >
    > current delivery (10mA).
    >
    > So, questions:
    >
    >  - Do I need to isolate the -5v if the +5v and ground are isolated ?
    > I'd think so, but better to ask :)
    >
    >  - Is there a better way to do it ? Bound to be - this is just the
    > first one I've come across :)
    >
    > Cheers
    >    Simon.


    I'm not sure what these "ORP" probes are - if they're anything like
    the pH
    electrodes I've used, they are _extremely_ high impedance; and you
    need
    to compare their output voltage with the potential of the solution
    they are
    measuring. Why 10 probes? Might the solution be at different
    potentials,
    and if so, by how much?

    IF the solution are at nearly the same potential, you may be able
    to get
    away with some good differential amplifiers and dispense with
    isolation -
    assuming, of course, that you aren't concerned with galvanically
    isolating the
    solution from the output. But that might be doable with one or two
    digital
    isolation devices.
     
    cassiope, Jan 12, 2012
    #3
  4. On Thu, 12 Jan 2012 08:25:57 -0800 (PST), Simon <>
    wrote:

    >Hi all,
    >
    >So, I want to put 10 pH/ORP probes onto a microcontroller, with each
    >probe being galvanically isolated from its peers so that the
    >measurements of different probes aren't interfering with each other.
    >Being more of a digital than an analogue guy, I'm not really sure how
    >to proceed...
    >
    >Another requirement is that it's a small solution - there's not much
    >space to play with here, about 0.5" x 1.5" per probe PCB... And of
    >course, the cheaper the better, given that I want 10 of them!
    >
    >One chip I found to do the isolation is the adum5241 (http://
    >www.analog.com/en/interface/digital-isolators/adum5241/products/product.html)
    >from analog devices, I could get an isolated GND, +5v, dIn and dOut
    >from that, which is what I need, but I also want -5v for the probe,
    >and I'm not sure how to go about getting it. If I try to tack on a +5v-
    >>-5v conversion after the adum5241, I think I'll over-run its maximum

    >current delivery (10mA).


    You can use an ICL7660 charge pump to get -5V.

    The whole galvanic isolation thing can be a bit tricky, especially
    when it comes to hundreds-of-megohm source impedance millivolt-level
    signals so I'm not going to say that this will work for you. Suggest
    you pay attention to the asymmetrical source impedance of the ORP/Ph
    electrodes and design accordingly..
     
    Spehro Pefhany, Jan 12, 2012
    #4
  5. Simon

    Simon Guest

    On Jan 12, 9:45 am, cassiope <> wrote:
    > On Jan 12, 8:25 am, Simon <> wrote:
    >
    >
    >
    >
    >
    > > Hi all,

    >
    > > So, I want to put 10 pH/ORP probes onto a microcontroller, with each
    > > probe being galvanically isolated from its peers so that the
    > > measurements of different probes aren't interfering with each other.
    > > Being more of a digital than an analogue guy, I'm not really sure how
    > > to proceed...

    >
    > > Another requirement is that it's a small solution - there's not much
    > > space to play with here, about 0.5" x 1.5" per probe PCB... And of
    > > course, the cheaper the better, given that I want 10 of them!

    >
    > > One chip I found to do the isolation is the adum5241 (http://www.analog..com/en/interface/digital-isolators/adum5241/product......)
    > > from analog devices, I could get an isolated GND, +5v, dIn and dOut
    > > from that, which is what I need, but I also want -5v for the probe,
    > > and I'm not sure how to go about getting it. If I try to tack on a +5v->-5v conversion after the adum5241, I think I'll over-run its maximum

    >
    > > current delivery (10mA).

    >
    > > So, questions:

    >
    > >  - Do I need to isolate the -5v if the +5v and ground are isolated ?
    > > I'd think so, but better to ask :)

    >
    > >  - Is there a better way to do it ? Bound to be - this is just the
    > > first one I've come across :)

    >
    > > Cheers
    > >    Simon.

    >
    > I'm not sure what these "ORP" probes are - if they're anything like
    > the pH
    > electrodes I've used, they are _extremely_ high impedance; and you
    > need
    > to compare their output voltage with the potential of the solution
    > they are
    > measuring.  Why 10 probes?  Might the solution be at different
    > potentials,
    > and if so, by how much?


    ORP is a similar probe, yes - it's an Oxidation/Reduction potential
    probe.

    The pH solutions are likely to be similar (within +/- 1 pH at most)
    and it's for measuring / controlling the pH of an aquarium when you're
    introducing chemicals at various places in the water column.

    >
    > IF the solution are at nearly the same potential, you may be able
    > to get
    > away with some good differential amplifiers and dispense with
    > isolation -
    > assuming, of course, that you aren't concerned with galvanically
    > isolating the
    > solution from the output.  But that might be doable with one or two
    > digital
    > isolation devices.


    I doubt there'd be much correlation in the readings between an ORP
    probe and a pH probe, though, and I wouldn't want the addition of one
    to affect existing readings. The same system will be reading
    temperature (since pH is temperature-dependent) although that's of
    lesser concern due to the nature of the probe.


    On Jan 12, 11:46 am, Spehro Pefhany
    <> wrote:

    >
    > You can use an ICL7660 charge pump to get -5V.
    >


    That's one I hadn't come across, and for the my needs, it would appear
    to produce a good -5v (the required load should be very low due to the
    massive impedance of the probe). Thanks!


    On Jan 12, 9:42 am, Tim Wescott <> wrote:

    >
    > * Go looking for a higher power version of the same chip.
    >


    And, of course, there is indeed one such - in fact it handles 0.5A.
    This still wouldn't be sufficient for the original 5v -> -5v converter
    chip (which can need 1->2A (!!) on startup), but the ICL chip doesn't
    seem to suffer from that.

    Combining the two suggestions, seems like that would work :)

    Thanks for all the help :)

    Simon
     
    Simon, Jan 12, 2012
    #5
  6. Simon

    Bruce Varley Guest

    "Simon" <> wrote in message
    news:...
    > Hi all,
    >
    > So, I want to put 10 pH/ORP probes onto a microcontroller, with each
    > probe being galvanically isolated from its peers so that the
    > measurements of different probes aren't interfering with each other.
    > Being more of a digital than an analogue guy, I'm not really sure how
    > to proceed...
    >
    > Another requirement is that it's a small solution - there's not much
    > space to play with here, about 0.5" x 1.5" per probe PCB... And of
    > course, the cheaper the better, given that I want 10 of them!
    >
    > One chip I found to do the isolation is the adum5241 (http://
    > www.analog.com/en/interface/digital-isolators/adum5241/products/product.html)
    > from analog devices, I could get an isolated GND, +5v, dIn and dOut
    > from that, which is what I need, but I also want -5v for the probe,
    > and I'm not sure how to go about getting it. If I try to tack on a +5v-
    >>-5v conversion after the adum5241, I think I'll over-run its maximum

    > current delivery (10mA).
    >
    > So, questions:
    >
    > - Do I need to isolate the -5v if the +5v and ground are isolated ?
    > I'd think so, but better to ask :)
    >
    > - Is there a better way to do it ? Bound to be - this is just the
    > first one I've come across :)
    >
    > Cheers
    > Simon.


    I can see all sorts of potential 'gotchas' in this scenario. With Hi-Z
    inputs, things like transients from switching power supply devices like the
    7660 could cause problems unless layout and packaging is just right. As
    other posts have suggested, things are a lot simpler if you can avoid
    isolation of the inputs, is the CMV really so high that you need it? If you
    don't, then you can use a single power supply that can be well packaged to
    avoid any noise or spikes if it incorporates switching. In fact, I've always
    found that for audio (similar need for power rail cleanliness), a simple
    rectifier/filter/linear reg arrangement is hard to beat.
     
    Bruce Varley, Jan 14, 2012
    #6
  7. Simon

    Simon Guest

    On Jan 13, 9:17 pm, "Bruce Varley" <> wrote:
    > "Simon" <> wrote in message
    >
    > news:...
    >
    >
    >
    >
    >
    >
    >
    >
    >
    > > Hi all,

    >
    > > So, I want to put 10 pH/ORP probes onto a microcontroller, with each
    > > probe being galvanically isolated from its peers so that the
    > > measurements of different probes aren't interfering with each other.
    > > Being more of a digital than an analogue guy, I'm not really sure how
    > > to proceed...

    >
    > > Another requirement is that it's a small solution - there's not much
    > > space to play with here, about 0.5" x 1.5" per probe PCB... And of
    > > course, the cheaper the better, given that I want 10 of them!

    >
    > > One chip I found to do the isolation is the adum5241 (http://
    > >www.analog.com/en/interface/digital-isolators/adum5241/products/produ...)
    > > from analog devices, I could get an isolated GND, +5v, dIn and dOut
    > > from that, which is what I need, but I also want -5v for the probe,
    > > and I'm not sure how to go about getting it. If I try to tack on a +5v-
    > >>-5v conversion after the adum5241, I think I'll over-run its maximum

    > > current delivery (10mA).

    >
    > > So, questions:

    >
    > > - Do I need to isolate the -5v if the +5v and ground are isolated ?
    > > I'd think so, but better to ask :)

    >
    > > - Is there a better way to do it ? Bound to be - this is just the
    > > first one I've come across :)

    >
    > > Cheers
    > >   Simon.

    >
    > I can see all sorts of potential 'gotchas' in this scenario. With Hi-Z
    > inputs, things like transients from switching power supply devices like the
    > 7660 could cause problems unless layout and packaging is just right. As
    > other posts have suggested, things are a lot simpler if you can avoid
    > isolation of the inputs, is the CMV really so high that you need it? If you
    > don't, then you can use a single power supply that can be well packaged to
    > avoid any noise or spikes if it incorporates switching. In fact, I've always
    > found that for audio (similar need for power rail cleanliness), a simple
    > rectifier/filter/linear reg arrangement is hard to beat.


    What I've decided to do (because even the solution above works out to
    be expensive, given the number of probes) is to prototype a couple of
    boards and see if I actually need the isolation. If there's no
    measurable difference I'll proceed without. If there is, I'll have to
    spend some time figuring out / learning more about how to fix it...

    Cheers
    Simon.
     
    Simon, Jan 14, 2012
    #7
  8. Simon wrote:

    > On Jan 13, 9:17 pm, "Bruce Varley" <> wrote:
    >> "Simon" <> wrote in message
    >>
    >> news:...
    >>
    >> > Hi all,

    >>
    >> > So, I want to put 10 pH/ORP probes onto a microcontroller, with each
    >> > probe being galvanically isolated from its peers so that the
    >> > measurements of different probes aren't interfering with each other.
    >> > Being more of a digital than an analogue guy, I'm not really sure how
    >> > to proceed...

    >>
    >> > Another requirement is that it's a small solution - there's not much
    >> > space to play with here, about 0.5" x 1.5" per probe PCB... And of
    >> > course, the cheaper the better, given that I want 10 of them!

    >>
    >> > One chip I found to do the isolation is the adum5241 (http://
    >> >www.analog.com/en/interface/digital-

    isolators/adum5241/products/produ...)
    >> > from analog devices, I could get an isolated GND, +5v, dIn and dOut
    >> > from that, which is what I need, but I also want -5v for the probe,
    >> > and I'm not sure how to go about getting it. If I try to tack on a +5v-
    >> >>-5v conversion after the adum5241, I think I'll over-run its maximum
    >> > current delivery (10mA).

    >>
    >> > So, questions:

    >>
    >> > - Do I need to isolate the -5v if the +5v and ground are isolated ?
    >> > I'd think so, but better to ask :)

    >>
    >> > - Is there a better way to do it ? Bound to be - this is just the
    >> > first one I've come across :)

    >>
    >> > Cheers
    >> > Simon.

    >>

    There have been a few useful suggestions for Isolated DC-DC amplifiers
    already so I won't add to that list. However, as someone who has always had
    to deal with having to provide full galvanic isolation for inputs from
    plant, I can offer some advice on the power supply side. Trying to get small
    isolated power on the plant side can be a real pain in the neck if you are
    doing it on an individual channel basis. A better solution is to provide a
    multi-output power pack where all the DC outputs are isolated from each
    other. This usually requires a custom built power pack but that works out
    cleaner and more stable than the individual chip based solutions.

    --
    ********************************************************************
    Paul E. Bennett...............<email://>
    Forth based HIDECS Consultancy
    Mob: +44 (0)7811-639972
    Tel: +44 (0)1235-510979
    Going Forth Safely ..... EBA. www.electric-boat-association.org.uk..
    ********************************************************************
     
    Paul E. Bennett, Jan 14, 2012
    #8
  9. Simon

    rickman Guest

    On Jan 14, 1:52 am, Simon <> wrote:
    > On Jan 13, 9:17 pm, "Bruce Varley" <> wrote:
    >
    >
    >
    >
    >
    >
    >
    >
    >
    > > "Simon" <> wrote in message

    >
    > >news:...

    >
    > > > Hi all,

    >
    > > > So, I want to put 10 pH/ORP probes onto a microcontroller, with each
    > > > probe being galvanically isolated from its peers so that the
    > > > measurements of different probes aren't interfering with each other.
    > > > Being more of a digital than an analogue guy, I'm not really sure how
    > > > to proceed...

    >
    > > > Another requirement is that it's a small solution - there's not much
    > > > space to play with here, about 0.5" x 1.5" per probe PCB... And of
    > > > course, the cheaper the better, given that I want 10 of them!

    >
    > > > One chip I found to do the isolation is the adum5241 (http://
    > > >www.analog.com/en/interface/digital-isolators/adum5241/products/produ...)
    > > > from analog devices, I could get an isolated GND, +5v, dIn and dOut
    > > > from that, which is what I need, but I also want -5v for the probe,
    > > > and I'm not sure how to go about getting it. If I try to tack on a +5v-
    > > >>-5v conversion after the adum5241, I think I'll over-run its maximum
    > > > current delivery (10mA).

    >
    > > > So, questions:

    >
    > > > - Do I need to isolate the -5v if the +5v and ground are isolated ?
    > > > I'd think so, but better to ask :)

    >
    > > > - Is there a better way to do it ? Bound to be - this is just the
    > > > first one I've come across :)

    >
    > > > Cheers
    > > > Simon.

    >
    > > I can see all sorts of potential 'gotchas' in this scenario. With Hi-Z
    > > inputs, things like transients from switching power supply devices like the
    > > 7660 could cause problems unless layout and packaging is just right. As
    > > other posts have suggested, things are a lot simpler if you can avoid
    > > isolation of the inputs, is the CMV really so high that you need it? If you
    > > don't, then you can use a single power supply that can be well packaged to
    > > avoid any noise or spikes if it incorporates switching. In fact, I've always
    > > found that for audio (similar need for power rail cleanliness), a simple
    > > rectifier/filter/linear reg arrangement is hard to beat.

    >
    > What I've decided to do (because even the solution above works out to
    > be expensive, given the number of probes) is to prototype a couple of
    > boards and see if I actually need the isolation. If there's no
    > measurable difference I'll proceed without. If there is, I'll have to
    > spend some time figuring out / learning more about how to fix it...
    >
    > Cheers
    > Simon.


    I'm pretty confident that you don't need the isolation between
    probes. Are they all going into the same fish tank, or into different
    tanks in a multiple aquarium setup?

    I would ask what is the purpose of combining them into one
    instrument? If you can make the instrument small and inexpensive, it
    might be better to make 10 separate instruments. That has some
    advantages such as cheaper prototyping and ease of repair by just
    replacing a single inexpensive unit with a spare rather than having to
    debug and repair a more costly multiple input device.

    Rick
     
    rickman, Jan 15, 2012
    #9
  10. Simon

    cassiope Guest

    On Jan 12, 2:36 pm, Simon <> wrote:
    > On Jan 12, 9:45 am, cassiope <> wrote:
    >
    >
    >
    > > On Jan 12, 8:25 am, Simon <> wrote:

    >
    > > > Hi all,

    >
    > > > So, I want to put 10 pH/ORP probes onto a microcontroller, with each
    > > > probe being galvanically isolated from its peers so that the
    > > > measurements of different probes aren't interfering with each other.
    > > > Being more of a digital than an analogue guy, I'm not really sure how
    > > > to proceed...

    >
    > > > Another requirement is that it's a small solution - there's not much
    > > > space to play with here, about 0.5" x 1.5" per probe PCB... And of
    > > > course, the cheaper the better, given that I want 10 of them!

    >
    > > > One chip I found to do the isolation is the adum5241 (http://www.analog.com/en/interface/digital-isolators/adum5241/product......)
    > > > from analog devices, I could get an isolated GND, +5v, dIn and dOut
    > > > from that, which is what I need, but I also want -5v for the probe,
    > > > and I'm not sure how to go about getting it. If I try to tack on a +5v->-5v conversion after the adum5241, I think I'll over-run its maximum

    >
    > > > current delivery (10mA).

    >
    > > > So, questions:

    >
    > > >  - Do I need to isolate the -5v if the +5v and ground are isolated ?
    > > > I'd think so, but better to ask :)

    >
    > > >  - Is there a better way to do it ? Bound to be - this is just the
    > > > first one I've come across :)

    >
    > > > Cheers
    > > >    Simon.

    >
    > > I'm not sure what these "ORP" probes are - if they're anything like
    > > the pH
    > > electrodes I've used, they are _extremely_ high impedance; and you
    > > need
    > > to compare their output voltage with the potential of the solution
    > > they are
    > > measuring.  Why 10 probes?  Might the solution be at different
    > > potentials,
    > > and if so, by how much?

    >
    > ORP is a similar probe, yes - it's an Oxidation/Reduction potential
    > probe.
    >
    > The pH solutions are likely to be similar (within +/- 1 pH at most)
    > and it's for measuring / controlling the pH of an aquarium when you're
    > introducing chemicals at various places in the water column.
    >
    >
    >
    > > IF the solution are at nearly the same potential, you may be able
    > > to get
    > > away with some good differential amplifiers and dispense with
    > > isolation -
    > > assuming, of course, that you aren't concerned with galvanically
    > > isolating the
    > > solution from the output.  But that might be doable with one or two
    > > digital
    > > isolation devices.

    >
    > I doubt there'd be much correlation in the readings between an ORP
    > probe and a pH probe, though, and I wouldn't want the addition of one
    > to affect existing readings. The same system will be reading
    > temperature (since pH is temperature-dependent) although that's of
    > lesser concern due to the nature of the probe.
    >
    > On Jan 12, 11:46 am, Spehro Pefhany
    >
    > <> wrote:
    >
    > > You can use an ICL7660 charge pump to get -5V.

    >
    > That's one I hadn't come across, and for the my needs, it would appear
    > to produce a good -5v (the required load should be very low due to the
    > massive impedance of the probe). Thanks!
    >
    > On Jan 12, 9:42 am, Tim Wescott <> wrote:
    >
    >
    >
    > > * Go looking for a higher power version of the same chip.

    >
    > And, of course, there is indeed one such - in fact it handles 0.5A.
    > This still wouldn't be sufficient for the original 5v -> -5v converter
    > chip (which can need 1->2A (!!) on startup), but the ICL chip doesn't
    > seem to suffer from that.
    >
    > Combining the two suggestions, seems like that would work :)
    >
    > Thanks for all the help :)
    >
    > Simon


    I'm with Rick on not needing isolation between channels. The
    electrodes shouldn't
    alter the solution being tested; and unless you have an incredible
    voltage gradient
    through the channel (you haven't suggested anything like that), they
    should all
    be well within the common-mode range of a decent set of amplifiers.
    What are you
    using for a ground reference? One or more than one electrodes? What
    kind of
    electrode?
     
    cassiope, Jan 15, 2012
    #10
  11. Simon

    Simon Guest

    On Jan 15, 11:26 am, rickman <> wrote:
    > On Jan 14, 1:52 am, Simon <> wrote:
    >
    >
    >
    >
    >
    >
    >
    >
    >
    > > On Jan 13, 9:17 pm, "Bruce Varley" <> wrote:

    >
    > > > "Simon" <> wrote in message

    >
    > > >news:....

    >
    > > > > Hi all,

    >
    > > > > So, I want to put 10 pH/ORP probes onto a microcontroller, with each
    > > > > probe being galvanically isolated from its peers so that the
    > > > > measurements of different probes aren't interfering with each other..
    > > > > Being more of a digital than an analogue guy, I'm not really sure how
    > > > > to proceed...

    >
    > > > > Another requirement is that it's a small solution - there's not much
    > > > > space to play with here, about 0.5" x 1.5" per probe PCB... And of
    > > > > course, the cheaper the better, given that I want 10 of them!

    >
    > > > > One chip I found to do the isolation is the adum5241 (http://
    > > > >www.analog.com/en/interface/digital-isolators/adum5241/products/produ...)
    > > > > from analog devices, I could get an isolated GND, +5v, dIn and dOut
    > > > > from that, which is what I need, but I also want -5v for the probe,
    > > > > and I'm not sure how to go about getting it. If I try to tack on a +5v-
    > > > >>-5v conversion after the adum5241, I think I'll over-run its maximum
    > > > > current delivery (10mA).

    >
    > > > > So, questions:

    >
    > > > > - Do I need to isolate the -5v if the +5v and ground are isolated ?
    > > > > I'd think so, but better to ask :)

    >
    > > > > - Is there a better way to do it ? Bound to be - this is just the
    > > > > first one I've come across :)

    >
    > > > > Cheers
    > > > >   Simon.

    >
    > > > I can see all sorts of potential 'gotchas' in this scenario. With Hi-Z
    > > > inputs, things like transients from switching power supply devices like the
    > > > 7660 could cause problems unless layout and packaging is just right. As
    > > > other posts have suggested, things are a lot simpler if you can avoid
    > > > isolation of the inputs, is the CMV really so high that you need it? If you
    > > > don't, then you can use a single power supply that can be well packaged to
    > > > avoid any noise or spikes if it incorporates switching. In fact, I'vealways
    > > > found that for audio (similar need for power rail cleanliness), a simple
    > > > rectifier/filter/linear reg arrangement is hard to beat.

    >
    > > What I've decided to do (because even the solution above works out to
    > > be expensive, given the number of probes) is to prototype a couple of
    > > boards and see if I actually need the isolation. If there's no
    > > measurable difference I'll proceed without. If there is, I'll have to
    > > spend some time figuring out / learning more about how to fix it...

    >
    > > Cheers
    > >    Simon.

    >
    > I'm pretty confident that you don't need the isolation between
    > probes.  Are they all going into the same fish tank, or into different
    > tanks in a multiple aquarium setup?
    >
    > I would ask what is the purpose of combining them into one
    > instrument?  If you can make the instrument small and inexpensive, it
    > might be better to make 10 separate instruments.  That has some
    > advantages such as cheaper prototyping and ease of repair by just
    > replacing a single inexpensive unit with a spare rather than having to
    > debug and repair a more costly multiple input device.
    >
    > Rick


    Well, they'll be in a mixture of the above. Multiple sump-tanks and
    display-tanks. Possibly with the same closed system of water, but also
    possibly with different water systems (a quarantine tank, a low-
    salinity hospital tank, a sump-and-a-display tank would all be
    different systems), for example. Add in places where you might be
    monitoring extra pH / salinity / ORP (right next to where you're
    changing those values using external systems) and the number of places
    adds up.

    The "instrument" (I call it a 'node') part *is* separate, (and a lot
    more complex), but these are intended to be plug-in modules to this
    monitoring 'node' on a network of nodes. There is then a head unit
    that interacts with the nodes and displays readings / controls.

    Perhaps I am over-worrying about it. As I mentioned above, I'm going
    to prototype a couple and see if they're reliable without the
    isolation. I was initially concerned because all the commercial
    devices do provide galvanic isolation, so I was assuming it would be
    necessary...

    Simon
     
    Simon, Jan 16, 2012
    #11
  12. Simon

    cassiope Guest

    On Jan 15, 5:40 pm, Simon <> wrote:
    > On Jan 15, 11:26 am, rickman <> wrote:
    >
    >
    >
    > > On Jan 14, 1:52 am, Simon <> wrote:

    >
    > > > On Jan 13, 9:17 pm, "Bruce Varley" <> wrote:

    >
    > > > > "Simon" <> wrote in message

    >
    > > > >news:...

    >
    > > > > > Hi all,

    >
    > > > > > So, I want to put 10 pH/ORP probes onto a microcontroller, with each
    > > > > > probe being galvanically isolated from its peers so that the
    > > > > > measurements of different probes aren't interfering with each other.
    > > > > > Being more of a digital than an analogue guy, I'm not really surehow
    > > > > > to proceed...

    >
    > > > > > Another requirement is that it's a small solution - there's not much
    > > > > > space to play with here, about 0.5" x 1.5" per probe PCB... And of
    > > > > > course, the cheaper the better, given that I want 10 of them!

    >
    > > > > > One chip I found to do the isolation is the adum5241 (http://
    > > > > >www.analog.com/en/interface/digital-isolators/adum5241/products/produ...)
    > > > > > from analog devices, I could get an isolated GND, +5v, dIn and dOut
    > > > > > from that, which is what I need, but I also want -5v for the probe,
    > > > > > and I'm not sure how to go about getting it. If I try to tack on a +5v-
    > > > > >>-5v conversion after the adum5241, I think I'll over-run its maximum
    > > > > > current delivery (10mA).

    >
    > > > > > So, questions:

    >
    > > > > > - Do I need to isolate the -5v if the +5v and ground are isolated?
    > > > > > I'd think so, but better to ask :)

    >
    > > > > > - Is there a better way to do it ? Bound to be - this is just the
    > > > > > first one I've come across :)

    >
    > > > > > Cheers
    > > > > >   Simon.

    >
    > > > > I can see all sorts of potential 'gotchas' in this scenario. With Hi-Z
    > > > > inputs, things like transients from switching power supply devices like the
    > > > > 7660 could cause problems unless layout and packaging is just right.. As
    > > > > other posts have suggested, things are a lot simpler if you can avoid
    > > > > isolation of the inputs, is the CMV really so high that you need it? If you
    > > > > don't, then you can use a single power supply that can be well packaged to
    > > > > avoid any noise or spikes if it incorporates switching. In fact, I've always
    > > > > found that for audio (similar need for power rail cleanliness), a simple
    > > > > rectifier/filter/linear reg arrangement is hard to beat.

    >
    > > > What I've decided to do (because even the solution above works out to
    > > > be expensive, given the number of probes) is to prototype a couple of
    > > > boards and see if I actually need the isolation. If there's no
    > > > measurable difference I'll proceed without. If there is, I'll have to
    > > > spend some time figuring out / learning more about how to fix it...

    >
    > > > Cheers
    > > >    Simon.

    >
    > > I'm pretty confident that you don't need the isolation between
    > > probes.  Are they all going into the same fish tank, or into different
    > > tanks in a multiple aquarium setup?

    >
    > > I would ask what is the purpose of combining them into one
    > > instrument?  If you can make the instrument small and inexpensive, it
    > > might be better to make 10 separate instruments.  That has some
    > > advantages such as cheaper prototyping and ease of repair by just
    > > replacing a single inexpensive unit with a spare rather than having to
    > > debug and repair a more costly multiple input device.

    >
    > > Rick

    >
    > Well, they'll be in a mixture of the above. Multiple sump-tanks and
    > display-tanks. Possibly with the same closed system of water, but also
    > possibly with different water systems (a quarantine tank, a low-
    > salinity hospital tank, a sump-and-a-display tank would all be
    > different systems), for example. Add in places where you might be
    > monitoring extra pH / salinity / ORP (right next to where you're
    > changing those values using external systems) and the number of places
    > adds up.
    >
    > The "instrument" (I call it a 'node') part *is* separate, (and a lot
    > more complex), but these are intended to be plug-in modules to this
    > monitoring 'node' on a network of nodes. There is then a head unit
    > that interacts with the nodes and displays readings / controls.
    >
    > Perhaps I am over-worrying about it. As I mentioned above, I'm going
    > to prototype a couple and see if they're reliable without the
    > isolation. I was initially concerned because all the commercial
    > devices do provide galvanic isolation, so I was assuming it would be
    > necessary...
    >
    > Simon


    Oh.... and you know that the quiescent potential of all of these tanks
    is
    nearly the same (i.e. guaranteed to be within a few volts at all
    times)
    because?...
     
    cassiope, Jan 17, 2012
    #12
  13. Simon

    Simon Guest

    On Jan 16, 7:06 pm, cassiope <> wrote:
    > On Jan 15, 5:40 pm, Simon <> wrote:
    >
    >
    >
    >
    >
    >
    >
    >
    >
    > > On Jan 15, 11:26 am, rickman <> wrote:

    >
    > > > On Jan 14, 1:52 am, Simon <> wrote:

    >
    > > > > On Jan 13, 9:17 pm, "Bruce Varley" <> wrote:

    >
    > > > > > "Simon" <> wrote in message

    >
    > > > > >news:..com...

    >
    > > > > > > Hi all,

    >
    > > > > > > So, I want to put 10 pH/ORP probes onto a microcontroller, witheach
    > > > > > > probe being galvanically isolated from its peers so that the
    > > > > > > measurements of different probes aren't interfering with each other.
    > > > > > > Being more of a digital than an analogue guy, I'm not really sure how
    > > > > > > to proceed...

    >
    > > > > > > Another requirement is that it's a small solution - there's notmuch
    > > > > > > space to play with here, about 0.5" x 1.5" per probe PCB... Andof
    > > > > > > course, the cheaper the better, given that I want 10 of them!

    >
    > > > > > > One chip I found to do the isolation is the adum5241 (http://
    > > > > > >www.analog.com/en/interface/digital-isolators/adum5241/products/produ...)
    > > > > > > from analog devices, I could get an isolated GND, +5v, dIn and dOut
    > > > > > > from that, which is what I need, but I also want -5v for the probe,
    > > > > > > and I'm not sure how to go about getting it. If I try to tack on a +5v-
    > > > > > >>-5v conversion after the adum5241, I think I'll over-run its maximum
    > > > > > > current delivery (10mA).

    >
    > > > > > > So, questions:

    >
    > > > > > > - Do I need to isolate the -5v if the +5v and ground are isolated ?
    > > > > > > I'd think so, but better to ask :)

    >
    > > > > > > - Is there a better way to do it ? Bound to be - this is just the
    > > > > > > first one I've come across :)

    >
    > > > > > > Cheers
    > > > > > >   Simon.

    >
    > > > > > I can see all sorts of potential 'gotchas' in this scenario. WithHi-Z
    > > > > > inputs, things like transients from switching power supply devices like the
    > > > > > 7660 could cause problems unless layout and packaging is just right. As
    > > > > > other posts have suggested, things are a lot simpler if you can avoid
    > > > > > isolation of the inputs, is the CMV really so high that you need it? If you
    > > > > > don't, then you can use a single power supply that can be well packaged to
    > > > > > avoid any noise or spikes if it incorporates switching. In fact, I've always
    > > > > > found that for audio (similar need for power rail cleanliness), asimple
    > > > > > rectifier/filter/linear reg arrangement is hard to beat.

    >
    > > > > What I've decided to do (because even the solution above works out to
    > > > > be expensive, given the number of probes) is to prototype a couple of
    > > > > boards and see if I actually need the isolation. If there's no
    > > > > measurable difference I'll proceed without. If there is, I'll have to
    > > > > spend some time figuring out / learning more about how to fix it...

    >
    > > > > Cheers
    > > > >    Simon.

    >
    > > > I'm pretty confident that you don't need the isolation between
    > > > probes.  Are they all going into the same fish tank, or into different
    > > > tanks in a multiple aquarium setup?

    >
    > > > I would ask what is the purpose of combining them into one
    > > > instrument?  If you can make the instrument small and inexpensive, it
    > > > might be better to make 10 separate instruments.  That has some
    > > > advantages such as cheaper prototyping and ease of repair by just
    > > > replacing a single inexpensive unit with a spare rather than having to
    > > > debug and repair a more costly multiple input device.

    >
    > > > Rick

    >
    > > Well, they'll be in a mixture of the above. Multiple sump-tanks and
    > > display-tanks. Possibly with the same closed system of water, but also
    > > possibly with different water systems (a quarantine tank, a low-
    > > salinity hospital tank, a sump-and-a-display tank would all be
    > > different systems), for example. Add in places where you might be
    > > monitoring extra pH / salinity / ORP (right next to where you're
    > > changing those values using external systems) and the number of places
    > > adds up.

    >
    > > The "instrument" (I call it a 'node') part *is* separate, (and a lot
    > > more complex), but these are intended to be plug-in modules to this
    > > monitoring 'node' on a network of nodes. There is then a head unit
    > > that interacts with the nodes and displays readings / controls.

    >
    > > Perhaps I am over-worrying about it. As I mentioned above, I'm going
    > > to prototype a couple and see if they're reliable without the
    > > isolation. I was initially concerned because all the commercial
    > > devices do provide galvanic isolation, so I was assuming it would be
    > > necessary...

    >
    > > Simon

    >
    > Oh.... and you know that the quiescent potential of all of these tanks
    > is
    > nearly the same (i.e. guaranteed to be within a few volts at all
    > times)
    > because?...


    Well, I don't, do I? As I mentioned way back in the first post, this
    isn't what I'm good at - I'm more familiar with the digital side of
    things, but I thought that was what the isolation was helping me
    with... Then I have people telling me that it probably won't matter...

    FWIW, it is fairly common practice to put grounding probes into tanks
    when you're measuring across multiple water systems, but I'd prefer
    that wasn't necessary. A grounding probe might help with stray
    voltage, but it'll introduce a current (from source to probe) that a
    fish might choose to sleep in, and while I don't know if that would
    harm any given fish, it's not desirable if I can avoid it...

    I do have multiple ADCs available per probe, would it be possible to
    do some sort of differential measurement to cancel out the offset ?
    Since the probe acts as sort of a tiny battery in the water, would it
    be as simple as putting an ADC on both leads of the pH probe, rather
    than measuring one lead and grounding the other ?

    Simon.
     
    Simon, Jan 17, 2012
    #13
  14. Simon

    rickman Guest

    On Jan 15, 8:40 pm, Simon <> wrote:
    > On Jan 15, 11:26 am, rickman <> wrote:
    >
    >
    >
    >
    >
    >
    >
    >
    >
    > > On Jan 14, 1:52 am, Simon <> wrote:

    >
    > > > On Jan 13, 9:17 pm, "Bruce Varley" <> wrote:

    >
    > > > > "Simon" <> wrote in message

    >
    > > > >news:...

    >
    > > > > > Hi all,

    >
    > > > > > So, I want to put 10 pH/ORP probes onto a microcontroller, with each
    > > > > > probe being galvanically isolated from its peers so that the
    > > > > > measurements of different probes aren't interfering with each other.
    > > > > > Being more of a digital than an analogue guy, I'm not really surehow
    > > > > > to proceed...

    >
    > > > > > Another requirement is that it's a small solution - there's not much
    > > > > > space to play with here, about 0.5" x 1.5" per probe PCB... And of
    > > > > > course, the cheaper the better, given that I want 10 of them!

    >
    > > > > > One chip I found to do the isolation is the adum5241 (http://
    > > > > >www.analog.com/en/interface/digital-isolators/adum5241/products/produ...)
    > > > > > from analog devices, I could get an isolated GND, +5v, dIn and dOut
    > > > > > from that, which is what I need, but I also want -5v for the probe,
    > > > > > and I'm not sure how to go about getting it. If I try to tack on a +5v-
    > > > > >>-5v conversion after the adum5241, I think I'll over-run its maximum
    > > > > > current delivery (10mA).

    >
    > > > > > So, questions:

    >
    > > > > > - Do I need to isolate the -5v if the +5v and ground are isolated?
    > > > > > I'd think so, but better to ask :)

    >
    > > > > > - Is there a better way to do it ? Bound to be - this is just the
    > > > > > first one I've come across :)

    >
    > > > > > Cheers
    > > > > >   Simon.

    >
    > > > > I can see all sorts of potential 'gotchas' in this scenario. With Hi-Z
    > > > > inputs, things like transients from switching power supply devices like the
    > > > > 7660 could cause problems unless layout and packaging is just right.. As
    > > > > other posts have suggested, things are a lot simpler if you can avoid
    > > > > isolation of the inputs, is the CMV really so high that you need it? If you
    > > > > don't, then you can use a single power supply that can be well packaged to
    > > > > avoid any noise or spikes if it incorporates switching. In fact, I've always
    > > > > found that for audio (similar need for power rail cleanliness), a simple
    > > > > rectifier/filter/linear reg arrangement is hard to beat.

    >
    > > > What I've decided to do (because even the solution above works out to
    > > > be expensive, given the number of probes) is to prototype a couple of
    > > > boards and see if I actually need the isolation. If there's no
    > > > measurable difference I'll proceed without. If there is, I'll have to
    > > > spend some time figuring out / learning more about how to fix it...

    >
    > > > Cheers
    > > >    Simon.

    >
    > > I'm pretty confident that you don't need the isolation between
    > > probes.  Are they all going into the same fish tank, or into different
    > > tanks in a multiple aquarium setup?

    >
    > > I would ask what is the purpose of combining them into one
    > > instrument?  If you can make the instrument small and inexpensive, it
    > > might be better to make 10 separate instruments.  That has some
    > > advantages such as cheaper prototyping and ease of repair by just
    > > replacing a single inexpensive unit with a spare rather than having to
    > > debug and repair a more costly multiple input device.

    >
    > > Rick

    >
    > Well, they'll be in a mixture of the above. Multiple sump-tanks and
    > display-tanks. Possibly with the same closed system of water, but also
    > possibly with different water systems (a quarantine tank, a low-
    > salinity hospital tank, a sump-and-a-display tank would all be
    > different systems), for example. Add in places where you might be
    > monitoring extra pH / salinity / ORP (right next to where you're
    > changing those values using external systems) and the number of places
    > adds up.
    >
    > The "instrument" (I call it a 'node') part *is* separate, (and a lot
    > more complex), but these are intended to be plug-in modules to this
    > monitoring 'node' on a network of nodes. There is then a head unit
    > that interacts with the nodes and displays readings / controls.
    >
    > Perhaps I am over-worrying about it. As I mentioned above, I'm going
    > to prototype a couple and see if they're reliable without the
    > isolation. I was initially concerned because all the commercial
    > devices do provide galvanic isolation, so I was assuming it would be
    > necessary...
    >
    > Simon


    Yes, I can see where the isolation could be important. I understand
    that you want it low cost. Lowest cost would perhaps be to use
    separate instruments. Power isolation is very inexpensive if you use
    wall warts. Data isolation adds to the cost.

    Perhaps you can provide isolated power which is relatively easy and
    use a wireless approach to the data. Bluetooth or Zigbee are low
    power and can run on batteries. The measurements you are talking
    about need to be taken rather seldom I believe. So batteries might
    not be a bad approach. With wireless data connections you also
    eliminate all that messy wiring... other than the wall warts. But
    then every aquarium has power nearby already.

    Is this something that will remain in place or are you talking about
    dipping a probe to take a measurement and then removing it?

    Rick
     
    rickman, Jan 17, 2012
    #14
  15. Simon

    Simon Guest

    On Jan 17, 4:36 am, rickman <> wrote:
    > On Jan 15, 8:40 pm, Simon <> wrote:
    >
    >
    >
    >
    >
    >
    >
    >
    >
    > > On Jan 15, 11:26 am, rickman <> wrote:

    >
    > > > On Jan 14, 1:52 am, Simon <> wrote:

    >
    > > > > On Jan 13, 9:17 pm, "Bruce Varley" <> wrote:

    >
    > > > > > "Simon" <> wrote in message

    >
    > > > > >news:..com...

    >
    > > > > > > Hi all,

    >
    > > > > > > So, I want to put 10 pH/ORP probes onto a microcontroller, witheach
    > > > > > > probe being galvanically isolated from its peers so that the
    > > > > > > measurements of different probes aren't interfering with each other.
    > > > > > > Being more of a digital than an analogue guy, I'm not really sure how
    > > > > > > to proceed...

    >
    > > > > > > Another requirement is that it's a small solution - there's notmuch
    > > > > > > space to play with here, about 0.5" x 1.5" per probe PCB... Andof
    > > > > > > course, the cheaper the better, given that I want 10 of them!

    >
    > > > > > > One chip I found to do the isolation is the adum5241 (http://
    > > > > > >www.analog.com/en/interface/digital-isolators/adum5241/products/produ...)
    > > > > > > from analog devices, I could get an isolated GND, +5v, dIn and dOut
    > > > > > > from that, which is what I need, but I also want -5v for the probe,
    > > > > > > and I'm not sure how to go about getting it. If I try to tack on a +5v-
    > > > > > >>-5v conversion after the adum5241, I think I'll over-run its maximum
    > > > > > > current delivery (10mA).

    >
    > > > > > > So, questions:

    >
    > > > > > > - Do I need to isolate the -5v if the +5v and ground are isolated ?
    > > > > > > I'd think so, but better to ask :)

    >
    > > > > > > - Is there a better way to do it ? Bound to be - this is just the
    > > > > > > first one I've come across :)

    >
    > > > > > > Cheers
    > > > > > >   Simon.

    >
    > > > > > I can see all sorts of potential 'gotchas' in this scenario. WithHi-Z
    > > > > > inputs, things like transients from switching power supply devices like the
    > > > > > 7660 could cause problems unless layout and packaging is just right. As
    > > > > > other posts have suggested, things are a lot simpler if you can avoid
    > > > > > isolation of the inputs, is the CMV really so high that you need it? If you
    > > > > > don't, then you can use a single power supply that can be well packaged to
    > > > > > avoid any noise or spikes if it incorporates switching. In fact, I've always
    > > > > > found that for audio (similar need for power rail cleanliness), asimple
    > > > > > rectifier/filter/linear reg arrangement is hard to beat.

    >
    > > > > What I've decided to do (because even the solution above works out to
    > > > > be expensive, given the number of probes) is to prototype a couple of
    > > > > boards and see if I actually need the isolation. If there's no
    > > > > measurable difference I'll proceed without. If there is, I'll have to
    > > > > spend some time figuring out / learning more about how to fix it...

    >
    > > > > Cheers
    > > > >    Simon.

    >
    > > > I'm pretty confident that you don't need the isolation between
    > > > probes.  Are they all going into the same fish tank, or into different
    > > > tanks in a multiple aquarium setup?

    >
    > > > I would ask what is the purpose of combining them into one
    > > > instrument?  If you can make the instrument small and inexpensive, it
    > > > might be better to make 10 separate instruments.  That has some
    > > > advantages such as cheaper prototyping and ease of repair by just
    > > > replacing a single inexpensive unit with a spare rather than having to
    > > > debug and repair a more costly multiple input device.

    >
    > > > Rick

    >
    > > Well, they'll be in a mixture of the above. Multiple sump-tanks and
    > > display-tanks. Possibly with the same closed system of water, but also
    > > possibly with different water systems (a quarantine tank, a low-
    > > salinity hospital tank, a sump-and-a-display tank would all be
    > > different systems), for example. Add in places where you might be
    > > monitoring extra pH / salinity / ORP (right next to where you're
    > > changing those values using external systems) and the number of places
    > > adds up.

    >
    > > The "instrument" (I call it a 'node') part *is* separate, (and a lot
    > > more complex), but these are intended to be plug-in modules to this
    > > monitoring 'node' on a network of nodes. There is then a head unit
    > > that interacts with the nodes and displays readings / controls.

    >
    > > Perhaps I am over-worrying about it. As I mentioned above, I'm going
    > > to prototype a couple and see if they're reliable without the
    > > isolation. I was initially concerned because all the commercial
    > > devices do provide galvanic isolation, so I was assuming it would be
    > > necessary...

    >
    > > Simon

    >
    > Yes, I can see where the isolation could be important.  I understand
    > that you want it low cost.  Lowest cost would perhaps be to use
    > separate instruments.  Power isolation is very inexpensive if you use
    > wall warts.  Data isolation adds to the cost.
    >
    > Perhaps you can provide isolated power which is relatively easy and
    > use a wireless approach to the data.  Bluetooth or Zigbee are low
    > power and can run on batteries.  The measurements you are talking
    > about need to be taken rather seldom I believe.  So batteries might
    > not be a bad approach.  With wireless data connections you also
    > eliminate all that messy wiring... other than the wall warts.  But
    > then every aquarium has power nearby already.
    >
    > Is this something that will remain in place or are you talking about
    > dipping a probe to take a measurement and then removing it?
    >
    > Rick


    Well, ideally it ought to stay in place and provide constant readings
    - in fact one of the ideas I had for the thing was to have the head-
    unit (graphical, user-friendly) have reminders to recalibrate the
    probes after user-set intervals because they tend to drift over time
    if left submerged.

    The circuit at http://www.emesystems.com/OL2ph.htm seems to use the
    ICL7660 to source the -5v supply for a pH probe and they supply
    monitoring systems. If I put an ADuM5402 (http://search.digikey.com/us/
    en/products/ADUM5402ARWZ/ADUM5402ARWZ-ND/1873720) in-between the main
    microcontroller and the slave one that's taking the readings, I could
    presumably generate an isolated +/- 5V @ up to 500mA, which ought to
    be plenty... I also get 2 data lines in each direction which I can use
    for {outputs: uart-tx, chip-select}, and {inputs: uart-rx, sensor-
    present}. Getting expensive when you scale the probe numbers, but if
    that's what it takes ... :)

    I'd thought about wireless before, but I didn't really want to make
    them battery powered. The readings are "infrequent" - take a reading
    once per minute and average for a 5-minute period. I'd hate to go away
    on vacation and come back to find out the batteries had run out
    though... It's not even worth thinking about what damage a pH
    Controller can do if the pH readings are skewed... So, battery backup,
    yes. Battery powered, no :)

    Cheers
    Simon.
     
    Simon, Jan 17, 2012
    #15
  16. Simon

    rickman Guest

    On Jan 17, 1:03 pm, Simon <> wrote:
    > On Jan 17, 4:36 am, rickman <> wrote:
    >
    >
    >
    >
    >
    >
    >
    >
    >
    > > On Jan 15, 8:40 pm, Simon <> wrote:

    >
    > > > On Jan 15, 11:26 am, rickman <> wrote:

    >
    > > > > On Jan 14, 1:52 am, Simon <> wrote:

    >
    > > > > > On Jan 13, 9:17 pm, "Bruce Varley" <> wrote:

    >
    > > > > > > "Simon" <> wrote in message

    >
    > > > > > >news:...

    >
    > > > > > > > Hi all,

    >
    > > > > > > > So, I want to put 10 pH/ORP probes onto a microcontroller, with each
    > > > > > > > probe being galvanically isolated from its peers so that the
    > > > > > > > measurements of different probes aren't interfering with eachother.
    > > > > > > > Being more of a digital than an analogue guy, I'm not really sure how
    > > > > > > > to proceed...

    >
    > > > > > > > Another requirement is that it's a small solution - there's not much
    > > > > > > > space to play with here, about 0.5" x 1.5" per probe PCB... And of
    > > > > > > > course, the cheaper the better, given that I want 10 of them!

    >
    > > > > > > > One chip I found to do the isolation is the adum5241 (http://
    > > > > > > >www.analog.com/en/interface/digital-isolators/adum5241/products/produ...)
    > > > > > > > from analog devices, I could get an isolated GND, +5v, dIn and dOut
    > > > > > > > from that, which is what I need, but I also want -5v for the probe,
    > > > > > > > and I'm not sure how to go about getting it. If I try to tackon a +5v-
    > > > > > > >>-5v conversion after the adum5241, I think I'll over-run its maximum
    > > > > > > > current delivery (10mA).

    >
    > > > > > > > So, questions:

    >
    > > > > > > > - Do I need to isolate the -5v if the +5v and ground are isolated ?
    > > > > > > > I'd think so, but better to ask :)

    >
    > > > > > > > - Is there a better way to do it ? Bound to be - this is justthe
    > > > > > > > first one I've come across :)

    >
    > > > > > > > Cheers
    > > > > > > >   Simon.

    >
    > > > > > > I can see all sorts of potential 'gotchas' in this scenario. With Hi-Z
    > > > > > > inputs, things like transients from switching power supply devices like the
    > > > > > > 7660 could cause problems unless layout and packaging is just right. As
    > > > > > > other posts have suggested, things are a lot simpler if you canavoid
    > > > > > > isolation of the inputs, is the CMV really so high that you need it? If you
    > > > > > > don't, then you can use a single power supply that can be well packaged to
    > > > > > > avoid any noise or spikes if it incorporates switching. In fact, I've always
    > > > > > > found that for audio (similar need for power rail cleanliness),a simple
    > > > > > > rectifier/filter/linear reg arrangement is hard to beat.

    >
    > > > > > What I've decided to do (because even the solution above works out to
    > > > > > be expensive, given the number of probes) is to prototype a couple of
    > > > > > boards and see if I actually need the isolation. If there's no
    > > > > > measurable difference I'll proceed without. If there is, I'll have to
    > > > > > spend some time figuring out / learning more about how to fix it....

    >
    > > > > > Cheers
    > > > > >    Simon.

    >
    > > > > I'm pretty confident that you don't need the isolation between
    > > > > probes.  Are they all going into the same fish tank, or into different
    > > > > tanks in a multiple aquarium setup?

    >
    > > > > I would ask what is the purpose of combining them into one
    > > > > instrument?  If you can make the instrument small and inexpensive, it
    > > > > might be better to make 10 separate instruments.  That has some
    > > > > advantages such as cheaper prototyping and ease of repair by just
    > > > > replacing a single inexpensive unit with a spare rather than havingto
    > > > > debug and repair a more costly multiple input device.

    >
    > > > > Rick

    >
    > > > Well, they'll be in a mixture of the above. Multiple sump-tanks and
    > > > display-tanks. Possibly with the same closed system of water, but also
    > > > possibly with different water systems (a quarantine tank, a low-
    > > > salinity hospital tank, a sump-and-a-display tank would all be
    > > > different systems), for example. Add in places where you might be
    > > > monitoring extra pH / salinity / ORP (right next to where you're
    > > > changing those values using external systems) and the number of places
    > > > adds up.

    >
    > > > The "instrument" (I call it a 'node') part *is* separate, (and a lot
    > > > more complex), but these are intended to be plug-in modules to this
    > > > monitoring 'node' on a network of nodes. There is then a head unit
    > > > that interacts with the nodes and displays readings / controls.

    >
    > > > Perhaps I am over-worrying about it. As I mentioned above, I'm going
    > > > to prototype a couple and see if they're reliable without the
    > > > isolation. I was initially concerned because all the commercial
    > > > devices do provide galvanic isolation, so I was assuming it would be
    > > > necessary...

    >
    > > > Simon

    >
    > > Yes, I can see where the isolation could be important.  I understand
    > > that you want it low cost.  Lowest cost would perhaps be to use
    > > separate instruments.  Power isolation is very inexpensive if you use
    > > wall warts.  Data isolation adds to the cost.

    >
    > > Perhaps you can provide isolated power which is relatively easy and
    > > use a wireless approach to the data.  Bluetooth or Zigbee are low
    > > power and can run on batteries.  The measurements you are talking
    > > about need to be taken rather seldom I believe.  So batteries might
    > > not be a bad approach.  With wireless data connections you also
    > > eliminate all that messy wiring... other than the wall warts.  But
    > > then every aquarium has power nearby already.

    >
    > > Is this something that will remain in place or are you talking about
    > > dipping a probe to take a measurement and then removing it?

    >
    > > Rick

    >
    > Well, ideally it ought to stay in place and provide constant readings
    > - in fact one of the ideas I had for the thing was to have the head-
    > unit (graphical, user-friendly) have reminders to recalibrate the
    > probes after user-set intervals because they tend to drift over time
    > if left submerged.
    >
    > The circuit athttp://www.emesystems.com/OL2ph.htmseems to use the
    > ICL7660 to source the -5v supply for a pH probe and they supply
    > monitoring systems. If I put an ADuM5402 (http://search.digikey.com/us/
    > en/products/ADUM5402ARWZ/ADUM5402ARWZ-ND/1873720) in-between the main
    > microcontroller and the slave one that's taking the readings, I could
    > presumably generate an isolated +/- 5V @ up to 500mA, which ought to
    > be plenty... I also get 2 data lines in each direction which I can use
    > for {outputs: uart-tx, chip-select}, and {inputs: uart-rx, sensor-
    > present}.  Getting expensive when you scale the probe numbers, but if
    > that's what it takes ... :)
    >
    > I'd thought about wireless before, but I didn't really want to make
    > them battery powered. The readings are "infrequent" - take a reading
    > once per minute and average for a 5-minute period. I'd hate to go away
    > on vacation and come back to find out the batteries had run out
    > though... It's not even worth thinking about what damage a pH
    > Controller can do if the pH readings are skewed... So, battery backup,
    > yes. Battery powered, no :)
    >
    > Cheers
    >    Simon.


    Wireless doesn't have to be battery powered. A small wall wart will
    provide isolated power. But the question is does wireless buy you
    much? I assume you need a power strip near each aquarium anyway while
    running cable to each pH sensor can be a pain.

    Rick
     
    rickman, Jan 17, 2012
    #16
  17. "Simon" <> wrote in message
    news:...
    > I'd thought about wireless before, but I didn't really want to make
    > them battery powered. The readings are "infrequent" - take a reading
    > once per minute and average for a 5-minute period. I'd hate to go away
    > on vacation and come back to find out the batteries had run out
    > though... It's not even worth thinking about what damage a pH
    > Controller can do if the pH readings are skewed... So, battery backup,
    > yes. Battery powered, no :)


    If you use a WiFly module from Roving Networks (RN-131 for example), you can
    setup a to do an AD conversion every few minutes and send this data to a
    server in various formats. While idle, it draws 4uA. It will run directly
    off two penlite batteries for more than a year.

    Meindert
     
    Meindert Sprang, Jan 18, 2012
    #17
  18. Simon

    Simon Guest

    On Jan 18, 2:18 am, "Meindert Sprang" <>
    wrote:
    > "Simon" <> wrote in message
    >
    > news:...
    >
    > > I'd thought about wireless before, but I didn't really want to make
    > > them battery powered. The readings are "infrequent" - take a reading
    > > once per minute and average for a 5-minute period. I'd hate to go away
    > > on vacation and come back to find out the batteries had run out
    > > though... It's not even worth thinking about what damage a pH
    > > Controller can do if the pH readings are skewed... So, battery backup,
    > > yes. Battery powered, no :)

    >
    > If you use a WiFly module from Roving Networks (RN-131 for example), you can
    > setup a to do an AD conversion every few minutes and send this data to a
    > server in various formats. While idle, it draws 4uA. It will run directly
    > off two penlite batteries for more than a year.
    >
    > Meindert


    Just checking the price on those ... *cough*! !*&%^&* *splutter*! $39
    each! => $390 just for the transport of data!

    Am I being naive to do (as above)

    "The circuit at http://www.emesystems.com/OL2ph.htm seems to use the
    ICL7660 to source the -5v supply for a pH probe and they supply
    monitoring systems.

    If I put an ADuM5402 (http://search.digikey.com/us/
    en/products/ADUM5402ARWZ/ADUM5402ARWZ-ND/1873720) in-between the main
    microcontroller and the slave one that's taking the readings, I could
    presumably generate an isolated +/- 5V @ up to 500mA, which ought to
    be plenty... I also get 2 data lines in each direction which I can
    use
    for {outputs: uart-tx, chip-select}, and {inputs: uart-rx, sensor-
    present}."

    In quantities of 25, an ICL7660 is $1.26, an ADuM5402 is $8.95 =>
    $10.21 per probe, not $39

    If the guy supplies these for a living, and he's using the 7660 to
    generate the -5v, I'm happy to do the same [grin]. I know there's the
    circuit layout to contend with, and I'll try to make the isolated part
    of the PCB as separate as possible from the non-isolated part. The
    plan would be to do something like:

    [connection] <==> [isolation] <==> [ADC / conditioning circuit]

    .... so I'd run the ADC (actually an AVR microcontroller) and the op-
    amps on the same circuit, and just do the isolation as we traverse
    back to the host board. That ought to make the PCB geometry easy to
    manage.

    Simon.
     
    Simon, Jan 18, 2012
    #18
  19. Simon

    Rob Gaddi Guest

    On Wed, 18 Jan 2012 14:57:21 -0800 (PST)
    Simon <> wrote:

    > On Jan 18, 2:18 am, "Meindert Sprang" <>
    > wrote:
    > > "Simon" <> wrote in message
    > >
    > > news:....
    > >
    > > > I'd thought about wireless before, but I didn't really want to make
    > > > them battery powered. The readings are "infrequent" - take a reading
    > > > once per minute and average for a 5-minute period. I'd hate to go away
    > > > on vacation and come back to find out the batteries had run out
    > > > though... It's not even worth thinking about what damage a pH
    > > > Controller can do if the pH readings are skewed... So, battery backup,
    > > > yes. Battery powered, no :)

    > >
    > > If you use a WiFly module from Roving Networks (RN-131 for example), you can
    > > setup a to do an AD conversion every few minutes and send this data to a
    > > server in various formats. While idle, it draws 4uA. It will run directly
    > > off two penlite batteries for more than a year.
    > >
    > > Meindert

    >
    > Just checking the price on those ... *cough*! !*&%^&* *splutter*! $39
    > each! => $390 just for the transport of data!
    >
    > Am I being naive to do (as above)
    >
    > "The circuit at http://www.emesystems.com/OL2ph.htm seems to use the
    > ICL7660 to source the -5v supply for a pH probe and they supply
    > monitoring systems.
    >
    > If I put an ADuM5402 (http://search.digikey.com/us/
    > en/products/ADUM5402ARWZ/ADUM5402ARWZ-ND/1873720) in-between the main
    > microcontroller and the slave one that's taking the readings, I could
    > presumably generate an isolated +/- 5V @ up to 500mA, which ought to
    > be plenty... I also get 2 data lines in each direction which I can
    > use
    > for {outputs: uart-tx, chip-select}, and {inputs: uart-rx, sensor-
    > present}."
    >
    > In quantities of 25, an ICL7660 is $1.26, an ADuM5402 is $8.95 =>
    > $10.21 per probe, not $39
    >
    > If the guy supplies these for a living, and he's using the 7660 to
    > generate the -5v, I'm happy to do the same [grin]. I know there's the
    > circuit layout to contend with, and I'll try to make the isolated part
    > of the PCB as separate as possible from the non-isolated part. The
    > plan would be to do something like:
    >
    > [connection] <==> [isolation] <==> [ADC / conditioning circuit]
    >
    > ... so I'd run the ADC (actually an AVR microcontroller) and the op-
    > amps on the same circuit, and just do the isolation as we traverse
    > back to the host board. That ought to make the PCB geometry easy to
    > manage.
    >
    > Simon.


    If your supply voltages aren't critical then for a few more parts you coulduse an ADuM1401 or 1402 at $2.50/ instead and power the whole thing open loop from a transformer with an 11V P-P secondary and a pair of diodes to make a half-bridge rectifier. You could drive all the transformer primaries in parallel.

    --
    Rob Gaddi, Highland Technology -- www.highlandtechnology.com
    Email address domain is currently out of order. See above to fix.
     
    Rob Gaddi, Jan 19, 2012
    #19
  20. Simon

    Guest

    On 18 Jan., 23:57, Simon <> wrote:
    > On Jan 18, 2:18 am, "Meindert Sprang" <>
    > wrote:
    >
    >
    >
    >
    >
    >
    >
    >
    >
    > > "Simon" <> wrote in message

    >
    > >news:....

    >
    > > > I'd thought about wireless before, but I didn't really want to make
    > > > them battery powered. The readings are "infrequent" - take a reading
    > > > once per minute and average for a 5-minute period. I'd hate to go away
    > > > on vacation and come back to find out the batteries had run out
    > > > though... It's not even worth thinking about what damage a pH
    > > > Controller can do if the pH readings are skewed... So, battery backup,
    > > > yes. Battery powered, no :)

    >
    > > If you use a WiFly module from Roving Networks (RN-131 for example), you can
    > > setup a to do an AD conversion every few minutes and send this data to a
    > > server in various formats. While idle, it draws 4uA. It will run directly
    > > off two penlite batteries for more than a year.

    >
    > > Meindert

    >
    > Just checking the price on those ... *cough*! !*&%^&* *splutter*!  $39
    > each! => $390 just for the transport of data!
    >
    > Am I being naive to do (as above)
    >
    > "The circuit athttp://www.emesystems.com/OL2ph.htmseems to use the
    > ICL7660 to source the -5v supply for a pH probe and they supply
    > monitoring systems.
    >
    > If I put an ADuM5402 (http://search.digikey.com/us/
    > en/products/ADUM5402ARWZ/ADUM5402ARWZ-ND/1873720) in-between the main
    > microcontroller and the slave one that's taking the readings, I could
    > presumably generate an isolated +/- 5V @ up to 500mA, which ought to
    > be plenty... I also get 2 data lines in each direction which I can
    > use
    > for {outputs: uart-tx, chip-select}, and {inputs: uart-rx, sensor-
    > present}."
    >
    > In quantities of 25, an ICL7660 is $1.26, an ADuM5402 is $8.95 =>
    > $10.21 per probe, not $39
    >
    > If the guy supplies these for a living, and he's using the 7660 to
    > generate the -5v, I'm happy to do the same [grin]. I know there's the
    > circuit layout to contend with, and I'll try to make the isolated part
    > of the PCB as separate as possible from the non-isolated part. The
    > plan would be to do something like:
    >
    > [connection] <==> [isolation] <==> [ADC / conditioning circuit]
    >
    > ... so I'd run the ADC (actually an AVR microcontroller) and the op-
    > amps on the same circuit, and just do the isolation as we traverse
    > back to the host board. That ought to make the PCB geometry easy to
    > manage.
    >
    > Simon.


    these?

    http://search.digikey.com/us/en/products/VAT1-S5-D5-SMT/102-1385-2-ND/989840
    http://search.digikey.com/us/en/products/H11L1S(TA)/1080-1201-2-ND/2675692

    -Lasse
     
    , Jan 19, 2012
    #20
    1. Advertising

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