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Pimp my pH input

Discussion in 'Electronic Basics' started by Fox one! Fox one!, Jun 20, 2007.

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  1. Hi,

    I am trying to debug the first stage of a pH input circuit. The first stage
    is just a voltage follower made from a TL084

    The pH probe centre electrode is connected directly to the non-inverting
    input of one opamp of a TL084 (this connection does not touch the PCB, it's
    all in mid air)
    The pH probe centre electrode also has a 33pF ceramic capacitor to ground,
    close to the TL084
    The pH probe outer electrode is connected to ground
    There is a 1M feedback resistor from the output of the opamp to the
    inverting input.

    When I connect a pH probe, the output of this first stage is about 75mV.
    According to the TL084 datasheet it has an input offset voltage of 6mV so I
    can't understand where the 75mV comes from . I have tried 2 different pH
    probes, one old one new, from different manufacturers, one with pointy tip
    and one with round bulb, but they both give the same result so I don't think
    it's the ph sensor.

    The power supply I think is my problem, I decided to use the +10V and -10V
    from a MAX232 rather than use a chip like a ICL7660. The MAX232 can only
    supply the +V supply of the TL084 with 5.77V and the -V supply with -3.88V.

    I am trying to calculate whether this is the problem. With this power
    supply, the 'ground' for the opamp is actually at 0.945V relative to circuit
    ground. With the pH probe in a pH 7 calibration solution, the output
    between electrodes should be close to 0V, so in this case from the opamp's
    point of view both inputs are at -0.945V, and the only signal on the output
    should be the offset voltage + (common mode voltage * CMRR).

    The CMRR on the datasheet is 86dB, or 1 / 20,000. If the common mode
    voltage on the inputs is 0.945V the contribution to the output should only
    be 47uV i.e. nothing.

    So, why is the first stage giving 75mV? Should I use the supply-voltage
    rejection ratio instead? It's also 86dB so adding them together still
    doesn't explain it.

    The output of the first stage is connected via a 510K resistor to the next
    stage so it is hard to imagine that that is affecting it.

    Any ideas would be much appreciated.
     
  2. Tom Biasi

    Tom Biasi Guest

    Can you post the circuit somewhere and the specs on the probe?

    Tom
     
  3. A pH probe set is a kind of electro-chemical cell, with one
    fairly low resistance connection to the solution (the
    reference probe that makes a connection to the solution with
    minimal voltage drop) and one very high resistance
    connection that includes the voltage offset produced by the
    cell. The hydrogen ion (naked protons that are much smaller
    than any atom) concentration of the solution determines the
    voltage produced by the cell, as they diffuse through the
    glass bulb of the measurement probe and produce a net
    voltage across the glass membrane.

    You must pass nearly zero current through either of these
    connections to get an undistorted measure of the voltage
    difference produced by the protons. It sounds like you have
    a very high resistance load (very limited current through)
    the measurement probe, but how about the reference probe.
    If the solution has other ground paths, you may be passing
    current through it, and getting some voltage drop through
    it. Is the test solution in a glass container (and
    electrically isolated from any current path that includes
    the reference probe) or is it grounded through other means
    than just the reference probe?
     
  4. Mr.pH

    Mr.pH Guest

  5. Hi Tom,

    Here is the circuit.



    1M
    ___
    .-----------|___|---------.
    | |
    | |
    | 5.77V |
    | |
    | | |
    | | |
    | | |
    pH probe | |\| |
    .-----. '-----------|-\ |
    | G | | >----------o-------------
    |_-_-_|--------------------|+/
    | | | |/| TL084
    '--o--' --- |
    | --- |
    | | |
    |
    0V 0V |

    -3.88V


    (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)
     
  6. Are the DC supplies batteries or AC derived supplies? If
    the latter, you have to include the AC paths through the
    inter winding capacitances back to the line and from there
    to ground and from there, back to the zero volt node.

    Have you checked to see if there is any AC hum on the opamp
    output?
     
  7. Thankyou that is a good page, I have read it before. My circuit is very
    similar except it is unity gain. The only reason I have the resistor in the
    feedback at all is that I read on this page:
    http://www.ibiblio.org/obp/electricCircuits/Semi/SEMI_8.html (search for
    "ph") that if the resistor has the same impedance as the probe, the voltage
    across it due to the opamp leakage current will match and thus cancel the
    voltage error due to the same leakage through the probe.

    I forgot to mention to Tom the specs of the probe, but no I don't know what
    they are. I am trying to make it work with typical probes, which from what
    I've heard are round bulb types and have a impedance of about 400Mohms.
     
  8. Fox one! Fox one! wrote:
    (snip)
    The probe resistance is not known and very high, compared to
    1 meg. You need an opamp that has a bias current so low
    that it produces an insignificant drop across the 100 meg or
    so probe resistance. Then you can eliminate the feedback
    resistor. I would go for an opamp with sub pico ampere bias
    current. perhaps the LMC6001 with 25 fempto amps of bias
    current.
    http://www.national.com/ds/LM/LMC6001.pdf
     

  9. My understanding of the probes is that the junction and the reference are
    all in the one package with only two terminals. In my circuit one terminal
    is connected to ground and the other directly to the non-inverting input of
    a TL084, so that should be zero current shouldn't it?

    It does respond to ph changes, I should have mentioned this:
    In a pH 7 buffer the output is 75mV (expecting 0mV)
    In a pH 4 buffer the output is 195mV (expecting 180mV, as output should
    change roughly 60mV for each pH)

    The test solutions are in disposable plastic cups with no other wires in
    them.

    ..
     
  10. That is just comp[act [packaging. There are still two
    distance probes that contact the solution, one through a
    direct electrical connection through a fluid bridge, and one
    through a membrane.
    I see. That simplifies things from a DC standpoint. Is the
    cup sitting on a metal sink or a non conductive surface? In
    a separate post, I have questions for you about the possible
    hum being injected through the supply.
     
  11. I've noticed that if I ground the ph input the opamp gives 1.7mV!! Maybe
    the probe is the problem?
     
  12. Good point I just thought since the space is there I would put in the
    largest value I have in my parts pile. I tried bypassing it (I am beginning
    to get superstitious now) and it made no difference (of course I knew it
    wouldn't).
    Ah yes I have seen that part but this is a project for a hobby community on
    the net, so I am trying to use parts you can get from the local electronics
    store in case they ever have to repair it. My local store stocks these
    which seem like they are the right type (FET input):

    CA3130 5pA bias current
    CA3140 10pA
    TL084 30pA (what I am using)
    TL074 65pA

    By my calculations, assuming the probe has an impedance of around 675Mohm
    (based on 'standard glass', hemispherical shape on this page:
    http://www.asi-sensors.com/learning/lrn_ph_glass.aspx) then a worst case
    30pA leakage current would cause a voltage drop of 20mV. That is
    surprisingly large but still doesn't explain where 75mV is coming from.
     
  13. I've tried a DC supply and an AC supply (my PCB supports both) but both give
    the same results. In normal operation I'd expect it to be used with an AC
    supply, and for it to be installed in an earthed metal case, with circuit
    ground connected to the case by the screw mounting points. i.e. circuit
    ground should aways be mains earth

    I've probed it with my CRO and no I can't see any mains frequency on the
    output at all.

    Thanks for your help.
     

  14. They are sitting on top of my computer. I tried putting a book between them
    and lifting the cup upwards but that made no difference.
     
  15. Then I am fresh out of leads to follow. Sorry. Perhaps
    your probe set is just contaminated or the reference probe
    plug clogged and is producing the 75 mV offset, no matter
    what amplifier is used to buffer it. I have seen probe sets
    go bad this way. You might try leaving the probe in the pH
    7 buffer solution for a few days and see if the offset
    drifts in either direction. No need to keep the amplifier
    on all the time, though it shouldn't hurt.
     
  16. Guest

    Or your opamp has a lot higher bias current than it is supposed to
    have. Replace the probe with a 10 meg resistor and see how high the
    output goes.
     
  17. Sheesh that was the problem! I don't have a 10meg resistor but I replaced
    the TL084 and voila, it works. I could swear I already tried this, god
    knows I changed everything else... Thanks a lot for your help.
     
  18. :)

    Live long and prosper.
     
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