pH circuit

Discussion in 'General Electronics Chat' started by newhobby, Jul 23, 2014.

  1. newhobby

    Thread Starter New Member

    Mar 26, 2014
    24
    0
    Hi,

    I'm having a hard time figuring out what is wrong with this circuit.
    Can anyone spot something wrong with it?
    I've built about a dozen of them and I only got a couple to work as intended.
    It is supposed to work as a unity op amp buffer to give me a voltage generated from a pH probe.
    The DC-DC converter is an isolated +/- 5V output.
    I'm supposed to get 150mV and I get over 3V in the output pin.
    Thank you in advance.

    RI
     
  2. JohnInTX

    Moderator

    Jun 26, 2012
    2,348
    1,029
    pH measurement involves very high impedances.

    I would look for any source of leakages in the circuit between the probe and the amplifier. What is the leakage in C43? Anything on the board that can conduct even uA is going to be a problem - including fingerprints and in rare cases, airflow over the board (one of mine is wrapped in anti-static bubble wrap for just that purpose when on the bench). The redoubtable Bob Pease once told of running his PCBs through a dishwasher to remove surface contaminants. Never tried that...

    Here's an apnote that describes some issues.

    What problems in particular are you having?
     
    Last edited: Jul 23, 2014
  3. newhobby

    Thread Starter New Member

    Mar 26, 2014
    24
    0
    But even with or without C43, I get the same results.
    The voltage in the output pin is not as expected, which is around 150mV.
    I made sure to locate the IC as close as I could from the BNC connector.
     
  4. JohnInTX

    Moderator

    Jun 26, 2012
    2,348
    1,029
    Sorry. I see in the first post your problem is that you get way out of range voltage on the output yes? If so, I'd visit the offset connection between pins 1-5. Not sure they can be shorted.
     
  5. newhobby

    Thread Starter New Member

    Mar 26, 2014
    24
    0
    Correct. It goes way out of the expected range.
    I don't think it is the offset, because if I disconnect the ph probe and short out the center pin to 0V, I get 0V as expected in the output.
    Besides, I also tried with different op amps, such as tl082, which doesn't have offset pins with same results.
    It's driving me crazy.
    I also tried adding resistors to make it a gain of 10 instead of unity gain and also the same thing... Output is way out of range.
     
  6. wayneh

    Expert

    Sep 9, 2010
    12,153
    3,059
    I'd try powering it with a couple 6V or 9V batteries, connected in series to give ±9V and "ground" at their junction. I question whether the PSU is causing your problem.

    Do you have bypass caps on the power supply pins? One for each pole to ground, 0.1µF each.
     
  7. newhobby

    Thread Starter New Member

    Mar 26, 2014
    24
    0
    Yes, I have bypass of ceramic 100nF and also tantalum 10uF for both positive and negative.
    I have also tried with a non isolated circuit with same results :(
    I used ICL7660 to generate the negative voltage.
     
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