• about isolator for PH probe

    From Piyush Kamdi@21:1/5 to All on Sun Jul 31 21:55:46 2022
    i use the analog isolator from atlast scintific but it is quiet expensive so suggest me the cheap isolator circuit in india . (for ph probe )

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  • From Ricky@21:1/5 to Piyush Kamdi on Sun Jul 31 23:25:22 2022
    On Monday, August 1, 2022 at 12:55:48 AM UTC-4, Piyush Kamdi wrote:
    i use the analog isolator from atlast scintific but it is quiet expensive so suggest me the cheap isolator circuit in india . (for ph probe )

    By "isolator", I assume you mean a high input impedance amplifier? I know pH probe have a very high impedance, so nearly any current draw will create an error. I don't know how high the impedance needs to be. This may require a circuit with a
    bootstrap and guard rings. Do you have a schematic? Perhaps you can make your own. Have you looked for sample circuits?

    --

    Rick C.

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  • From Martin Brown@21:1/5 to Ricky on Mon Aug 1 10:54:41 2022
    On 01/08/2022 07:25, Ricky wrote:
    On Monday, August 1, 2022 at 12:55:48 AM UTC-4, Piyush Kamdi wrote:
    i use the analog isolator from atlast scintific but it is quiet
    expensive so suggest me the cheap isolator circuit in india . (for
    ph probe )

    Their OEM pH board doesn't look unreasonably priced to me:

    https://atlas-scientific.com/embedded-solutions/ph-oem-circuit/

    By "isolator", I assume you mean a high input impedance amplifier? I
    know pH probe have a very high impedance, so nearly any current draw
    will create an error. I don't know how high the impedance needs to
    be. This may require a circuit with a bootstrap and guard rings.

    A working approximation is around 1G worst case.

    You would probably get away without a guard band provided that the
    humidity and ambient temperature isn't too high. Many opamps the input
    bias current rises quickly with temperature above 25C ambient.

    Do you have a schematic? Perhaps you can make your own. Have you
    looked for sample circuits?

    If you don't care too much about thermal drift or precision calibration
    then almost any modern FET input opamp will be adequate in a unity gain
    voltage follower configuration. eg #9.3 in TL071 datasheet

    https://www.ti.com/lit/ds/symlink/tl071.pdf

    Or a more recent automotive part like

    https://www.ti.com/lit/ds/symlink/ina333-q1.pdf

    If you do care about precision calibration and have to ask this question
    then you are probably much better off with a commercial product.

    --
    Regards,
    Martin Brown

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