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Lmp7721 as transimpedance single supply amp

david ericson
Prodigy 30 points
Other Parts Discussed in Thread: LMP7721

Hi,

I have an lmp7721 eval, I would like to use it as a single ended amp to amplify the ph range over 7.0.

The range is negative and is 0mv to approx 250mv.  Ph probes are very high impedance.

Can you advise the best mode? I assume transimpedance . Can you advise me the optimal configuration in terms of the feedback resistor and other necessary components?

Thank you,

David Ericson

  • 0
    TI__Guru 62456 points
    Hello David,

    This seems similar to a previous post..

    e2e.ti.com/.../1730514

    Please see AN-1852, "Designing With pH Electrodes"

    www.ti.com/.../getliterature.tsp

    The transimpedance amplifier input impedance would be too low. A non-inverting amplifier is usually used, and this also allows you to add gain if necessary.

    Since you want to use the 'negative" side, you would have to wire the probe backwards so that the amplifier sees a positive voltage. Depending on the electrode style used, this may be an issue since this may make the outer shield of the coax conductor become the sensitive input conductor - which would cause noise pickup and leakage (destroying the "guarding" effect).

    As described in AN-1852, a bias is usually applied to the common end of the electrode to allow for the bipolar output of the sensor. Not convenient, but high-impedance circuits are never simple...

    Regards,