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OPA855DSGEVM: OPA855DSGEVM

Phillip Snyder
Prodigy 50 points
Part Number: OPA855DSGEVM
Other Parts Discussed in Thread: OPA858, OPA855

Tool/software:

I am using the OPA855 in a TIA circuit. The input is a capacitive sensor with a capacitance of approximately 50pf with resulting currents estimated around 50pAmps. The issue I have is that an offset exists on the V- of -200mV measured at (In). I also have the circuit constructed just as shown with no sensor connected between the (In) and GND, and I get the same -200mV offset. I have ordered the OPA858 to see if that works better. 

Supply voltages are +2.5V and -2.5V.

Any thoughts on this?

Schematic of what I am doing is shown.

Thank You,

Phillip Snyder

  • 0
    TI__Intellectual 2700 points

    Hi Phillip,

    OA855 works on the minimum gain of 7V/V so if you dont connect the sensor, the gain will be 1V/V and for this gain the OPA855 or OPA858 will not be stable. It will either rail or will oscillate. I see a 10Gohm resistor in the feedback of OPA855, do you really need a TIA gain of 10G?

    Regards
    Anant