enter image description here I need to interface an AC coupled sensor with rest of my system. The sensor I am working with produces approx. 10V peak with 10V of DC offset but it has a very small series resistance about 50fento.

Sensor operates at about 35KHz. My initial thought is to use an op-amp in inverting configuration for ease of bias setting (reference voltage at +IN). I also want this circuit to work as low pass filter.

The problem I face are reasonable component values.

I do not want to use feedback resistor value larger than 10Meg ideally I would like to reduce it while maintaining -3dB at 35KHz.

This however means that I would need to reduce C(feed) to maintain fc at 35KHz which leads to lower output amplitude in pass band.

Is there any other topology I could use?

My goal is to keep SNR in my system as high as possible and noise as low as possible.

  • \$\begingroup\$ Will your sensor mind being run into a virtual ground? What does "045pF" mean numerically? \$\endgroup\$
    – Andy aka
    Commented Mar 24, 2022 at 14:14
  • \$\begingroup\$ Apologies it is missing a decimal point. It meant to be 0.45p. \$\endgroup\$
    – Wintermute
    Commented Mar 24, 2022 at 14:54

1 Answer 1


You will want a very low input capacitance load CMOS Op Amp to work as a Charge Amplifier or TransImpedance Amp. (TIA) This research shows how your BW and Cs can be loaded without the usual pF load attenuation. Rin values of 1000 Gohm are possible in commercial packages but I have not looked for the lowest Cin specs except for one which is 100 fF.

The research design shows Cin < 100 fF.


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