Yes, you are right. One more question, do you have any suggestion if I want to move the cut off frequency to higher?
Hello Liping,
This low-pass cut-off of 1.38MHz is due to the thevenin equivalent resistance (Rth) of input divider ~50M and input common-mode capacitance of OPA810 of 2 pF. There can be two ways to push this cut-off to higher values:
1. Reduce input resistors proportionally to get the same attenuation but lower Rth and higher filter cut-off.
2. If 1Mohm DC impedance is absolutely necessary, you may try realizing an all pass response by adding capacitors in parallel with input resistors such that R1C1=R2C2. Now by default C2 is 2 pF (internal to OPA810) which gives much much smaller C1 not possible to realize on the board. Hence, we use a 1 pF in parallel with 950kohm and scale up the capacitor C2 (including the 2pF from OPA810).
Here is the input impedance and gain response vs frequency:
As can be seen the cut-off frequency increases to 121MHz but the input impedance reduces with increase in signal frequency (10kohm at 16.73 MHz).
Regards,
Bharat Agrawal
