I inherited a opamp measurement circuit that is supposed to be able to measure differential voltages of up to 1000 V with a bandwidth of 1 MHz and a protective impedance between the HV and SELV side. The 0-1000 V on the primary side are converted to 1.5 V - 3.5 V on the secondary. It is basically a differential amplifier with 9 megaohm resistors so that no lethal currents can flow.
The 1.5 V offset is applied via the (66 kohm//330 kohm) to (33 kohm//100 kohm) voltage divider and to avoid troubles caused by the common mode the same impedance is used in the feedback path.
To prevent troubles with the input capacitance of the opamp or the capacitance of the diodes 220 pF capacitors are used to add a defined capacity (again in both paths because of possible common mode problems otherwise).
The problems with these capacitors is that they would limit the bandwidth to 40 kHz and add a pole at around 1 MHz which they circumvented with a 15 pF in the feedback.
When I tried making their circuit (which worked for them, they used the same opamps and very similar caps) the frequency response of mine always has a notch at around 200 kHz which I can't explain myself.
Does anyone have any idea which parasitic or stupidity of mine could cause such a deviation from the simulated frequency response.
!! Update #2:
C3 & C4 are there to improve the steepness of the filter. But as andy aka pointed out they drive the gain of the opamp up at high frequencies until the 15pF get active. Could this be the problem?
Could it be possible that the GBP of the opamp wont be enough at high frequencies because the gain also increases? How would I simulate for that?