An Integrator Reset Circuit not working properly

Question

I am trying to integrate the output of a Rogowski coil , with an inverting integrator with a reset circuit. I want to have the voltage level 0V during the off time of my switch, however after reset interval, the output voltage does not stay at zero and drops to some negative value.

During the entire time after the reset event, the input is zero, so there is no apparent source for this negative value.. Can someone please answer why does this happen?

EDIT 1:As per the answer by Spehro Pehfany, the problem indeed was the charge injection due to the JFET. However, I do not get the same problem with a non-inverting integrator, what might be the reason for that. I used the same device there too.

Answer 1

The gate source capacitance of the J113 JFET is a few pF (5pF max) so you'd expect the gate voltage delta to be reflected in the output voltage by about the ratio with your 66pF cap.

Which is what you are seeing.

If you don't like that, you can design or buy an analog switch with less charge injection. A good one might have typical injection in the 0.5pC range. Yours is almost 50x worse.