I am following this circuit:
to measure the capacitance. The opamp is OPA604 with supply voltage +-5V, Cx and Cf is 5pF, Rf is 10Mohm, the diode is 1N4007 and the output capacitor is 1nF. I also have 3 CD4051AN analog multiplexer/demultiplexer at 2 sides of Cx to change the capacitors and between the diode and the output capacitor to discharge the output capacitor. Then I connect Vi to about 100kHz sine wave signal.
The signal at the output of op-amp seem to have a low frequency noise, which makes the whole output seemed to be moving up and down:
And the signal after the diode still have the sinewave there, which I cannot understand.
Then I tried changing the 10M resistor by a 636k resistor, and the low frequency noise at the op-amp output just disappeared. The signal between the diode and the output capacitor also seemed to be much more constant.
So could anyone please explain to me where is the low frequency noise come from? And also how can there is a sine wave between the diode and the capacitor (since the diode forbid any discharge to the capacitor). Any pointing is also greatly appreciated. Thanks in advance.