I have an AD4661 IC in a design right now (dual package op amp). I had this working fine, but was required to make a small modification that could be causing an issue I don't totally understand. The input to this amp is ~50mV sine wave. The feedback path has a voltage divider consisting of a 2k resistor and a 20k pot (AD5272-20). When the 5272 is configured with it's minimum impedance I would theoretically pass the input sine wave through with very little gain applied. This worked when I DID NOT have the cap between the 2k resistor and ground. When I added this cap my signal got fuzzy. You can see below in the screen shot from the scope.
Channel 1 (yellow) is the output of the 4661. Channel 2 (blue) is the input of the 4661.
I've attached a pdf of the schematic.
The problem occurs when the AD5272-20 is set to it's minimum resistance (it's a pot) allowing for the least amount of gain. You can see the signal is very "fuzzy" on the output compared to the input. When i record the data with the ADC in the product this is being developed for I get very fuzzy data. I've also attached a screen shot of that.
So I'm trying to figure out how to clean this output up if possible. When I increase the resistance of the ad5272-20 the gain increases and this goes away. I'm worried that there is a design flaw here now since I added that capacitor. I need to decouple this amp from ground because it is applying a gain to ONLY the AC part of an input signal. The input of this amp normally sits at 1.65V. I only want to amplify the AC signal on the input which is why I added this cap.
Here is an example of what I have gotten in the past and what I would expect. I apply a gain of 5 to the same signal and out put looks great. This is data recorded with my ADC and plotted in the same exact manner as the distorted data with no gain applied.