I'm building a guitar audio processor. I inherited this circuit from someone else's project. I am seeing a significant amount of noise on the input. I digitize the input using a 16b ADC and clip to 12b to eliminate quantization noise. However, I was still seeing a lot of noise (+/- 7mV) even when the input signal prior to the preamp is clamped to 0V. If I bypass the preamp and clamp the input to the ADC to 0V I get a clean 0V signal. So, I know the source of the noise is the preamp. The schematic is attached. As you can see the analog input (SW_JACK_IN) is passed through a CAP to remove DC and then a 1.65V DC is added to put the signal mid-rail of a 3.3V range. The input is a basic unamplified audio signal from a guitar, so not much more than a few 10's of mV. It then passes through a low-pass filter set for about 20KHz, a opAMP amplifier and a second low-pass filter also at about 20KHz. The final analog signal is then digitized through an ADC at 96Khz.
When I do a SPICE simulation of this circuit I see a huge sensitivity to noise. I'm trying to figure out how to reduce the noise. I suspect the problem is in the opAmp amplifier. Would I be better off just doing a voltage follower prior to the amplier to get a high-impedance input prior to amplifying it?