Quote Originally Posted by ronsimpson View Post
Audio has a large dynamic range. I wanted to get as much range as possible. I needed to “zero” at 512 (10 bit). Resistors at 1% and even 0.1% did not always get me to zero. I also tried resistors to get within 1% of zero and then used the PWM output to move the ˝ voltage point +/- a little. I know in software I could have move the zero point to correct for this error. I wanted to do all the math slightly faster than the ADC could work and there was no time.
I can live with the 1% (even 2%)....quiescently, I was seeing jitter of just over 1%.(ie readings of 127,127,126,127,17,126, etc)

The accuracy of your ADC 'zero' point is obviously going to be down to how you approach DC biasing your AC signal feeding into the ADC pin - as a late suggestion, have you tried a DC blocking cap between your AC signal input & your chosen ADC pin? Obviously you then have to rebias the ADC pin - but 2 x 1% tolerance resistors between your PIC VCC & Gnd should ...with the the resistor junction connected to your ADC pin, should get you incredibly close to a reading of 512.