Sampling rate of a PIC. Please Help!

[misterT]

Still have to work things out about the noise I'm getting.

What kind of low-pass filter you have? Could you post schematics?

If you want low noise with 8kHz sampling rate and a 2nd order anti-aliasing filter, your cut-off frequency can not be anywhere near 4kHz. Even if you have 5th order filter, your cut-off frequency should be around 1kHz in order to get close to ideal SNR of -60dB at 4kHz stop-band. I am assuming a 10bit ADC resolution and 0dB pass-band gain.

You should use active filters with SAR ADC. If you use passive RC filter, use "small" resistor values and longer Tacq (because of the relatively large capacitance of the SAR converter).

 

[misterT]

I have to correct myself about this one because there is more about the subject than what I said:

You should use active filters with SAR ADC. If you use passive RC filter, use "small" resistor values and longer Tacq (because of the relatively large capacitance of the SAR converter).

Here is what you should do if you want good signal to noise ratio:
https://www.electro-tech-online.com/custompdfs/2012/08/Improving20SAR-ADC20Accuracy.pdf
https://www.electro-tech-online.com/custompdfs/2012/08/00699b.pdf