Other Parts Discussed in Thread: ADS124S08
I have designed pH measuring device. It is 4-20mA loop powered, isolated using flyback converter. It has STM32, ADS124S08 as ADC and LMP91200 as front end. All running on 3.3V. Vref is connected to 3.3V.
I'm trying to have accuracy better than 1mV but experiencing some oscillation:
1. INP and VCM shorted on PCB and no cable connected, gives me pretty acceptable 200uVpp noise (measured by reading ADC)
2. when I connect RG58 cable ~1m shorted at the far end and INP and VCM shorted on PCB I start to have 3mVpp kind of sinusoidal noise on VOCM line. The VOUT is also affected but it's 500uVpp. It looks like below (x axis is number of sample and it's 20sps - ADC set to: single sampling, 400sps, sinc3, chop on.
3. the filter between LMP and ADC is: 1kR on VOCM and VOUT, 10nF on each ADC input to gnd, also 100nF between ADC inputs but currently not fitted.
How to get rid of that noise/oscillation?