The same RC low-pass filter, the same -3dB cutoff frequency, different resistor resistance, but the measured ADC value is very different;

We have a product, in a separate PCB board through the MIC acquisition of sound signals, and through the 10cm or so long wire connected to another piece of MCU ADC port, analog-to-digital conversion.

A resistor is placed on the acquisition board near the op-amp output and a capacitor is placed on the MCU board near the ADC input to form an RC low-pass filtering circuit to filter out the high-frequency signals.

The sampling rate of the ADC is 8kSa/s, so the -3dB cutoff frequency of the RC low-pass filter is set to 3.4kHz, the resistor R is selected as 4.7kΩ, and the capacitor C is selected as 10nF.

The ADC conversion was performed with the STM32F051 processor, shorting the signal to the reference ground on the acquisition board, but the values obtained had random jitter of up to several tens of AD values, up to 80 AD values.

Even with the capacitor at the ADC input shorted directly to ground, the random jitter values remain.

I think there should be interference from high frequency signals between the ground of the reference power supply of the ADC and the ground to which the input capacitor is connected.

The capacitance capacity of the ADC input port should be increased. In order to achieve the same cutoff frequency, the resistance value of the resistor needs to be reduced.

When R=4.7kΩ and C=10nF, the maximum jitter is 80 ADs.
When R=470Ω, C=100nF, the maximum jitter is 20 AD values, the
When R=470Ω, C=220nF, the maximum jitter is 3 AD values, and
When R=150Ω and C=220nF, the maximum jitter value is 6 AD values.

And according to the -3dB cutoff frequency of the low-pass filter = 1/(2*π*R*C), the cutoff frequency is 1539Hz when R=470Ω, C=220nF, which is a little lower, and 4844Hz when R=150Ω, C=220nF.

For the signal output from the op-amp, the resistor and capacitor in series form a low-pass filter;
For the noise coupled from the PCB board, PCB alignment and connecting wire resistance and capacitance constitute low-pass filtering, the resistance value is very small, the need to increase capacitance to reduce the low-pass filtering effect, filtering out more high-frequency noise signals.