When calculating the input-referred noise and filtering to a 24-bit sigma-delta ADC (MCP3561), I hit a roadblock. Knowing the noise spectrum density (nV/sqrt(Hz)) of a signal noise source, like thermal noise, the noise in uVrms (microvolt root mean square) is calculated using the bandwidth of the signal. The latter number should be the noise floor that determines the effective resolution. Therefore I need to know what bandwidth (frequency span) applies in my scenario:
The ADC will do internal oversampling to form a single one-shot reading. The signal is from a thermistor where the bias power will be pulsed at 10 Hz for the duration of ADC, hence the signal itself is assumed constant for the measurement period and I suppose the signal frequency is 10 Hz. But 0-10 Hz can't reasonably be the frequency span to calculate noise from, as the ADC is inactive most of the time.
Is the frequency span from 0 to the oversampled sampling time (in the kHz range, but just one sample)?
Or is the frequency span from 0 to the internal ADC sampling rate (in the MHz range)? But I read that higher ADC speeds reduce the noise problem, which isn't consistent with this idea. And while the ADC runs at 4.9 MHz, the sampling time for each oversampling level is specified as 3x the expected (for example OSR=128 takes 78 us = 12.8 kHz = 4.9 MHz / (3 * 128)) so even the internal frequency is unclear to me.
It's also easy to imagine even higher noise frequencies affecting the measurement, but maybe any high-frequency noise is filtered in the ADC and included in the ADC datasheet ENOB? Maybe some Nyquist limit is involved.
The goal is to understand the practical measurement resolution under different Vref values and oversampling levels, and to optimize the filters. It seems to me that the slow signal may be filtered until thermal noise is the limit, but to know this I need the thermal noise of the signal as uVrms.