Noise simulation of transimpedance amplifier in LTSpice - What stop frequency should I use?

Question

I'm doing a noise simulation of a transimpedance amplifier (TIA) in LTSpice for calculating the total RMS noise. The bandwidth is approximately 100 kHz. As long as the simulation stop frequency is higher than the bandwidth of the circuit I get, as expected, the same value of the RMS noise.

Now to the actual question - if I have a delta-sigma ADC with a 100 Hz digital lowpass filter sampling the output of the TIA, can I simply set the stop frequency in the simulation to 100 Hz to get a reasonably good approximation of the RMS noise of the sampled and digitally filtered signal?

Answer 1

The total RMS noise will be based off of the smallest bandwidth of the system (this is why we filter to reduce noise). As long as the ADC has a frequency cutoff (and doesn't alias frequencies), you can use 100Hz for the cutoff of the noise measurement.

This means the bandwidth of the simulation needs to be whatever the lower end is of your noise (usually 0.1Hz is good enough) and the high end 100Hz. So this will give you a reasonable estimate for the analog noise before the ADC. The ADC will also contribute noise, and if it does you need to include a noise source with the ADC noise (or just take your final number and sum it (by the sum of the squares) to the ADC noise).

Also, there is more than one way to sum up noise than changing the extent of the simulation, you can also use a .meas statement. https://www.analog.com/en/resources/technical-articles/ltspice-integrating-noise-over-a-bandwidth.html