I would like to read an analog input from temperature sensor or pressure sensor that measure the value from a slow process with let's say 1s of rise time. I know that using a low pass filter is a common approach when designing a circuit for reading analog values, however I am not sure what would be the approach towards choosing the cut off frequency.
I know the answer would be that it depends on the type of noise that you are expecting, therefore in addition I'd like to provide some more details.
Let's say we'd like to filter out the common radio frequencies. What would be the common cut off frequency in this case? Would a cut off frequency of 1MHz be sufficient?
What if the system is located near live wires with with distance of 10cm and relatively high currents of 1-3A. I guess we would also like to filter out the 50-60Hz. What would be the reasonable cut off frequency in this case? Would a cut off frequency of 10Hz be sufficient?
Choosing that low cut off frequencies brings its own problems. The low pass filter will introduce slow rise times and will prevent measuring fast dynamics in processes. A common 10Hz low pass filter would have a rise time of ~10ms, which means we would require at least 50ms before measuring a stable analog value on a step response. Since my process is pretty slow, I can comfortably introduce a 500ms sample time.
Is filtering the 50Hz noise with a low pass filter really necessary. Can I only filter the RF spectrum with a low pass filter and introduce additional digital filter(software window filter) to filter out the 50Hz. Would this be a better approach?
I know that ADC input capacitance should be considered as well for a precise measurement, but I don't think it is really necessary with such low sampling times of 0.5s. But I am not 100% sure.