I may be pushing physics here so please tell me if I am. I have a signal I'm trying to anti-alias at 50Hz (sampling at 100Hz). The problem is my input signal is essentially an Amplitude Modified square wave that matches the sampling frequency with a very short duty cycle (~2-5%, settable). I'm timing my samples so that they only read the pulses at their peak after a settling time ts. I just realized though that my 50Hz cuttoff passive AA LPF will filter out the square wave and make ts far too long.
We've been banging our heads against the wall on this one, but here's what we want (don't know if it exists):
A Low Pass Filter that can be turned on or off quickly. Desired sequence of events is as followed:
- Signal input is at 0V (off duty cycle). Filter can be on or off during this time.
- Filter is turned off so signal can quickly rise to it's max amplitude (on duty cycle).
- After ts, Filter is turned on again so that it is now being filtered at that higher level taking whatever value the signal is at as a sort of initial value.
- After a few ms, the signal has been sampled by the ADC and the filter is turned off so the signal can return to 0V.
I dream of the circuit that can accomplish it, but cannot figure out how to pull it off. Does this exist? If so, what should I be googling?