I need to capture the waveform of a low-amplitude signal that sits on top of a slow-varying, higher-amplitude component. I'm thinking of using an ADC with two channels, and feed one of them with a low-pass filtered version of the signal and the other one with an amplified, high-pass filtered version of the signal. That would increase the apparent resolution of my ADC. Am I wrong? Can you foresee any problems with this?
I forgot to say I have to capture the low-frequency component as well (the algorithm needs the average value of the signal).
The "high"-frequency component goes from 0.01 hertz to 10 hertz. The low-frequency component is mainly the average value of the signal, but it may change, slowly. The faster-changing component may have an amplitude 100 times smaller than the maximum average value. The microcontroller we will use has a 12-bit ADC (I cannot change that), but with many channels.
Best Answer
This is a very good idea. The BioTac tactile sensors from Syntouch do this very same thing. They have a pressure sensor inside them which captures both the low frequency part of the signal at about 50 sps, and the high frequency components amplified and sampled at 2000 sps. This works beautifully.
However, I don't know if you can actually combine these two signals to create a higher resolution, I.E. more bits. You may be able to with some clever signal processing, but it wouldn't be trivial.
Another way to increase the ADC resolution is by oversampling. If you take 16 12-bit samples (and assuming there's at least one LSB of noise) then you really have increased the effective resolution.