Can we sample two signals simultaneously using the ADC (analog to digital) of AVR mircocontroller?
Tell us more about your signals. If they're slow enough, you may be able to get away with using on the onboard ADC. You didn't say which AVR model you're using, but most AVRs I've used had a maximum sampling frequency of around 4kHz. If you need to sample two signals with it, the ADC is multiplexed, so cut that in half. Unless your signals are significantly slower than that (and without trying it, I would guess a factor of 10 would be pushing it), I would go with a two-channel external ADC. The goal is to sample fast enough that relative to the slower signal, the samples were taken at approximately the same time. The multiplexer in the onboard and some external ADCs will take some finite time to switch. This needs to be considered. The uC clock also needs to be running fast enough that it can complete the necessary math before the next samples.