I have a system where I need to pieces of information from a single analog signal. The first is a time-average of the signal, over the course of around 0.25 seconds. The second is a detailed breakdown of the signal at approximately 135 kHz.
A little more detail: the input signal is correlated with a 1500 Hz square wave and broken down in to "in phase" and "out of phase", where everything in phase arrives when the square wave is high, and everything out of phase correlates to when the square wave is low. I've achieved this already with the 135 kHz sampling, and I can easily distinguish between in and out of phase. However, the averaging uses a lot of computing overhead and it's not strictly necessary to be sampling this fast all the time. I can't pause frequently enough to make the averaging efficient, and holding all that data in an array overwhelms my memory.
The reason I need the high-resolution measurement is because I need to know how well-correlated the signals actually are, since it's possible that my input signal could lag behind the reference wave slightly. This piece of information is actually critical to my setup, since it tells me important things about the stuff I'm measuring. Ideally, I'd construct a system where the slow measurement is essentially uninterrupted and the fast measurement happens a few times per second.
If I were to use two separate ADCs, I figure I could use one to do the "slow" measurements, use on-chip oversampling, and get a high-SNR signal that I can then casually average and report normally. Another ADC could do the high speed measurements and retrieve the phase separation between my signal and my 1500 Hz reference.
My question is about best practices. How do I make sure that when one ADC is measuring, it doesn't interfere with the other ADC? I've read a little about using buffer op-amps to isolate the signals, would that be the way to move forward? If I can reliably segregate the input into two ADCs, I can work out the details of the math later.