I have a signal that is a sine wave plus an offset. I would like to measure the dc component and possibly also subtract it from the signal later. The signal is very slow (10th of Herz) and the offset changes on the order of minutes. I thought it should be possible to get a time average with an integrator, but the layouts that I find are all using periodic resets. If I use the standard integrator with an opamp that has a capacitor as negative feed back, and I add a leak resistor over the capacitor, does this give me a time average of the input voltage with an exponential dependence on the history of the input signal? Are there better ways to do this?