I'm using a basic opamp driven mosfet to implement a precision low current sink for an LED:
This seems to work just fine within the range that I want it (10mA or so). I've seen other circuits that put an lpf in the feedback loop (R3 was 1K in the one I saw):
I'm trying to understand how that works. With the values of 100n and 10k, the -3dB point is 160Hz. What I don't understand is why the capacitor is connected to the opamp output instead of to ground. Wouldn't that divert noise above 160Hz back into the output? Of course, there'd be a phase shift as well but I cannot see why you wouldn't connect the capacitor to ground.
I'm guessing this is a really basic opamp circuit in a slightly different form but I'm not understanding what that form takes. I may have it backwards as high frequencies at the output of the opamp will "short" back to the inverting input and reduce the opamps output.