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Following the directions in the datasheet for the Linear LTC1968, I implemented a Sallen-Key filter with buffer as per the attached schematic.
When I model the circuit in LTSpice, it works fine and is stable. On the PCB, however, it nicely oscillates with a 400 kHz, 1V p-p sine wave. I've tried and I can't make it do that in LTSpice. It's obviously a layout problem, but any suggestions on where to look and how to fix would be greatly appreciated. On the board, all components are SMT. What I did notice is that R11, R12, and R13 are set close together, and putting a finger close to them affects the oscillation, usually increasing it, so it's probably capacitive coupling somewhere.
Hi Paul,
if you can live with the offset voltage then you can use the NE5532, of course, and everything is fine. :-)
That's the big problem with these simulations, you never know what's actually going on... Because of this, I allways carry out simplified phase stability analysis which only contain the open loop output impedance and input capacitance of OPAmp:
Kai
