On 7/20/2011 9:00 AM, V G wrote: > On Wed, Jul 20, 2011 at 8:31 AM, Joe Wronskiw= rote: > >> I think the "shifting up and down" is the 10 khz, as the period looks >> like it could be about 100 uS. The faster wave has to be some other >> oscillation. Try a cap across the feedback resistor. Looks like this >> op amp needs some compensation; the app notes should suggest something. >> > Hi! I put a 10pF capacitor across like you said. That fixes the problem. = But > why is it needed? The datasheet also says something about over-compensati= on. > What is that? I'm not very up on the theory, but it has to do with phase shift and=20 gain at different frequencies. The cap cuts down the high freq gain to=20 suppress the oscillation. Michael W's suggestion about the input R is=20 related to the same principal, I think. This is an area that makes me=20 wish I had read more of Bob Pease's articles, but of course they're=20 archived somewhere. The general topic is "compensation" Most op amps=20 used to have an input pin for this, as well as 2 other pins for offset. Joe W --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .