darwint

thanks again bro. you've been a great help. i found that the reason for the oscillation was stray capacitance at the output as low as 20pF which causes my opamp to become unstable.

hi pal, this is the basic circuit. only my circuit has both inputs at gnd. another interesting find of mine is the ciruit won't oscillate at output with 1kohm load. the datasheet though guarantees the specs at 100ohm load. thanks.

"Is it correct to say that some noise being picked up by the part on the ckt system triggeres the oscillation? Do you agree with my theory? Not noise but its input picking up its output, or its output delayed by stray capacitance which causes its feedback to be positive at high frequencies, or its supplies not solid at very high frequencies. " Honestly I am still a bit confused about "at very high frequencies" since I got both inputs at 0v or ground, as opposed to having a high freq input. Would appreciate if you can clarify that up further for me.

Thanks again. The part is actually ad8390, a low power high output current differential amplifier, and i don't see anything about how much capacitance it can drive in the datasheet. darwin

you've been very helpful buddy, thanks so much! i greatly appreciate your point by point reply. as a new member i hope i can also be a good contributor to this forum.

Thanks pal for the welcome and reply! I still don't know how to post pics here, but it's basically an inv voltage feedback amplifier with Rf=10k and Rin=1k. The output has 100ohms load, and doesn't drive a capacitor. I said both inputs are grounded because I am testing the circuit for the Vos and the supply current when input is zero. The circuit is on some kind of a test board. Supplies are properly bypassed by 0.1uF. I am thinking that stray capacitance at the output might be causing it to oscillate, because the part itself has been tested on the bench without oscillation, with the theory behind that the stray capacitance adds additional phase shift to the amplifier causing it to be unstable. Is it correct to say that some noise being picked up by the part on the ckt system triggeres the oscillation? Do you agree with my theory?

hello guys. i got an op amp circuit that oscillates at the output at 80MHz, ~70mVp-p with both inputs at ground. I am just supposed to be getting the Vos at the output. AMplifiers gain is 10. I was thinking this might be due to some capacitance loading at the output. It'd be great to hear some of your ideas why my op amp oscillates. darwin