I had an oscillation problem when testing my fabricated chip. As shown in these figures, the transient waveforms of the op amp outputs got severe oscillations. Ch1 and Ch2 are the two output nodes and the red curve is differential output after math operation.
the oscillation frequency for the two channels seem to be different, the lower one is around 20KHz and 133KHz for the higher one.
More about the op amp, it is a fully-differential class AB power amplifier designed for audio applications, it is connected with a closed-loop gain of 2. It is driving a 16 Ohm + 200 pF load and is supplied by a power supply of 1.2V
I suspects the amp is not stable. But during the pre-layout simulation, I checked the stability conditions of the amplifier and ensured it has good phase margin (>60 Deg) and gain margin (<-20 dB)
Please advice me on any possible source for this problem or any diagnostic test needs to done. Thanks in advance!