Osicllations on IC30 output
During the testing of FmcDac600mDDS I have found a problem with IC30 (operational amplifier).
Description fo the problem
I connect synchronous clocks to phase detector (IC29) inputs. The phase offset between input clocks may differ. I expect to measure a constant voltage on the ADC (IC19). Voltage depends on the input clock phase offset.
It turns out that if the phase offset is "low" I measure a sine wave. Sine amplitude is around 300 mV P-P and its DC offset roughly correspond to an expected constant voltage. A sine wave is generated if DC offset is below ~2.2V. Above 2.2V the output is constant as expected. Please note that output sine frequency is different than the input sawtooth frequency.
It turns out that the problem is caused by IC30 OpAmp. It generates sine wave even though its input voltage is constant (checked with the oscilloscope) - PD filter seems to work.
I'm attaching oscillograms:
PD filter input:
PD filter output:
The problem seems to be caused by OpAmp output capacitance C108. Removing it causes the oscillations to stop. I have verified that the correct capacitor was mounted on the tested boards.