There is optional outside clock input to the gyros, does this control gyro's mechanical oscillation frequency or just for ADC converter?
TLDR;
I have an application where there are 2x ICM-42688 on same mezzanine board. On first batch of boards we found that some gyros have much more RMS noise than "typical" values in datasheet.
When looking at actual samples noise seemed very periodic. When looking at FFT of samples it was same frequency for both gyros, but effected some axes more that others (z-axis was often the worst).
On different boards, the noise was often different frequencies, but often between 10-150Hz. Gyro sample rate is set to 500Hz, and 2nd-order filter at ~100Hz.
After lots investigation of possible "common" electrical noise sources, I believe I've determined the noise is caused by interference between thy gyros themselves. If I don't enable either gyro (through register settings) the other's output signal is noise free. While the noise could be electrical, the supplies of each gyro is behind a ferrite bead, and adding extra supply filter capacitance made little difference on FFT peak magnitude.
I believe that the interference is caused a "beat frequency" different between mechanical oscillators in each gyro. The mechanical elements in the gyro report there oscillation frequency is 25-29Hz. The beat frequency is based on difference two different frequencies, and that's why noise source seems to be at 10Hz-150Hz.
In current design, the IMUs are about 9mm apart, I can increase this in future designs.
However, I'm wondering if is possible to sync the two mechanical oscillators. There is an outside clock input to the gyros, does this drive mechanical oscillation, or just ADC sampling?