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ISO122 output envelope/ripple

Other Parts Discussed in Thread: ISO122

Hi all, 

I am using ISO122 in my circuit to isolate a analog signal from 20mV to 10V range.

I found if I measure the ISO122 output, I could see two things:

1. the 500KHZ oscillations with 30mVpp (expected, not an issue with low pass filter)

2. However, even i short the input to GND, I could see the envelope/common-mode ripple of this 500KHz is not steady, which itself has a  5mVpp ripple at much lower frequency. After the low pass filter in next stage this envelop give me trouble reflected as a ADC ripple.

I would like to ask if this common-mode ripple is expected on this IC or Not?? any one have the same experience, or any idea where this common-mode ripple comes, PS i used a linear Power supply for testing, so should not be coupled from PS. 

Many Thanks!!