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ISO224: Appearance of an instantaneous voltage offset at cold temperature

Part Number: ISO224
Other Parts Discussed in Thread: LM7301,


We have a design using the LM7301 in a follower configuration and a ISO224 to isolate the input from the rest of the circuits.
All parts are industrial temperature grade.

The primary side of the system is powered at +-15V.
The input signal is 60 Hz / 5 Vpeak.  Far away from the rail of both the LM7301 and ISO224.

On about half of the channels in the systems, when we do a cold temperature test at around -20 Celcius, there is an offset (couple of mv) that appears on the signal at the end of our analog circuit going to an ADC.

And the offset appears very fast.....  One 60 Hz cycle is ok, then the next has the offset...   

The interesting point is when we apply an oscilloscope probe at the output of the follower, this offset disapears !!!!  Our probes have a 50 pF capacitive loading.

Does anyone have an explanation ?   

Here is a part of our schematic....

And the result when we do a quick freeze on the LM7301 without the probe. 

60 Hz signal is in blue a long time period and only lower side visible to zoom on the offset.