ADUM 3190 Impendace problem

Hello,

we used the evaluation board to test the ADUM 3190. The image on the left side shows the input cirquit of the ADUM 3190.

We removed the resistors between GND1 and GND2.

It works fine with R=1.5k.

The problem with this low R is the low input impendance. Therefore we increased R to 750k. Our source has an impendance of 71.5k.

With this high R the cirquit is no longer working.

Could that be? What do we overlook.

Regards

Alexander

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