HF R800 internal mic does not shut off when using external mic

[franklyray]

What are the ideal settings for using an external mixer feeding the MIC input on my HF R800 camcorder. 

I am using a Behringer mixer L&R Main out to the camcorder but I get a mean buzz.

The output of the camera is coming into my BlackMagic ATEM Mini Pro via HDMI from the camera.

Critical timing on this. Need for church Sunday.