ADM3260

I'm using the ADM3260 in a new design.  VDD primary = 5V   Visol. = 5V The output level(SDA and SCL) at the line (isolated output) signal swing is 5V to 800mV. I have a 1k5 pull-up on both SDA and SCL. I've measured the output relative to the ADM3260 Isolated Gnd. The input logic  SDA and SCL signal swing is 5V to 0V.

How do I reduce GND offset?  Why do i have 800mV GND offset?

Thanks