AD7685 sdo outputs on wrong ref scale


I am using the AD7685 in CS mode 3-wire with busy indicator.  I have Vref and VDD tied to 5V (with caps as recommended), and VIO tied to 3.3V.  SDI is tied to 3.3V also, and SDO has a 47.5k pullup to 3.3V.

The sclk is about 1MHz, with CNV pulsing every 20us or so.

On IN+ there is 3.79V, and IN- has 1.27V, for a differential input voltage of 2.52V.  I would expect this to be around 0x8106 on sdo output, but instead the output is 0x55BB.

Why isn't the output code matching the values in table 8. of the datasheet?



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