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AD7685 sdo outputs on wrong ref scale

Question asked by Timothy89 on Sep 8, 2014
Latest reply on Sep 9, 2014 by Ryan_Curran

Hello,

 

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?

 

Thanks,


Tim

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