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[AD7793] Zero not at center for bipolar readings with bias voltage

Question asked by AndreasKl on Jul 7, 2015
Latest reply on Jul 8, 2015 by AndreasKl

I'm trying to read a heatflow sensor with a AD7793. The sensor has two wires and generates a voltage from -10mV to 10 mV between them. Its resistance is 180 Ohm.

 

In terms of configuration, I use the internal reference with the AD7793, a gain of 128, and the built in bias voltage.

 

Here is my problem:

When I read the sensor with a multimeter, I measure 2.8 mV and if I reverse + and -, I measure -2.8 mV as I would expect. However,  with the AD7793, I measure 2.3 mV and -3.3 mV if I reverse + and -. When I short the input of the AD7793 I still read get a reading of -0.5 mV as long as even just a single wire of the heatflow sensor touches the input. I get the same result if I touch the shorted input with my finger. Only when I short the input without anything touching it, I get a reading of -0.02 mV. If I deactivate the bias voltage, I get 0.00 mV for every scenario above.

 

Am I doing something wrong with how I try to use the bias voltage? The ADC clearly gets the range right but it seems that my zero point is shifted.

 

I've tried with another AD7793 but got the same result.

 

Any help is greatly appreciated. Thanks!

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