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Differential voltage signal measurement using ADUCM360 is resulting in erroneous measurement ?

Question asked by srinivasa on May 31, 2018
Latest reply on Jun 4, 2018 by barryzhang

Hello,

 

I am using the evaluation board of ADUCM360 from Analog devices, recently i have come across a strange error as per description below.

For this wheatstone bridge, I have excited the bridge with 1mA current source and used a precise R reference resistor of 500 ohms.

 

When voltages are measured by switching in the ADC (single ended / differential mode)

 

1.  AIN1 to AGND the voltage value converted from ADC count is as expected

2.  AIN2 to AGND the voltage value converted from ADC count is as expected

3. AIN2 - AIN1, the differential voltage measured has a difference of approximately ~25mV which is inducing an error while estimating the unknown resistance RX.

 

With single ended inputs, the voltages are proper and the unknown RX matches with expectations; However with differential input measurement the RX has huge discrepancies.

 

I measured the voltage with multimeter also at the circuit level (in breadboard) the values are correct, but ADC is showing a lesser value when measured in differential mode; I also tried to change the PGA gain to 2 and 4 but the results are same.

 

Could you please help me locate the error ?, Is it due to lead resistances ? or is it some noise which is making this problem?

 

Thanks in advance.

 

With Best Regards

Srinivas

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