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RTD to AD7783 Sigma Delta ADC problem

Question asked by jmalves on Feb 11, 2012
Latest reply on Feb 16, 2012 by RobK

Good day all, we are using a AD7783 A/D to read temperatures from RTD (Pt100) probes.

We configured the AD7783 as slave and the PGA gain set to 16 because we are using the input analog signal (from RTD) range from 0 to 160mV.

An embedded microprocessor on the same board is the Master and it draws the ADC data from the AD7783 via its SPI port.

We read a short brief on the net saying that by swapping the excitation current (200uA) via the ADC pin IPin (pin 8) and then performing some maths we can effectively zero/cancel reading errors that are introduced because of the line resistances between the RTD and the AD7783.

Apparently two consecutive readings are to be take, one with IPin high and another with IPin low then the data obtained from the ADC is to be post processed.

We are not understanding how to post process this data in order to compensate and effectively zero the errors due to line resistances.

 

The RTD (Pt100) being used is a three wire device.

 

Please can someone indicate how to post process the data or what the algorithm is so that we can perform that because currently the lines feeding the RTDs are quite long and we do notice we are getting errors in our temperature readings.

 

Thanking you all in advance.

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