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AD7793 temp. measurement: antialiasing filter leads to bias

Question asked by AndreasKl on Jan 27, 2015
Latest reply on Jan 28, 2015 by jtee

Hi,

 

I have a circuit with an AD7793 as described in the attached schematic which I use to measure temperatures with thermistors and RTDs. For testing purposes, I replaced what would normally be a thermistor with a 10k resistor (R11; 0.1% tolerance). I use a 20k resistor (also 0.1% tolerance) as reference resistor (R12) and I use one IOUT channel with 0.210 mA to excite the resistors. My gain is 1 and my rate is 4 Hz.

 

I read that an antialiasing filter is recommended for the input signal, so I added the recommended filter to the ADC input as seen in the schematic.

 

However, when I read the value of the test resistor R11 with this configuration, I obtain a value of 10,077 Ohm. If I remove the capacitor C1, I obtain a value of 10,005 which seems much more precise.

 

Should I not use the antialiasing filter or did I do something wrong?

 

Thanks a lot for your advice,

Andreas

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