I have setup a 4-wire RTD sensing circuit using MSP430F6723, where I am reading the RTD voltage levels on MSP's 24-bit SigmaDelta ADC. I am using a gain of 64 and oversampling rate of 128.
I measured the voltage levels on the RTD leads using multimeter and it reads 13.5mV, and the ADC reads 7132.
Then I replaced the RTD leads with a 0.1% precision 100Ohm resistor, and the multimeter shows a 11.9mV reading. The ADC shows 6790.
As per the Pt100 RTD table, the ratio of voltage values above correspond to the correct room temperature (approx 35 degree Celsius), but the ratio of the ADC readings does not match that.
13.5/11.9 = 1.134
7132/6790 = 1.050
Multiplying the above values with 100 to get the resistance value from RTD and referring to the table on the below link-
http://www.intech.co.nz/products/temperature/typert.html
Please help me with suggestions on what could be going on here, and how do we verify correct values.
The offset value would be too high (although only 0.5% of full scale offset is mentioned in the datasheet).
Thanks & regards,
Archana
