Dinesh, I don't see anything wrong using this schematic. However there are a couple of things to note in the construction of this circuit. What type of resistor is RREF? I asked this in my last post, because the accuracy is directly related to the accuracy of this resistor. If this resistor drifts, then the system measurement will drift as well. This resistor should be something like a 0.01% accuracy and 5ppm/C drift type of resistor. Additionally, the IDAC current mismatch may also appear as a gain error because IDAC1 drives the RTD and IDAC1+IDAC2 drives the reference resistor. You can remove this error through chopping or swapping the IDAC current sources. Are there any other devices connected to the RTD lead connections? Any leakages from a multiplexer or protection diode will also create an error in the measurement. Looking at the excel files, I think I see what you're doing now. Column A is the reading from a temperature meter. Column B is the ADC average of 8 readings. Column C and D are the min and max from a set of 10 readings, where I think you remove the min and max to get the 8 that you average. The next four columns are the calculation that you use to get the 24 bit data (which you don't need to really show). Column I is the calculation of the resistance based on column B. Now, I'm not sure how you calculate the temperature, but the ADC reading looks a little low. With 16.2°C, I would guess that the RTD would be about 106.3Ω. In your RTD measurement there is some strange change over time, but the values all seem a bit low. With the 22.5°C reading I would expect an RTD resistance of about 108.8Ω and again it appears a bit low. Also, in that set of data, there are a few readings that appear abnormally high. At this point, I'm not sure what that is. It might be a communication error, but that would have given some larger min/max changes. With this data, the two temperatures you use are too close together to determine if this is a gain error or offset in the measurement. I would guess this is a gain error caused by the IDAC mismatch, but I can't be sure. You can try doing the chopping of IDAC currents to see if it helps. As for the strange behavior in the 22_50 data, I'm not sure what that is yet. However, as I mentioned in a previous post, I think you should start with measuring precision resistors and seeing what error you get. Using precision resistors you can set up the ADC measurement. Then you can also take voltage measurements across the precision resistor and the reference resistor with a precision multimeter. This way you can directly compare the ADC measurement with the multimeter results. I've used this method to determine the source of errors in other ADC measurements. Joseph Wu
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