Hi Saumitra,
I think your problem is in how you are determining and using the slope. I think it would be best if you looked through this document:
http://www.ti.com/tool/TIDA-00095
This TI Designs is a temperature transmitter design using the ADS1220. You should get even better results using the ADS1247. We also have a TI Precision Design for 3-wire RTDs using the ADS1247, but it is not quite as comprehensive with respect to how the calculations and calibration is accomplished. With the ADS1247, the offset correction can be done using the SELFOCAL command following device configuration at startup. So what you really need to focus on is gain error of the ADS1247 and how that corresponds to the result.
In your calculations it is initially assumed that the current and resistor is ideal. In the ideal case, the outcome of the transfer function is 1/2 of the reference current flows through the RTD. By removing the current from the equation (IDAC current in numerator and 2* IDAC current in the denominator) you simply have the ratio of the resistances. The ability to evaluate your system will be based on the precision of the measurement of your calibration resistors. If you read through the document mentioned above, you will notice that very precise external measurements are used in determing the value of resistances used. Here is a similar table in a different form from another data set:
The Gain Calibration Calculation is the (Measured Resistor (value) * PGA GAIN * (2^24 -1)) / (2 * 2 * Reference Resistor (value)) in codes. The slope is then the ratio of the codes determined in the above equation divided by the codes returned from the ADC. In other words, the theoretical divided by the real. In this table there was also offset correction, but this is not necessary for the ADS1247 as the offset is taken care of internally by the ADC.
You will notice that only one resistor value was used for gain calibration. Remember that this was really a two point calibration where the first point is 0. This simplifies the slope equation calculation. Is this the only way to calibrate the system? No, there are a number of ways and this is just one of them. If you want to use a different method that is fine but make sure that the calculations make sense for the end result.
Best regards,
Bob B