Resolution of the card would be defined by the bits in the analog to digital converter (ADC). With the PXI-6221 this is 16 bits of resolution. Correct me if I am wrong, but I think what you are interested in is the
code width, or smallest theoretical detectable change in voltage. This will change depending on the gain settings or amplification prior to the conversion.
Assuming that you have set up your SCXI 1102 to have a small enough range (<100mV), the gain should be 100 giving you a code width of 20V/(100x2^16)=3uV. Refer to the document linked above and scroll to the section on code width to get more information about the formula. In summary the PXI-6221 has a full range of 20V, the SCXI a gain of 100, and the resolution is 16 bits. Theoretically a change in temperature which produces a 3uV difference in your RTD could be measured, however there are several factors which will play into the actual measurement including the calibration and accuracy factors discussed above.
Hope this is helpful,
Jennifer O.
