Conditioning a voltage using constant current for an ADC range
Making a constant-current source is, in principle, very easy if you have a precision voltage source. The basis for conditioning your RTDs is 







simulate this circuit &ndash; Schematic created using CircuitLab

In this case the current is set by R1, and is equal to V1/R1. The output of the first op amp is equal to the current times the RTD value, but is negative. So op amp 2 is used to invert the signal (assuming you want a positive voltage to the ADC), and this section may have some gain associated with it. R5 is used to provide an offset to compensate for the fact that the RTD does not produce a zero resistance at one end of the useful range.

You'll need to be careful of your resistors. If you're going to the lengths of using a platinum unit and a 16-bit ADC, this suggests that you're trying for very accurate and precise measurements. Temperature coefficients in the resistors can adversely affect this accuracy, but you can get low-drift resistors with at tempco in the range of 25 ppm/deg C.