I'm going to be using a PT100 RTD for immersion measurement of flowing water. Since lead length is not an issue and I will be calibrating to a reference thermometer I was going to use a simple 2-wire circuit and 1-5ma of driving current. The point is to run some current through the RTD and measure the voltage across it, which I will then amplify with an op-amp. So the stability of my current source will directly affect my temperature measurement--if my current source is +-10%, then that's all the accuracy I can expect. Is a simple LM7805 linear IC good enough for for something like this? Would you recommend a different circuit altogether?
If you're concerned about accuracy then a dedicated current source might be appropriate:
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The Quick Shield: breakout all 28 pins to quick-connect terminals