I can't help you with the exact details - Microchip has some pretty good RTD application notes, which will help you with the theory and practice. alan smith wrote: > So it turns out its a 3 wire RTD. So if I understand it correctly, one wire is the excitation voltage, one is return and one is the voltage drop across the RTD ? > > > Spehro Pefhany wrote: > Quoting Mike Harrison : > >> On Thu, 3 Jan 2008 09:55:38 -0800 (PST), you wrote: >> >>> I've used the 2 wire RTDs before...essentially measure the voltage >>> across the device for the temperature, but I've been asked to look >>> at 3 wire or full bridge RTD's. Now, they talked like they are >>> one and the same but I think a full bridge is a 4 wire? 4 wire >>> seems simple enough in that it has a constant current source to >>> get better accuracy but what about 3 wire devices? A current >>> source that dumps into a common ground? Just need some >>> clarification on that. >>> >> With 4 wire, you apply current on one pair and measure voltage >> differentially on the other. Easiest >> way is to apply a constant-ish current, and take the ADC reference >> as the voltage across a precision >> series resistor in the excitation leg - this gives a direct reading >> of resistance independent ( >> within reason) of the excitation current, avoiding the need for >> expensive voltage references. >> >> 3-wire makes the reasonable assumption that the voltage drop on both >> excitation wires is the same. >> >> Assume the wires to one end are A,B and the other is C >> >> Apply current to A, with C grounded >> Measure voltage across B & C - this is the RTD voltage plus the >> voltage drop on the C wire >> Measure voltage between A and B - this is the voltage drop on the A >> wire. Subtract this value from >> the BC reading to get the 'real' RTD voltage value. > > Another approach is to use a second current sink that's twice the > value of the main one, and just apply that to the compensation > lead. That causes the leadwire resistance to cancel out without > any calculation, and with a single-ended input, provided, of course, > that the leadwires are of equal resistance. Otherwise there's an > error proportional to the difference in the leadwire resistances. > With 100R (or less) sensors, narrow sensing ranges, and long runs > of relatively small-gauge wire, the leadwire resistance can be > substantial compared to the sensing range, so differences from > different lots of wire can have a significant effect. Nonetheless, > 4-wire sensing is rarely used outside of lab situations. > > Best regards, > Spehro Pefhany -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist