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. 


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