At 05:02 PM 1/11/2008, you 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 ?

It depends how you drive it, as I said there are a couple of ways.
You're measuring resistance, so you'll usually be feeding it a
current.

With what you have .. one wire is attached to one side of the RTD,
two wires to the other.  Call them A, B and B'

If you ground B', drive -1mA into B and +1mA into A, you can just
read the voltage on A.

Otherwise you can ground B', drive +1mA into A, and read the voltage on A,
minus double the voltage on B (the subtraction can be done digitally
or by analog means. Whichever way you go, it's usually safe to assume
that the compensation voltage will change slowly so you can filter
the cr*p out of it.

And there's another way- ground A, feed 1mA into B and read the voltage
from B', subtracting double the difference between B and B'.

And yet another way.. ground B', drive +1mA into A and +1mA into B,
then read the differential voltage A - B.

(1mA above is just an example typical of low resistance RTDs, but
obviously other values are possible).

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com



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