Imagine you had a thermocouple made of metal X and metal Y, and that=20
metal X had a Seebeck coefficient of 1uV/degree and metal Y had a=20
coefficient of 1.5uV/degree.  The coefficient of the XY thermocouple is=20
therefor .5uV per degree.

The hot end is at 125 degrees and the cold end is at 25 degrees.

You define the cold Y end as 0 volts.  The hot Y end is at 150uV.  The=20
hot X end must also be at 150uV, since they are connected.  The cold X=20
end is at 100uV negative relative to the hot X end, and therefor at=20
+50uV relative to the Y cold end.  You have an IC temperature sensor=20
here.  You also transition to copper in the PC board traces.  The traces=20
make their way across the board, which has a temperature differential=20
that causes the copper to exhibit +10uV difference.  The signal from the=20
X hot end is now 60uV, and the signal from the Y cold end is 10uV.  The=20
difference is still 50uV, so when you divide the 50uV by .5uV per degree=20
of our XY thermocouple, and add the 25 degrees of the cold junction, you=20
get 125 degrees for the hot end.=20

As long as the X and Y inputs of the A/D are at the same temperature,=20
and the X and Y junctions at the cold junction are the same, they will=20
cancel out. regardless of the temperature difference between the cold=20
junction and the A/D.

Kerry


Sean Breheny wrote:
> Is there really one reference junction, though? I am under the
> impression that any joint of dis-similar metals in the thermocouple
> circuit will cause a voltage difference which is temperature
> dependent. This includes even things like bond wire connections inside
> the IC used to read the thermocouple. So, I'm not sure that it makes
> sense to couple the cold junction sensor so closely with the TC
> connector unless you are also coupling it closely with the rest of the
> electronics in the circuit which reads the TC.
>
> I am speaking as a heavy user of TCs who has never designed a circuit
> to interface with them so perhaps someone with more experience here
> can chime in.
>
> Sean
>  =20



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