Olin Lathrop wrote:
> 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.
>=20
> Temperature differences don't matter anymore when both conductors are
> made of the same material, like both become copper traces on a PC board.

Sean is right. You have many different junctions throughout the circuit, an=
d
you need to make sure that corresponding junctions are at the same temperat=
ure
throughout. It never becomes a "don't care". However, in most cases,
corresponding junctions are pretty much implicitly at the same temperature
because of the physical constraints of building the circuit.

For example, there isn't going to be much temperature difference between th=
e
two junctions of gold bond wires to aluminum pads on the surface of the die
unless there's a significant source of localized heat on the die itself. A
similar argument holds for the junctions between the chip lead frame and th=
e
pads on the PCB, and so on throughout the circuit.

One feature of speciallized thermocouple connectors is that they have a low
thermal resistance path (but high electrical resistance) between the two
contacts, in order to keep them at the same temperature. As long as that's
true, it doesn't matter what that temperature actually is.

-- Dave Tweed
--=20
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