[volt-nuts] Some questions to zeners (thermoelectric effects)
vnuts at toneh.demon.co.uk
Mon Jan 28 08:48:43 EST 2013
On 27/01/2013 20:31, John Beale wrote:
> On 1/27/2013 9:36 AM, Ed Breya wrote:
>> I think your expectations are not realistic - even if you could make
>> such a
>> reference, you could not transport its voltage to the ADC without
>> thermoelectric effects causing error that would swamp the
>> performance. To
>> keep everything below the 1 ppm/deg C range you would have to put the
>> entire circuit in controlled temperature - the reference, the ADC,
>> and the
>> signal connection to the outside world.
Presumably, if the voltage reference uses an amplifier then a four wire
connection can be used to eliminate all the EMFs between the reference
and ADC other than those at the Kelvin connections at the ADC and
reference/amp so that the reference and the ADC don't have to be in
thermal equilibrium with each other. Could the sense wires be welded to
the ADC pins between the solder connection to the PCB and the package to
avoid the thermal EMFs of a solder joint?
> I assume the hardest connections to keep thermally equalized would be
> the terminals connecting your reference/ADC to an external device. If
> your voltmeter is limited to low voltages, optimizing this suggests
> the smallest and most closely-spaced connections possible, embedded in
> an insulating but thermally conductive matrix (ceramic?). Standard
> banana jacks with 3/4 inch spacing and surrounded by plastic, seem far
> from "small" or "closely spaced" or "well thermally coupled"
For a couple of data points, here's one manufacturer's approach to
dealing with thermal EMFs:
Their low thermal reversing switch uses plenty of copper and aluminium
to minimise thermal differences, claiming typical thermal offset of only
3nV, 10nV max! Its not clear (to me) though exactly where that 3nV is
being measured and how effective the 1.5mm thick copper lugs connecting
the reference source/DUT's terminals are at minimising temperature
differences between the terminals in the presence of normal air currents
in a typical Lab.
And this scanner: http://www.dataproof.com/scanner.pdf claims thermal
EMFs of less than 15nV typical (30nV max) using lots of aluminium to
keep relay contacts in thermal equilibrium.
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