[volt-nuts] Some questions to zeners (thermoelectric effects)

[Tony Holt]

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:

  http://www.hpd-online.com/reversing_switch.php

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.


Tony H