[volt-nuts] Some questions to zeners (thermoelectric effects)
John Beale
beale at bealecorner.com
Sun Jan 27 15:31:40 EST 2013
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.
I don't have the practical experience or measurements to back this up, but
I understand Seebeck thermoelectric effects are a function of the
temperature difference between dissimilar-metal junctions, and not absolute
temperature. So if you have perfectly balanced both the thermal mass and
the thermal conductivity to ambient of every bimetallic junction in your
circuit, there should be zero tempco of the system due to thermocouples,
regardless of both absolute ambient temperature and the rate of temperature
change with time.
So in theory, if you use a symmetrical circuit layout with balanced thermal
mass* and then surround your battery-operated device with a large enough
block of metal (to minimize both thermal gradient, and rate of change with
time), you can get d(temperature)/d(time) of the circuit and the associated
internal temperature differentials to be arbitrarily small. How practical
this "large metal block" would be to meet a <1 ppm/C tempco spec, I do not
know. Assuming you have avoided the copper oxide problem (Cu-CuO: 1000
uV/C) the worst thermocouple will be Cu-Kovar at 40 uV/C so layout at and
around the IC packages will be the most critical.
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".
* The "20-bit DAC" app note mentions this technique:
http://cds.linear.com/docs/Application%20Note/an86f.pdf
John Beale
www.bealecorner.com
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