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 _______________________________________________ volt-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
