I have a need for a +12Vdc, 20mA power source that is stable over temperature (it is a lab instrument, so it will likely see between 25 and 40C). It doesn't need to be precise or trimmed, it just needs to be the same voltage as temperature varies. I don't have a drift spec, I'm just trying to come up with a circuit that has as low of a drift as is feasible. I've Looked at the LM317 and it looks to be in the 500+ppm/C range (estimating the T/C of the voltage ref from the temp curve times the effective loop gain), the LM723 is 150ppm/C, but I wonder what could be done to get a lower drift.
The attached schematic is something I ginned up to see if I could improve on the COTS parts (the circuit is not built yet, its a strawman at the moment  I *think* I got the opamp polarity right). The divider resistors look to contribute about 12ppm/C of drift if their T/Cs are at opposite extremes. The opamp T/C would be about 0.72ppm/C (0.15ppm/C input offset drift * the effective loop gain).
The voltage ref T/C gets multiplied by the effective loop gain, which is about 4.8 here. Adding up the drift results gets me at about 109ppm/C (assuming that superposition holds).
So, the questions become: are my calculations at least in the ball park, and what improvements could be made to reduce the T/C of the circuit? If the basic circuit is valid, what compensation would be needed to clean it up?
Note that part of this effort is to come up with a circuit, while the other part is to learn something about T/C (so just buying something off the shelf only solves part of the problem).
Cheers,
Joe
The attached schematic is something I ginned up to see if I could improve on the COTS parts (the circuit is not built yet, its a strawman at the moment  I *think* I got the opamp polarity right). The divider resistors look to contribute about 12ppm/C of drift if their T/Cs are at opposite extremes. The opamp T/C would be about 0.72ppm/C (0.15ppm/C input offset drift * the effective loop gain).
The voltage ref T/C gets multiplied by the effective loop gain, which is about 4.8 here. Adding up the drift results gets me at about 109ppm/C (assuming that superposition holds).
So, the questions become: are my calculations at least in the ball park, and what improvements could be made to reduce the T/C of the circuit? If the basic circuit is valid, what compensation would be needed to clean it up?
Note that part of this effort is to come up with a circuit, while the other part is to learn something about T/C (so just buying something off the shelf only solves part of the problem).
Cheers,
Joe
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