Electronics > Metrology
LM399 based 10 V reference
David Hess:
--- Quote from: Kleinstein on October 30, 2024, 10:22:15 am ---From the stability and simplicity it is hard to beat the resistor from the 10 V output. Even when build with separate drive and sense it is still the preferred solution.
--- End quote ---
It does have the disadvantage of adding a second order error from positive feedback because the bias current increases with output voltage, but I assume this error is insignificant.
Ratiometric sensors, including strain gauge bridges and RTDs, may add this error deliberately to improve linearity, in some cases by a lot. I think bandgap references also use this error or something like it for bow correction over temperature.
Kleinstein:
The reference is stable and thus not really much of 2nd order error. There is a tiny bit of amplification of effect like the TC, but this is minute. With some 3 K from 10 V to provide 1 mA and 0.5 ohm typical impedance for the LM399 this would be some 1.0002 fold gain on the TC or similar drifts. So nothing to even notice. Even for the 1N82x zeners with there 20 times high impedance and 8 x higher current the positive feddback is still a non issue. What changes is that current source and TC of the resistor can start to matter.
With the LM399 with the rather low impedance the current source is not critical. So other solutions will also work, though more complicated and still most of them less accurate (even if it does no matter). With a roughly 14000 fold (7 V / 1 mA / 0.5 ohm) attenuation of drift the qualtiy of the resistor is not that critical (just don't use low grade carbon).
mawyatt:
--- Quote from: David Hess on October 30, 2024, 01:59:42 pm ---
--- Quote from: Kleinstein on October 30, 2024, 10:22:15 am ---From the stability and simplicity it is hard to beat the resistor from the 10 V output. Even when build with separate drive and sense it is still the preferred solution.
--- End quote ---
It does have the disadvantage of adding a second order error from positive feedback because the bias current increases with output voltage, but I assume this error is insignificant.
Ratiometric sensors, including strain gauge bridges and RTDs, may add this error deliberately to improve linearity, in some cases by a lot. I think bandgap references also use this error or something like it for bow correction over temperature.
--- End quote ---
A similar means (+ feedback) was employed with the Caprio Quad Cell based differential amplifier to improve linearity.
Best
senso:
Hi everyone!
Long time since I last had free time to play with electronics, and I just discovered a couple days ago 4 LM399 that I ordered from Aliexpress some 5-7 years ago.
Do they look fake?
They all have burn marks, seeming to have been extracted from some equipment. They all have gold plated leads, all have some solder in the end.
I would like to just make a basic PCB to learn how to use KiCAD since I no longer have Altium and Kicad 8 seems pretty nifty.
This would be just to say that I have a voltage reference and to leave them aging, maybe some day I will get a decent multimeter.
I read some 10 pages back on this topic, plus a couple of the initial ones, is there any "recommended" design, or just go with the basic from the datasheet, since I'm not hunting for 10.0000V..
Best regards.
Kleinstein:
The chips look legit as reclaimed parts. The LM399 are normally quite robust, but there could still be a damaged one.
The standard circuit is with amplification to some 10 V and providing the current for the zener from there. So a bit like the "portable calibrator" circuit in the data-sheet, but without the trimmer. Depending on the amplifier used one may not need the 200 K for the start up.
I could make sense to also have an extra output for buffered 7 V.
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