Electronics > Metrology

LM399 based 10 V reference

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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

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