Electronics > Metrology

Need Help with an LM399 Reference with Selectable Output Voltage

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Chance92:
Hi! I would like to build a 1mV to 10V voltage reference based on LM399. The circuit is based on fmaimon's design in the LM399 thread. I made some changes to make it a bit more versatile:

1. Added a voltage divider and a buffer to get voltages from 1mV to 10V. (I calculated the output resistance of the divider to be around 10 ohms, so I put a 10 ohm resistor on the buffer feedback. Is this resistor critical?)
2. Added a connector for battery connection.
3. Changed LM317 to 7815 to simplify the design a little bit. (Is this a good idea?)
4. Used a dedicated 7815 to power the heater in order to reduce the effect of battery voltage drop. (Is this necessary for maintaining stability?)
5. Added some input protection.

I have some used LM399s date back to 1989. Are they a better choice comparing to brand new LM399A?

I am thinking about putting the entire thing into a thermos flask. Is this a good idea?

Can I use this reference to test the stability of a 7.5 digit DMM? How can I improve the stability?

Suggestions and criticism are welcome.

TiN:
C6 should be C0G or film, not X7R. I'd add bigger bulk cap in parallel with C5 (e.g. 100-1000uF 25V/35V).
Also some smaller bulk caps at outputs of LDOs might be nice to have (22-100uF). Check LDO datasheet for recommended/max capacitance.

You need to route heater ground separate from signal ground. Also you should modify schematics to reflect star mecca ground for all signal nodes, this is important. Your connection of zener cathode to heater pin suggests that you missed this important point.

I'd get rid of switch (as unreliable contributor in signal path) and instead have multiple individual output taps (with opamp buffers if you have budget).

Why AD706 chosen?

Also consider new ADR1399, it has noise similar to that of LTZ1000 at fraction of cost/complexity.

There are no 0.2ppm/K resistors, suggest to specify real spec, not typical specmanship ;).

Flask is decent idea. Make sure to protect circuit well from air drafts on both sides.

After a year of ageing with constant power and careful calibration and some history data logging you'd be able to use such reference to calibrate any 7.5-digit DMM on fullscale for base 10V and maybe lower ranges. Stability and performance of your divider chain R6-R10 would be biggest error contributor in this design. You might want to consider specially designed resistor network, likes the ones Caddock makes.

PartialDischarge:
The offset of the AD706 is too high, not good.
The AD706 needs bipolar supply to go down to 1mV, even 1V output. For most op-amps you will need some negative rail for outputs near 100mV.
I'm missing some kind of "smallish" trimming capability
Don't use that switch, it adds too much resistance.


For an application were I need 10-100mV with at least 1% accuracy I'm building this circuit, it uses 2 gold plated pin headers for each switch range:

Andreas:
Hello,

I personally would also use more beefy batteries than PP3 blocks.
A alkaline battery with ~200 mAh will last not long especially as the voltage quickly drops from 9V to 8V within the first 10% of discharge.
So with 20-25 mA for the heater and 5 mA for each voltage regulator + some current for the buffers the voltage will fall out of regulation before the LM399 is heated to full precision. (~15-30 minutes).

with best regards

Andreas

iMo:
I would add an output transistor (or better two transistors for the current limiting) at the output of the U5..
PS: see below an example - it assumes your opamp is rail to rail.. You may put that stage at the U4 as well (to de-load the U4).. The max output current aprox 30mA..

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