Electronics > Metrology

10V ref LM399 project

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jonpaul:
Bonjour a tous!

After a long pause for travel, work, medical,  back to the VoltNuts....

Voila!  a 10.000 V ref box.  power ext 18V lab supply or  or 2  9V internal batteries.

 Adapted a German CircuitValley.com  LM399 board, inside cast Pomona box,

Added  15V  prereg 15V LM2940-15 LDO   3 way power switch ,  1N5818 Schottky polarity protect diodes,  LED and binding posts.

The  LM399 has a Styrofoam heat shield , removed for photos.

Shown with Keysight 34465A  1 yr, in CAL.    9.999968 V  after warmup.

Can replace the LT1001 ACNS easily as it is on gold-plated machine pin socket.

Seeking an independent CAL of the board.

Your thoughts greatly appreciated!

Bon Soiree,


Jon

Andreas:
Hello,
just some questions/notes:
- where is the 15V voltage regulator. I hope it is after the schottky diode at least for the mains input. There is a output voltage dependancy of the LM399 on heater voltage.

- Why do you use a FET with only +/-12V max gate voltage for a input voltage of ~18V?

- Why a Transient zener with 30 V (usually max operating voltage) which begins to conduct at ~40V

- Since a Transient zener usually creates a short on overloading I would put a fuse before the zener.

- The Schottky diodes at the output are unclear for me (perhaps the drawing is hard to read).

- I hope the trim range is very small for the output voltage (usually trimmers have rather high tempco).
  usually a different trimming scheme (similar to standard cell replacement in the data sheet) is used for precision trimming.

- before calibration I would at least do some run in phase for 2 kHrs

- and of course you should do some tempco tests for the whole cirquit

- the housing needs some markings (input voltage, output voltage and orientation of the package).
  the output voltage is dependant on orientation of package.
  I had some strange drift after putting the LM399 up side down accidently for some days after a calibration
  I had luck that it reversed after some weeks in normal operation.

with best regards

Andreas

Kleinstein:
The trim circuit is not ideal. A better place would be in parallel to the other resistor in the gain setting part and than with a series resistor for the coarse part.

The LED should get away with less than 10 mA off current - this helps with battery operation. Modern LED should get away with ~ 0.2-1 mA.
The LT1001 is a bit power hungy with some 2 mA typical. For battery operation I would consider a lower power OP-amp, like OPA207 (SMD only and thus not that practical for the existing PCB).

For judging on the cause of drift, it would help to also have acess to the raw 7 V ref voltage.

I personally would prefer a different connector for the supply and if wanted an adapter cable to connect to a lab supply if needed.

imo:
The major issue I see with the RoadRunner's reference board is the "zener" is not bootstrapped off the opamp's output (a "standard" today). It hangs on the Vcc (not stabilized) and the opamp makes 7->10V only.

r6502:
Hi Jon,

If you have it not already seen, have a look here:

LM399 based 10 V reference

May be, you are also interested in the new ADI ADR1399, then have a look here:

ADR1399 reference

Lots of useful info out there.

Guido

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