10V ref LM399 project - Page 1
Forum section: electronics / general

author: jonpaul
author_rank: Super Contributor
timestamp: January 30, 2023, 02:56:30 am
content: 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

author: Andreas
author_rank: Super Contributor
timestamp: January 30, 2023, 07:57:03 am
content: 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 calibrationI had luck that it reversed after some weeks in normal operation.with best regardsAndreas

author: Kleinstein
author_rank: Super Contributor
timestamp: January 30, 2023, 08:35:32 am
content: 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.

author: iMo
author_rank: Super Contributor
timestamp: January 30, 2023, 08:41:47 am
content: 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.

author: r6502
author_rank: Regular Contributor
timestamp: January 31, 2023, 11:49:26 pm
content: Hi Jon,If you have it not already seen, have a look here:LM399 based 10 V referenceMay be, you are also interested in the new ADIADR1399, then have a look here:ADR1399 referenceLots of useful info out there.Guido