Yet another (LM399) volt-reference - Page 1
Forum section: electronics / projects

author: wiss
author_rank: Frequent Contributor
timestamp: December 15, 2014, 08:14:06 pm
content: I have built a basic LM399 reference.I never really felt like ordering 100's of euros worth of resistors, so right now I intend to use the zener-voltage directly, measured with a high-impedance meter.R1, R4, R5 and R6 are 15 ppm 0.1%, U1 can be something reasonable, I'm using OP07 or OPA27 mostly right now.

author: Seekonk
author_rank: Super Contributor
timestamp: December 15, 2014, 08:53:56 pm
content: Somehow, I feel like I'm missing something.

author: wiss
author_rank: Frequent Contributor
timestamp: December 15, 2014, 08:59:21 pm
content: uhu?You make a bunch of them (6), but them in a box with a rotary switch so you can easily compare the individual drift.Each ref-board is small enough to be shipped as a letter.Apparently LM399 does not complain about being powered down.(and btw, I did find 18 399s in the trash the other month...)

author: Andreas
author_rank: Super Contributor
timestamp: December 16, 2014, 06:29:51 am
content: Quote from: wiss on December 15, 2014, 08:14:06 pmHello,so I understand that you are using the 7V output only as it is the only high stability point in the cirquit.Did you build the cirquit already?I think for stability (oscillations) it would be better to have someintegration capacitor from output of OP to the negative input.With best regardsAndreas

author: Seekonk
author_rank: Super Contributor
timestamp: December 16, 2014, 09:18:32 am
content: For those who want to argues such things, I was giving it a bump up before it was lost to oblivion in the foggy time warp. This was mentioned in the other thread, how does it start. It seems you are hoping for all those things you want to design out. A bump resistor could be added across the FET and it would have no further effect.Happened to look on ebay ans found gobs of 399's that were pull outs. Makes me wonder what to source of all these were and what kind if issues they have.

author: wiss
author_rank: Frequent Contributor
timestamp: December 16, 2014, 09:40:46 am
content: I'm taking measurements of them right now, layout, photos and data later.This circuit will self-start! Don't worry!Andreas: if the op is too fast there will be need for a compensation-cap, I have a foot-print for it on the board, with op07 and opa27 there seems to be no need, I smoke-tested with ca3140, and I think I mounted the compensation-cap on the first 2 boards(3140's natural state is oscillating... it can be hacked into an opamp)

author: wiss
author_rank: Frequent Contributor
timestamp: December 16, 2014, 07:33:12 pm
content: Photo of the board, a small travel-case I made and the drift since last night (measured with a Keithley 192)Andreas: No, I did not mount any compensation-caps on these boards, there is one on the prototype but not these (the foot-print is still there).

author: Andreas
author_rank: Super Contributor
timestamp: December 16, 2014, 08:09:14 pm
content: Quote from: wiss on December 16, 2014, 07:33:12 pmSo I think you will have to do some "run in" phase until the reference stabilizes before you can use it.The question is where the noise comes from.Is it the Keithley or your reference? Can you improve the noise by thermal shielding of the LM399?Usually the LM399 noise is typical around 4uVpp (some bad samples have more)so it should be not visible with your instrument (10uV) resolution.But of course this depends also on power supply quality.With best regardsAndreas

author: wiss
author_rank: Frequent Contributor
timestamp: December 16, 2014, 08:45:15 pm
content: The Keithley is rather noisy, this is measured withoutany filtering, one 100 ms every minute, with my 7075the noise is less than 10 uV at 1 s integration (one count flickering).The 399's come from power-supplys for a XPS, or something likethat, has been running 24/7 for 10 years, I rather believeit's the keithley that is drifting...

author: Dr. Frank
author_rank: Super Contributor
timestamp: December 17, 2014, 07:56:40 am
content: .. And .. are you still riding to Germany next Saturday?.. the volt-nuts party is already fixed for Saturday evening...Frank

author: wiss
author_rank: Frequent Contributor
timestamp: December 17, 2014, 09:05:00 am
content: No, all of Saturday I will on the road, about as for to Zwickau as to Frankfurt, ETA about 2300 if traffic is good...But, if you want, 3 of my Refs could participate! If you PM me your address and I mail them before 1800 today they will probably make it.

author: babysitter
author_rank: Frequent Contributor
timestamp: December 18, 2014, 06:51:40 am
content: Re Party: I dropped out, a nasty little virus took me out of order and then my wife sequentially, so need saturday to compensate for things that didn't happen.

author: Dr. Frank
author_rank: Super Contributor
timestamp: December 18, 2014, 10:50:05 am
content: Ok, well, that's a pity.. We'll have a 1 on 1 party only, then.Hendrik, get better again!Frank

author: babysitter
author_rank: Frequent Contributor
timestamp: December 18, 2014, 01:09:45 pm
content: Thank you, the upside is the bug leaves as abrupt as it appears. 24 hrs feeling sick, then rapid getting well.But still bad for the party this time. But such events should happen on a regular base, right ?

author: wiss
author_rank: Frequent Contributor
timestamp: December 18, 2014, 10:08:05 pm
content: Now I've figured out a way to better measure the noise and drift, both params will be lower than shown in the picture, any drift in both curves will be the DMM, and there's still some math to get to the ref-drift/noise. I will collect data over night (Christmas?) and come back on that.Anyway, noise is less than 5 uV p-p.

author: wiss
author_rank: Frequent Contributor
timestamp: December 24, 2014, 06:43:26 pm
content: Now some more measurements over a few days, tie-scale is in hours and the magenta-trace is the noise of the voltmeter, the other thre traces should be dominated by the noise and drift of either of the three references:

author: Andreas
author_rank: Super Contributor
timestamp: January 07, 2015, 08:14:34 pm
content: Hello,for me it is unclear what the y-axis is.relative error (ppm) or absolute drift in uV.With best regardsAndreas

author: wiss
author_rank: Frequent Contributor
timestamp: January 07, 2015, 08:37:03 pm
content: There are 3 references, vr1, vr2, vr3.There are 4 measurements vm1 = vr1 - vr3, vm2 = vr2 - vr1, vm3 = vr3 - vr2 and a short.In the last graph I plotted( vm1 - vm2 ) / 2 = vr1 - 1/2(vr3 - vr2)( vm2 - vm3 ) / 2 = vr2 - 1/2(vr1 - vr3)( vm3 - vm1 ) / 2 = vr3 - 1/2(vr2 - vr1)zeroversus hours.Ofcourse, the average of each voltage was subtracted before plotting. ..