LT1021CMH based mini-reference - Page 1
Forum section: electronics / general

author: Alex Nikitin
author_rank: Super Contributor
timestamp: January 31, 2017, 10:37:09 am
content: I've worked on that project slowly over half a year now and at last the reference is boxed and working quite well. It is built on a bit of a stripboard and as I only have enough bits to build one more, I am not planning to make a pcb for it. Most components are from eBay, the main exception is Pomona CuTe connectors. The reference is very simple, containing the LT1021CMH chip, a 20K multi-turn pot, few resistors and diodes for an additional temperature compensation (without it the tempco was over 10ppm/C, now it is around 1ppm/C), a reverse polarity protection diode on the P/S connector, a P/S capacitor, an LED, a 3-position switch and a nice VBF 10K/1.111111K divider in a hermetic case. This unit can provide 10V, 1V, 10K and 1K reference values with a reasonable accuracy and (hopefully) a decent long-term stability (better than 50ppm looks about right). The switch connects the output terminals to the voltage reference in one position, disconnects it in the middle and shorts the terminals in the third position. The divider is wired permanently to the output terminals providing 1V reverence from 10V output, so with the voltage reference disconnected, the resistance between two red terminals is either 10K or 1K. I will now measure the voltage and resistance values over a period of time (probably a couple of weeks) and my plan is to use this unit as a travelling reference. There is a space left for an output buffer to give the reference better load capabilities and a better short-circuit protection (right now it will survive a short, but most likely would suffer a hysteresis shift after that) so I will install it as a next step.CheersAlex

author: TiN
author_rank: Super Contributor
timestamp: January 31, 2017, 11:35:38 am
content: Your DMM is unhappy though.Nice little box. How about 10V noise? Also it's small enough to be fit into little heater box, do you plan running tempco tests as well?I'm in for travel comparisons, if you need victims.VHD divider from hifi-guy? I remember he had this value dividers in his shop.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: January 31, 2017, 11:51:50 am
content: Quote from: TiN on January 31, 2017, 11:35:38 amError 103 was left after a remote operation.Quote from: TiN on January 31, 2017, 11:35:38 amThe noise (at NPLC100) is below 1ppm p-p, the tempco is estimated from the overnight run - see the graph below - the temperature change was about 6C max-min, so the tempco is about +1ppm/C). And yes, the divider is from that source. I've bought several items from him, including two of these dividers specifically for that box.CheersAlex

author: Alex Nikitin
author_rank: Super Contributor
timestamp: January 31, 2017, 01:47:51 pm
content: I've added a buffer using OP97 (so now short circuits are not a problem) and run a quick test on NPLC1 to look at the noise. Here is the result for a 90 seconds run, the vertical scale 0.2ppm/div . I suppose it is OK for this kind of a reference. The current consumption is less than 4mA unloaded, the load regulation on the output terminals with a buffer is less than 3ppm/mA (<3mohm output resistance).CheersAlex

author: Alex Nikitin
author_rank: Super Contributor
timestamp: January 31, 2017, 03:42:56 pm
content: And here is another measurement for 80min at NPLC10, in the lab at about 23C room temperature, with 10K load placed on the output terminals for 15min.CheersAlex

author: Andreas
author_rank: Super Contributor
timestamp: January 31, 2017, 03:48:27 pm
content: Is this battery powered or mains connected?With best regardsAndreas

author: Alex Nikitin
author_rank: Super Contributor
timestamp: January 31, 2017, 03:53:37 pm
content: Quote from: Andreas on January 31, 2017, 03:48:27 pmRight now it is powered from mains through a simple switcher adapter. It can certainly run from a couple of 9V batteries in series. For what it should do it is good enough as it is. I might add an internal 15V LDO regulator though.CheersAlex

author: Andreas
author_rank: Super Contributor
timestamp: January 31, 2017, 04:02:38 pm
content: Hello,At least in the 2nd Diagram the noise seems to be rather high to me. (10uVpp).After my understanding the 0.1-10 Hz noise should be 6uVpp typical.With 10 NPLC (5 Hz) it should be even sligthly below.With best regardsAndreas

author: Alex Nikitin
author_rank: Super Contributor
timestamp: January 31, 2017, 04:18:26 pm
content: Quote from: Andreas on January 31, 2017, 04:02:38 pmHi Andreas,The power supply right now is not very good, without question, just something that was at hand in the lab. I will try a battery run and a different (linear stabilized) supply possibly tomorrow. The "typical" values in the datasheet however are not a guarantee of performance. The "typical" tempco for the LT1021CMH is shown as 5ppm/C with maximum 20ppm/C in the temperature range. In fact the one I'm using in this unit has a tempco over 10ppm/C around 23C, fortunately quite linear so I could correct it with a simple diodes chain.CheersAlex

author: Alex Nikitin
author_rank: Super Contributor
timestamp: February 06, 2017, 09:39:42 am
content: I am running the reference now from a nice linear 24V regulated supply however the 0.5ppm "jumps" are still present and I suppose that is the LT1021 chip itself. Still OK for the application intended. The VBF divider 10K/1K111111 is reasonably stable, below the results from the 68hours run over the weekend on the divider output giving 1V reference, the temperature dropped from +26C at the start to +20C at the lowest point, making the tempco a bit over +1ppm/C, plus a zoomed-in 1h graph to show the "jumps". Also attached the current schematics. The five diodes chain is selected for the particular reference (each diode gives about -2ppm tempco correction in this configuration, with the resistor R7 changed accordingly to keep the output at 10V). VR1 provides a fine trim. Output terminals switched by a three position switch, in one position only SW1.1 and SW1.2 closed, in the second position all switches are open and in the third only SW1.3 is closed, providing a short on the output to connect 10K and 1K111111 resistors in parallel, giving 1K reference value.CheersAlex

author: Alex Nikitin
author_rank: Super Contributor
timestamp: February 06, 2017, 02:45:59 pm
content: One more graph - 10K output measured over 15min (NPLC100 on HP3458A).CheersAlex

author: Andreas
author_rank: Super Contributor
timestamp: February 06, 2017, 03:18:01 pm
content: Quote from: Alex Nikitin on February 06, 2017, 09:39:42 amHello,looks like popcorn noise.I had also 1 extreme candidate (besides some other) in my LT1027DCLS8 thread.What NPLC did you use on the measurement?With best regardsAndreas

author: Alex Nikitin
author_rank: Super Contributor
timestamp: February 06, 2017, 03:32:43 pm
content: Quote from: Andreas on February 06, 2017, 03:18:01 pmHi Andreas,It is a pop-corn noise, however I think it is acceptable for this unit. I use NPLC100 most of the time for long-term logging, with some earlier graphs taken at NPLC1 and NPLC10 (noted in the relevant messages) . Here is another graph taken (at NPLC100) during a warm-up from a non-powered state on 10V output - 5ppm/div vertical and 1min/div horizontal. So far the stability is OK, I may try to run the unit in my improvised "Happy Reptile" temperature chamber to look at the tempco.CheersAlex

author: Kleinstein
author_rank: Super Contributor
timestamp: February 06, 2017, 08:39:42 pm
content: Even with some popcorn noise, the noise level is very good (e.g. 0.6 µV for the 1 h run).The steps are only like 200-300 nV - so hard to tell with a less sensitive meter.You also have to take into account that some of the observed noise is from the reference inside the meter.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: February 09, 2017, 10:13:42 am
content: I did a 23C-15C-45C-23C run in my "temperature chamber" on the 1010B reference and will post some graphs later. Few things transpired during that run.First, there is a noticeable lag between the reference itself and the temperature compensating diodes, as the diodes are placed on the board and not in a direct thermal contact with the case of the reference. It is only noticeable during reasonably fast temperature changes, as the diodes react faster than the reference, so if the temperature rises fast, the voltage can actually drop by 5-10 ppm before rising again some minutes later.Second, the tempco measured after the voltage is settled at a particular temperature is reasonably constant at +1...+1.5ppm/C over the whole 15C-45C range, so with a better temperature compensation circuit this could be adjusted closer to zero. I won't bother with this unit though, it is good enough for it's intended application as a travelling reference that could be posted cheaply.Third, the hysteresis was not readily measurable over that run, less than 1ppm variation after settling down at the same 23C temperature as at the start. It is also worth noting that the reference was transported at an outside temperature below 5C and did not exhibit any voltage shifts. The control run will be after the weekend when I'll bring it back to HP3458A.I will also run a proper temperature test for the internal 10K reference resistor. A preliminary estimate from few points is below 2ppm/C.CheersAlex

author: Alex Nikitin
author_rank: Super Contributor
timestamp: February 20, 2017, 12:38:49 am
content: Here is the second run in a "temperature chamber", this time with few "stops" - again, there is a visible lag on the thermal compensation, essentially no hysteresis, +1ppm/C tempco and a bit of a popcorn noise, still visible though the voltage was measured with the Keysight 34465A.CheersAlex

author: Kleinstein
author_rank: Super Contributor
timestamp: February 20, 2017, 04:57:43 pm
content: The popcorn noise look much stronger than in the earlier measurement. So maybe it is from a different source, like the DMM.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: February 20, 2017, 05:05:50 pm
content: Quote from: Kleinstein on February 20, 2017, 04:57:43 pmNot really, the popcorn noise is about the same 0.5ppm p-p. Vertical scale is 2ppm/div. Other "jumps" are from the thermal lag between the tempco correcting diodes and the reference IC.CheersAlex

author: Alex Nikitin
author_rank: Super Contributor
timestamp: February 27, 2017, 03:19:16 pm
content: I've measured the reference again with the HP3458A after three weeks and some temperature changes powered and not powered between 5C and 40C - the 10V output has drifted less than 3ppm. I suppose it is good enough for that reference.CheersAlex

author: Alex Nikitin
author_rank: Super Contributor
timestamp: March 02, 2017, 03:26:54 pm
content: Here is the latest from a 48 hours run with the room temperature variations between 21C and 24.5C. On a longer scale the popcorn noise type variations are about 1ppm p-p, which is still acceptable for this kind of a travelling reference, I think, as the stability with time, temperature and humidity variations appears to be good.CheersAlex

author: Alex Nikitin
author_rank: Super Contributor
timestamp: March 06, 2017, 03:34:36 pm
content: I've done a little update on the 1010B last week, adding a DMOS FET to bootstrap the supply voltage on the LT1021. The main reason was the supply voltage variation affecting the output too much due to the self-heating of the LT1021. With the FET in the circuit the supply voltage on the reference chip is 11.3V and does not change with the external voltage supplied from 13 to 28V. As a nice side effect the power dissipation in the LT1021 is halved and so is the warm-up drift (from 40ppm to 20ppm). The rest of the parameters seems to be unaffected.CheersAlex

author: Gyro
author_rank: Super Contributor
timestamp: March 06, 2017, 08:18:51 pm
content: Just looking at the LT1021 datasheet, I hadn't realised there was an LT1021-7 which does not include internal scaling resistors (or trim pin). The thing that surprised me is that its long term drift is quoted at a mere 7ppm/kh typ versus 15ppm for the other variants, marginally lower than the LT LM199 spec (8ppm/kh). Presumably this is for the H package only.I'm just curious that it's the lowest drift I've seen quoted for any IC reference. It's also quieter than the LM199 and with the benefit of a buffered output. Ok, the TC is higher than the LM199 butmuchlower than an unheated LM329. It looks as if it could work very well in a low temperature oven. Anybody tried it?

author: Kleinstein
author_rank: Super Contributor
timestamp: March 06, 2017, 09:03:54 pm
content: It looks like there is no (more) LT1021-7 in a metal case. At least LT does not have is for sail in small quantity.

author: Gyro
author_rank: Super Contributor
timestamp: March 06, 2017, 09:19:42 pm
content: Hmm, a good point (sadly). It's still just about possible to find genuine LM329H on ebay (and other LT1021H variants) but no sign of LT1021-7. An idea behind its time then.

author: Andreas
author_rank: Super Contributor
timestamp: March 06, 2017, 10:07:04 pm
content: Quote from: Kleinstein on March 06, 2017, 09:03:54 pmHow can you tell that?Arrow has more than 1000 LT1021DCH-7 stocked.And sells in single quantities.Besides this:The stated long term drift (the first 1000 hrs after soldering) usually is much higher than that what you can measure after a 10000 hrs run in phase.I normally measure 1-2 ppm/year for good hermetically references after that.So for me a T.C. larger than 1 ppm/K hurts much more since a compensation of T.C. is never perfect (perhaps a factor 10 improvement) when the compensation is outside the reference chip.With best regardsAndreas

author: Alex Nikitin
author_rank: Super Contributor
timestamp: March 07, 2017, 09:14:56 am
content: Quote from: Gyro on March 06, 2017, 08:18:51 pmYou need to be careful about datasheet figures, it is not something to rely on from my experience, especially in regard to the noise, tempco and long term stability. At best it is a rough guide. My examples of LM1021CMH are relatively noisy (noisier than a good LM399), with a rather large tempco (~10-15ppm/C) though the long term stability and hysteresis after one month appear to be very good. I have chosen this chip for a travelling reference mostly because it provides 10V with internal (i.e. placed in a hermetic case) resistors, so all critical bits (the reference, the step-up network and the resistor reference/divider) are in a hermetic packaging. OK, it is relatively noisy with 1ppm p-p of a pop-corn "jumps" but the long term stability appears to better than 1ppm/month (how much better only the time will tell) and it proved so far to be quite insensitive to travelling and temperature cycling from 5C to 40C.I have ten LM129AH in a hermetic packaging, perhaps I'll build the next reference with that one (or 2-3 in parallel, or maybe 3 in series for ~20V main reference voltage) and see how that will work. Another option is my JVR metal can JFET based reference, which appear to be very stable (less than 5ppm drift over more than a year, essentially less than I could measure reliably) and has a low tempco around room temperature. It does rely upon some good resistors though and doesn't have a particularly low noise (~0.5ppm p-p).CheersAlex

author: Gyro
author_rank: Super Contributor
timestamp: March 07, 2017, 10:07:09 am
content: QuoteOh absolutely. It just struck me as strange that LT would go to the 'trouble' of stating half the typical long term drift for the 7V version rather than just sticking with the more or less universal 15ppm/khr. It effectively implies that [Edit: the ratio drift of] the on chip resistive divider used on the other variants contributes typ 8ppm/khr, not something you would expect them to want to draw attention to.I was wondering how your JFET reference was doing, I hadn't seen an update for quite a while. It sounds way better than your initial expectations.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: March 11, 2017, 05:54:18 pm
content: The reference is on its way to one of the forum members in Netherlands, anybody else would like to be next to use/measure it?CheersAlex

author: SvanGool
author_rank: Regular Contributor
timestamp: March 16, 2017, 04:26:36 pm
content: QuoteThat is me and I just picked it up today.Two questions for Alex:Did you calibrate at 23°C or another temperature?Is R5 (200k) a low tc resistor, I saw it has quite some impact when temperature changes?

author: Alex Nikitin
author_rank: Super Contributor
timestamp: March 16, 2017, 05:10:04 pm
content: Quote from: SvanGool on March 16, 2017, 04:26:36 pm1) Yes, 23-24C ambient. It should be written on the label.2) 200K is MELF 0.1% , ± 15ppm/°C . It should not have any noticeable influence on the output v temperature. The temperature compensating diodes are right next to it though.CheersAlex

author: SvanGool
author_rank: Regular Contributor
timestamp: March 17, 2017, 12:49:10 pm
content: I received and unpacked it, thanks a lot Alex !I did some first experiments with the traveling standard with my HP 34401A (close to 1 year cal), Alex used a HP 3458A 002 for measuring the travelling standard, before sending it.Questions popping up, I am trying to imagine the possibilities of this travelling standard and I'm still learning:What does it mean if my HP 34401A is displaying 10.00038k (23C) when the travelling reference has 10.00045k on the label? Is this HP 34401A 10k range at least 10ppm (HP 3458A 002 accuracy) + 25ppm (0.0020% + 0.0005%, HP 34401A 24h accuracy) + 3ppm (travelling standard 1ppm noise + 2ppm 1C temp. accuracy) + 7 ppm (difference in meter readings) = +45/-31 ppm accurate for the next 24 hours?Would it mean, that if I would calibrate (assuming the travelling standard was stable since its first check with the HP 3458A 002) the HP 34401A with the travelling standards 10K and enter 10.00045 as calibration value, in that range, that the HP 34401A would have a minimum accuracy, for that 10k range, of 10ppm (HP 3458A 002 accuracy) + 25ppm (0.0020% + 0.0005%, HP 34401A 24h accuracy) + 3ppm (travelling standard 1ppm noise + 2ppm 1C temp. accuracy)= 38 ppm for the next 24 hours, in stead of the current 110 ppm (HP 34401A 1 Year 23C+/-5C accuracy)-25 ppm(24h 23C+/-1C ) = 85 ppm more worse minimum accuracy I have now?What would this mean for the HP 34401A 90 days and 1 year accuracy after 24 hours and 90 days, is it correct that e.g. the 1 year accuracy would become 110+10+3=123 ppm in stead of the 110ppm when I would officially re-calibrate the unit? So the price of using the traveling standard (in stead of officially calibrating) would be 13ppm for the involved ranges, but the calibration times would be reset again, for those ranges.Of course (for now when we don't know the exact behavior yet of the traveling standard) I would have to do the above exercise with my own LTC1021 voltage reference, VHP202Z 10K000 0.1% resistor and VHP202Z 1K0000 0.05% resistor, that I would measure simultaneously and would use those ones for calibrating when Alex has verified the standard completely again, on return. To mitigate the risk that I would ruin my current calibration, for those ranges, when the standard seems not to be stable upon return, assuming that my own references and meter have small aging drifts meanwhile.

author: ap
author_rank: Frequent Contributor
timestamp: March 17, 2017, 03:02:38 pm
content: Without having verified the detail ppm levels you state for each of the meters and cases, your logic is right. If you have a travelling standard with a certain total uncertainty (its cal value uncertainty, its possible temperature drift error, its aging related drift, anything else such as e.g. noise from surrounding; you need to establish all that), then you can use this reference to calibrate your 34401, and going from there, t0 starts again for your 34401. The uncertainty of that cal resistor adds to the time-depending uncertainties of the 34401.Be carefull when establishing the basic uncertainty of the resistor (you say 10ppm). The 3458A opt 002, btw, is no better doing resistance measurements than a standard 3458A. The 1 year 3458A uncertainty would be 10+0.5+3ppm absolute, if a standard cal was done as per cal manual of Keysight. They do have calibrations that are more precise than the 3ppm though. But if not done at Keysight, it may as well be worse. I have seen people calibrating a 3458A with a Fluke 5500...

author: lars
author_rank: Regular Contributor
timestamp: March 17, 2017, 05:56:23 pm
content: Hello Alex,I like your small box. Very clever use of the 10+1.111k with the switch! Even if I prefer NTC compensation, diodes also works very well.Your idea to send it around to get statistics is very good I think. Guess I should PM you.Hope the discussion on uncertainty will give good answers. That is one of the most interesting things for me. Having no volt nut friends nearby or access to sufficient good calibrated equipment getting low uncertainty is a problem for me (and many others).In December I think I wrote a long post in the Calibration club thread but hesitated to send it. TiN´s questions (provocation) about uncertainty really is important. Using GUM (Guide to Uncertainty of Measurements) for DIY is probably as useful as for my professional work, but how to get statistics???As far as I understand from my professional work: When I send eg a 34401 for calibration the uncertainty for the next 1 year will be the specification assuming the calibration lab says "pass"! An interesting discussion I have had with myself for my hobby is what say 34401 10k resistance that is off 7ppm at latest calibration and the last years have been say 10, 4, 15ppm. Can I conclude something?? (Of course I have characterized the aging, temperature and humidity sensitivity (takes long time)Lars

author: Kleinstein
author_rank: Super Contributor
timestamp: March 17, 2017, 06:29:05 pm
content: With many parts the drift follows a simple curve, often getting slower the older the part and moving in one direction. With some parts (e.g. voltage ref. chips) it is even known from experience in which direction and about as fast they usually drift. With high end parts, there is however the problem, that the readings during calibration also has some uncertainty and this may be more like random scattering (if it was a known trend, they should have corrected it). So you instrument history reflects both the instrument aging and the scattering and maybe drift of the calibration lab(s).For a meter send in to calibration, one has to distinguish between an calibration with adjustment and a pure check / reading. You only get the tighter short time specs if there was an adjustment or if you use the actual readings during calibration for correction. The direct reading from the display are not getting better from just a calibration that says still in specs (which?) without an adjustment.With some instruments (especially high end ones) it is also that the quality level of calibration can make a difference - not every calibration will bring you back to the stated short time specs.

author: SvanGool
author_rank: Regular Contributor
timestamp: March 28, 2017, 12:36:31 pm
content: I finished my measurements on Alex's LT1021CMH based "Travelling reference":Equipment used:6.5 Digit multimeter HP34401A:Last calibration: Dec 2014 (16 months over 1 year calibration accuracy)Settings: 10GOhm DCV input imp., 100PLC, manual range, 4-wire resistance measurementSpecified accuracy DCV 10.00000 V: 40 ppm (1 yr)Specified accuracy DCV 1.000000 V: 47 ppm (1 yr)Specified accuracy Res. 10.00000 KOhm: 110 ppm (1 yr)Specified accuracy Res. 1.000000 KOhm: 100 ppm (1 yr)Oregon temperature meter TWR800: accuracy 0.5°CRacal Dana 9232 Lineair Laboratory Power Supply:Output voltage: 15.16 VLoad regulation: <0.01%Ripple and noise: < 1 mVOutput impedance: <5 mOhm @ 1 kHzResult:10 VDC: 09.99999 V (@23.3 °C), HP3458A calibration: 10.00000 V, difference -1 PPM1 VDC: 1.000033 V (@23.3 °C), HP3458A calibration: 1.00004 V, difference -2 .. -12 PPM10 KOhm: 10.00037 K (@23.5 °C), HP3458A calibration: 10.00045 K, difference -8 PPM1 KOhm: 1.000093 K (@23.5 °C). HP3458A calibration: 1.00010 K, difference -2 .. -12 PPMRecommendations:Also specify the calibration for a lower temperature (e.g. 20°C), it was difficult to reach 23 °C in the cold North.Assess why it was not possible to get a stable 6.5 digit reading for the 1.000000 VDC and 1.000000 KOhm ranges on the HP3458A (on the HP34401A on "slow 6 digit", it was no problem).Consider to include wiring, to exclude measurement error addition by the wiring.Conclusion:if Alex verifies the calibration still to be intact on return of the reference at his place, it proves that the reference is behaving excellent and I can conclude that the accuracy of my equipment would be within 124 ppm or less, for these ranges again for the next year, if I would calibrate it now (see alsomy previous posting).I find this a very good result and it looks very promising for achieving the goal of this travelling standard.Thanks a lot Alex, very good job!Large fast picture of measurements.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: March 28, 2017, 12:49:45 pm
content: Thank you, good results!CheersAlex

author: Squantor
author_rank: Contributor
timestamp: April 02, 2017, 02:24:47 pm
content: Hello Alex,I am measuring the traveling standard as we speak. I am doing measurement runs but I am yet to bring my logging facility up.I am testing with the following requipment: HP34401A, Two Keithley 2700's and a Keithley 2000. Too bad all their calibration statuses are unknown. So I have no idea what my contribution will be, maybe just academic interest. I am measuring everything in my server room where it is a balmy 26 degrees centigrade, this is measured with a Klockis Ikea multifunctional clock, alarm, timer, temperature measurement thingy.More information coming up.

author: The Soulman
author_rank: Super Contributor
timestamp: April 19, 2017, 08:51:15 pm
content: How is the reference doing?I'm interested in the drift caused by shipping, temperature changes, etc.Only Alex can tell...

author: Squantor
author_rank: Contributor
timestamp: April 20, 2017, 07:15:31 am
content: I was really busy at work and other private matters. While measuring with my hp34401 calibration date unknown I noticed at 23C it was spot on with the reference! Very impressive! The cables that SvanGool made worked very well too. Did some quick tests with thermal EMF's with my recently repaired hp419a. It was about 200nV but not logged it for a longer period. I wanted to compare it to a battery of keithley 2002's that we have lying around but I could not find time due to overtime I have to put in these weeks.I will arrange with SvanGool to return the reference and first work on my own measurement setup to enable automated measuring and make a few extra references based on this design. Then if Alex is willing reship it to me to compare and measure again, maybe with the keithley 2002's from work.

author: SvanGool
author_rank: Regular Contributor
timestamp: May 06, 2017, 02:57:10 pm
content: Hello,I got the "Alex" LTC1021CMH reference back from Squantor and measured again with the same setup (@23.4 °C, 15.16V input, HP34401A 10GOhm DCV input imp., 100PLC, manual range, 4-wire resistance measurement)as here:https://www.eevblog.com/forum/metrology/lt1021cmh-based-mini-reference/msg1172332/#msg1172332after three hours warming up and same cabling.The results, 5 weeks after the first measurement:10 VDC: 10.00001 V, +2 PPM on first measurement, +1 PPM HP3458A1 VDC: 1.000033 V, 0 PPM on first measurement, -2 .. -12 PPM HP3458A10 KOhm: 10.00032 K, -5 PPM on first measurement, -13 PPM HP3458A1 KOhm: 1.000089 K, -4 PPM on first measurement, -6 .. -16 PPM HP3458AStill not bad at all.I will now send the unit back to Alex in the UK, who will get his (re-calibrated ?) HP3458A also back this week

author: The Soulman
author_rank: Super Contributor
timestamp: June 25, 2017, 05:05:26 pm
content: And?

author: Alex Nikitin
author_rank: Super Contributor
timestamp: June 25, 2017, 06:46:23 pm
content: Quote from: The Soulman on June 25, 2017, 05:05:26 pmThe reference is not back to me yet.CheersAlex

author: SvanGool
author_rank: Regular Contributor
timestamp: June 25, 2017, 07:19:33 pm
content: But it will be this weekI kept it to check also a HP3457A, I just repaired, which took a bit longer then expected, the last results:HP34401A (last calibration Dec 2014):@24.1 °C: 10.00005 V@24.1 °C: 1.000041 V@24.1 °C: 10.00023 K@24.1 °C: 1.000081 KHP3457A (last calibration Dec 2013):@23.7 °C: 10.00023 V@23.7 °C: 1.000047 V@23.8 °C: 10.00047 K@23.8 °C: 1.000084 KComment: the HP3457A has a nonadjustable 10 MOhm input impedance on the >=10V range and >10GOhm on the <10V range. On the HP34401A I used the >10 GOhm setting on all ranges.I'm curious for your HP3458A results.

author: The Soulman
author_rank: Super Contributor
timestamp: June 25, 2017, 08:13:19 pm
content: Thanks, looks like this little reference is holding up pretty well.I'm also interested to see the results on the 3458, but is has to cross the pond first...

author: Alex Nikitin
author_rank: Super Contributor
timestamp: June 25, 2017, 08:28:30 pm
content: Quote from: SvanGool on June 25, 2017, 07:19:33 pmThank you. In this case the input impedance at 10V does not matter, as the output impedance at 10V output is a fraction of an Ohm, and at 1V output the output impedance 1K, however both meters are at a high impedance input at that range.CheersAlex

author: Alex Nikitin
author_rank: Super Contributor
timestamp: June 29, 2017, 10:45:52 am
content: Yesterday I've received the reference back and today took it to the lab and after 2h warm-up measured it with the freshly calibrated (9 May 2017) HP3458A Opt 002 (after ACAL ALL):Here are the results @24C:10.000043V1.000044V10.00045K1.000096KThe wiring offset was less than 0.5uV. The popcorn noise "jumps" in the voltage are about 0.5ppm, as before.It looks like the reference drifted about +4ppm on 10V and 1V, with no visible drift on the resistance values (less than 1ppm) over the time period of about four months and some international travel.The calibration results for the HP3458A show +1ppm error on 10V and 1V DC gain.CheersAlex

author: Alex Nikitin
author_rank: Super Contributor
timestamp: July 03, 2017, 06:44:02 pm
content: Well, the 1010B reference is now available on loan to the forum members in the UK and Europe, the only conditions are to pay postage and to return the reference in a good state in 2-3 weeks time.The reference provides stable 10V, 1V, 10K and 1K - measured with better than 10ppm accuracy. In the UK I can provide a good 24V DC wall-wart supply, otherwise a reasonable quality 15-30V DC supply is required.CheersAlexP.S. Update - the reference is booked for the next two weeks or so. Please PM me if you would like to join the queue.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: July 26, 2017, 09:14:07 am
content: Another update after the reference has traveled to Italy and back. Here are the measurements results, using the same HP3458A Opt 002 after ACAL. The lab temperature was 24.5C.10.000040V1.000043V10.00044K1.000095KThe reference is again available.CheersAlex

author: Alex Nikitin
author_rank: Super Contributor
timestamp: July 26, 2017, 03:40:39 pm
content: Here is a 3 hour run today in the lab for 10V output. 1ppm/div vertical scaleCheersAlex

author: Alex Nikitin
author_rank: Super Contributor
timestamp: August 07, 2017, 10:50:31 am
content: The reference is booked for another two weeks.CheersAlex

author: Alex Nikitin
author_rank: Super Contributor
timestamp: November 03, 2017, 11:39:22 am
content: The 1010B reference is back in my hands and after a 2 hours warm up at 24C in the lab the HP3458A (after ACAL ALL) measures 10.00002V on 10V output. I will log the 1010B over the weekend and post a graph next week, together with the resistance measurements and 1V output value. The residual offset in the measuring system is less than 0.5uV.The reference will be available again at the end of next week.CheersAlex

author: Alex Nikitin
author_rank: Super Contributor
timestamp: November 06, 2017, 04:32:58 pm
content: The voltage readings are about -1ppm from what I've measured in August at the same temperature, both for 10V and 1V outputs. I suppose it is stable enough (the popcorn noise is still about 1ppm p-p) .CheersAlex

author: Alex Nikitin
author_rank: Super Contributor
timestamp: November 09, 2017, 12:16:35 pm
content: The latest measurements at 24C :10.00002V1.000041V10.00044K1.00010KThe reference is again available.CheersAlex

author: Alex Nikitin
author_rank: Super Contributor
timestamp: November 10, 2017, 12:01:06 pm
content: Here is the overnight measurement on 10V output with the temperature of the unit measured by a thermocouple. The tempco in this range of temperatures is about +1.25ppm/C .CheersAlex

author: Alex Nikitin
author_rank: Super Contributor
timestamp: March 01, 2018, 03:46:14 pm
content: The 1010B is back in my lab after another travel and again available. The measured values are holding inside +/- 5ppm for last six months or so, measured by the same HP3458A Opt 002.CheersAlex

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: March 13, 2018, 09:38:16 am
content: The reference just arrived in Belguim! I'll maybe shoot some video of it, unless you prefer me not to.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: March 13, 2018, 10:43:36 am
content: Quote from: TheUnnamedNewbie on March 13, 2018, 09:38:16 amNo problem with a video as long as you don't open the reference, it will ruin the calibration.CheersAlex

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: March 13, 2018, 10:48:26 am
content: Quote from: Alex Nikitin on March 13, 2018, 10:43:36 amOfcourse! I was not planning on opening it because of that reason.Thanks again for making this possible. Seeing my Keithley read 10.00007 has given me a lot more confidence in future measurements. Will start logging tonight.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: March 24, 2018, 09:57:14 am
content: Had a quite busy week so unfortunatly couldn't get as much data as I was hoping to, and haven't been able to make it to the post on Friday. However, I'll send it back on Monday or Tuesday.Sorry for going over my 2 week slot!

author: Alex Nikitin
author_rank: Super Contributor
timestamp: March 24, 2018, 12:22:12 pm
content: Quote from: TheUnnamedNewbie on March 24, 2018, 09:57:14 amNot a problem whatsoever, take your time, another week if you need it, there is no queue at the moment!CheersAlex

author: guenthert
author_rank: Frequent Contributor
timestamp: March 31, 2018, 12:09:30 pm
content: My HP34401 showed 10.00000V (+1/-2 over N=100, NLPC=10) at about 23°C (can't currently measure temperature with much confidence), which is quite astonishing given that it has not been calibrated in the last two years (if ever).My Datron Wavetek 1271 showed between 9.999,9752 to 9.999,9766V (min, max each average over 10). Not sure, whether the variance is due to low frequency noise of the DUT or the one of the internal reference or some draft across the contacts. My home 'lab' is not suitable for sub-ppm measurements. This result too is IMHO quite remarkable, given that the instrument was calibrated in 1993 (or at least nobody since bothered to update the calibration time stamp in the NVRAM).I think this goes to show that with some luck old instruments can be very stable and much more accurate then the nominal specification suggests.

author: ap
author_rank: Frequent Contributor
timestamp: March 31, 2018, 12:44:36 pm
content: Quote from: guenthert on March 31, 2018, 12:09:30 pmThere is a reason why many cal labs prefer old, aged precision instruments, such as the 3458A, 732 and 1281 (anyway not available new anymore) over new ones

author: Alex Nikitin
author_rank: Super Contributor
timestamp: May 25, 2018, 07:57:18 am
content: The 1010B is back in my lab after another travel to Europe and again available.It looks like my little unit holds the values really well, the difference from the values I've measured a month ago is below what I can measure reliably (HP3458A measures all four values at less than +/-2ppm difference).CheersAlex

author: borghese
author_rank: Regular Contributor
timestamp: May 25, 2018, 09:47:58 pm
content: I was the last user of the reference, and I used it to calibrate my two HP3456, previous calibration date unknown; surprisingly they were out of tolerance less than 20ppm; excellent and easy to repair instruments.Alex, maybe you could add the resistance value of 1.111k resistor so to have six digits of resolution; I estimated 1111.210 ohmThanks again AlexCheers

author: Alex Nikitin
author_rank: Super Contributor
timestamp: August 09, 2018, 09:51:07 am
content: The 1010B is back in the lab after a trip to Scotland, re-measured (with essentially the same results) and available again.CheersAlex

author: stijena1973
author_rank: Newbie
timestamp: May 31, 2019, 07:56:29 am
content: Are there some new data on the mini reference?Also did You keep track on resistor ratio separately from the reference voltage?If You did then You probably have some statistical data on the quality of the resistors internal to the chip...?

author: alex-sh
author_rank: Regular Contributor
timestamp: February 11, 2023, 12:02:42 pm
content: Hi,Reviving this old post.Is there any replacement for LT1021CMH?ThanksAlex

author: mycroft
author_rank: Regular Contributor
timestamp: February 11, 2023, 02:48:44 pm
content: Hi!I am not sure if this LT1021CMH is still in production but Arrow has some LT1021DCH-7https://www.arrow.com/en/products/lt1021dch-7/analog-devices.

author: Andreas
author_rank: Super Contributor
timestamp: February 11, 2023, 03:23:49 pm
content: Mhm,now more and more hermetically packaged devices run out of production.(except those SMD packages where the long term stability is still not proven).LT1021 is one option (perhaps as military version 5962-8860001GA)AD588JQ would be my choice after the AD587JQ is now near unobtanium.with best regardsAndreas

author: iMo
author_rank: Super Contributor
timestamp: February 11, 2023, 06:02:44 pm
content: I got LT1027LS8 in past - the smd ceramic one, still "in production", it seems.It is with 5V output, however.

author: SvanGool
author_rank: Regular Contributor
timestamp: February 11, 2023, 10:10:02 pm
content: On eBay the LT1021CMH/883 which I bought there in 2018 is still sold on the same site:https://www.ebay.com/itm/191123710347

author: alex-sh
author_rank: Regular Contributor
timestamp: February 16, 2023, 07:16:56 pm
content: Quote from: imo on February 11, 2023, 06:02:44 pmI saw it, but I am after 10V ref

author: alex-sh
author_rank: Regular Contributor
timestamp: February 16, 2023, 07:19:22 pm
content: Quote from: SvanGool on February 11, 2023, 10:10:02 pmThank you - will try this one.Though I was thinking about LT1021B-10 (2ppm/C)

author: randymatt444
author_rank: Contributor
timestamp: April 10, 2023, 06:37:14 pm
content: PCB for this design. Comments before I send it out please.

author: BD1QMP
author_rank: Contributor
timestamp: April 18, 2023, 07:07:43 am
content: I have some LT1021BCN8-10, the Temco is negative. cannot use 1n4148.

author: iMo
author_rank: Super Contributor
timestamp: April 18, 2023, 07:22:36 am
content: Why not? Put the diodes on the opposite side of the trim divider.. (low side)..

author: BD1QMP
author_rank: Contributor
timestamp: April 18, 2023, 11:12:10 am
content: Quote from: iMo on April 18, 2023, 07:22:36 amyou are right. need adjust the diode position.Temco of LT1021B is about 2ppm/degree, so I design some jumper to add in 1-5 diodes, but mine is negative, so I have to re-layout the pcb.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: April 19, 2023, 01:46:29 am
content: i use plastic, had luck to adjust tempco to nothing , drop in the small oil bath and just a heater. all this fit in metal thermos 0.5L wide one.so far after 2 weeks , for a month it just a fluctuation of few microvolts, and in overall it seems combination of ext. temp , imperfection of measurement etc ...i use 2 pins only to solder dip8 to the board , 2 other over like spring\hanging wire ...my point - i think plastic have some chance as well(yes yes plastic and crystal fused together, atmospheric pressure, imperfection you name it ....) but so far so good ...

author: iMo
author_rank: Super Contributor
timestamp: April 19, 2023, 07:39:08 pm
content: What kind of oil do you use?

author: GigaJoe
author_rank: Frequent Contributor
timestamp: April 19, 2023, 11:57:59 pm
content: it was a pure mineral oil from a drug store , i think transformer oil will fit , but when i poke around some suppliers, they said there no mineral transformer oil in use , instead some fluori...h.. something liquid.aaand NTC resistor indicated that simply heating up , not really good idea at all, due to internal temperature are balance between heat production and dissipation . so dissipation affected even by something lay nearby or more affected if on top ... so need some basic thermostat.

author: randymatt444
author_rank: Contributor
timestamp: April 26, 2023, 02:33:45 am
content: New PCB 24hr TestSo now trying to figure out why this looks so good.Also, I'm guessing the 34461A measurements is because of noise (?)

author: Andreas
author_rank: Super Contributor
timestamp: April 26, 2023, 04:42:49 am
content: Quote from: randymatt444 on April 26, 2023, 02:33:45 amHello,did you really use 100 NPLC integration time on the 34461A?My 34401A typically has less than 3 uVpp in this case.And your 34461A has obviously more T.C. than the voltage reference.with best regardsAndreasedit:could be also the FAN within the 34461A:https://www.eevblog.com/forum/metrology/dmm-adc-noise-comparison-testing-project/msg652325/#msg652325

author: randymatt444
author_rank: Contributor
timestamp: April 26, 2023, 04:58:57 am
content: Quote from: Andreas on April 26, 2023, 04:42:49 amYes, double checked. I have another 34461A and a 34401A I'll hook up in parallel to check it out. This is buffered with an OP07 so that should be fine.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: April 26, 2023, 05:49:43 am
content: How do i measure:preparing 2 oil bath in thermos , room temperature , and around +35-40stable 10V source , and DUT as 10V source. measure as difference between 2 source.with difference method i can record faction of microvolts, on 3457drop the DUT in room temp . oil bath, settle time around 10-15 min , record temperate , record diff. in voltagedrop the DUT in hot temp . oil bath, settle time around 10-15 min , record temperate , record diff. in voltagethen a simple mathi don't quite understanding of measuring how heater\air-conditioning acting for 24 hour , really ...

author: iMo
author_rank: Super Contributor
timestamp: April 26, 2023, 06:43:00 am
content: Quote from: randymatt444 on April 26, 2023, 02:33:45 amThe 34461A measurement looks to me like a typical one, TC of the meter could be less than 0.5ppm/C and the TC of the 1021 could be less than 3ppm/C. Also the noise with its 10uVpp looks ok to me. Your meter is 399 powered (5-10uVpp) and the noise of your 1021 could be 10uVpp as well..PS: in the excel you may calculate the standard deviation of your measurement, and you get "rms" noise. My guess it would be around 1uV (I got a running stddev upon last 100 samples typically between 0.6-1.2uV with 1021 epoxy refs and my 34401A).

author: iMo
author_rank: Super Contributor
timestamp: April 26, 2023, 07:22:46 am
content: FYI - here is a typical overnight run of two LT1021-10 epoxy (wired in parallel) with running stddev (over last 100samples)..

author: randymatt444
author_rank: Contributor
timestamp: April 26, 2023, 08:20:06 am
content: Quote from: iMo on April 26, 2023, 06:43:00 amWhat is the sample rate you are logging? I'm using 3.6s (1,000 per hour).

author: Kleinstein
author_rank: Super Contributor
timestamp: April 26, 2023, 08:39:07 am
content: The curve looks quite stable. The step like features (some 5 µV step size) could be due to the reference inside the 34401 DMM, it is about the expected size for the LM399.The epoxy version may still have slow drift from humidity effects.

author: iMo
author_rank: Super Contributor
timestamp: April 26, 2023, 09:39:40 am
content: Quote from: randymatt444 on April 26, 2023, 08:20:06 amI've been always logging with 100NPLC (aprox 4secs period).

author: randymatt444
author_rank: Contributor
timestamp: April 26, 2023, 09:56:06 am
content: Quote from: iMo on April 26, 2023, 09:39:40 amSo, with the Agilent DMM Utility set to 100 NPLC and sample set to "fastest" it's about 3.4sec, I set it to 3.6 so it would be 1K to 1 hr.I'm kind of new to the logging part so I'm questioning everything.

author: iMo
author_rank: Super Contributor
timestamp: April 26, 2023, 10:04:18 am
content: Quote from: Kleinstein on April 26, 2023, 08:39:07 amYep, those 5uV "steps" are due to popcorn of the LM399, the LT1021 may popcorn too, but with two LT1021 in parallel it comes from the 399, sure. There is always some TC with the 1021, humidity with epoxy, and most problematic is the hysteresis of the LT1021 in epoxy, at least with my samples. Up to 60ppm after a couple of months switched off (it takes aprox a month to get somewhere back with the voltage). I wonder how those various 1021-10 epoxy based cheapo references people produce and sell actually perform..

author: randymatt444
author_rank: Contributor
timestamp: May 11, 2023, 07:00:46 am
content: Quote from: Alex Nikitin on March 07, 2017, 09:14:56 amHow are you guys measuring and logging the temperature? I mean, manufacturer, model, type (RTD, Thermistor, Other) of the thermometer and such, as I'm looking.ThX-Randy

author: ejd.pol
author_rank: Contributor
timestamp: May 13, 2023, 01:29:25 pm
content: Hello,Thank you very much for sharing this design! The trick with the diodes to make temperature compensation is very neat.I am considering building (at least) one of these references too, as the accuracy and stability seem quite high.Before going into the build, I have a couple of questions, hope that you can answer them:-What is the dependency of the temperature compensation circuit on the particular device?For example, could it be that for another device the number of diodes should be changed? How about resistor values?If changes may be needed, how would one go about figuring out which changes to make?-Does it make sense, in addition to the temperature compensation, to protect the LT1021 from temperature swingsby a) insulation or b) encasing it in a temperature controlled enclosure?-Could it be useful to protect the device from physical stress on the leads?There are designs where the leads are not soldered directly to the PCB, but some thin and flexible wiring is inserted in between.The thin wires not only absorb the physical stress, they also constrain heat loss.Regardless of insulation or stress-relief wiring, the temperature compensation circuit shouldalways be kept in good thermal contact with the LT1021 itself, of course!Those are some first thoughts, I would be grateful to hear your insights!Kind regards,Evert-Jan

author: Andreas
author_rank: Super Contributor
timestamp: May 13, 2023, 02:15:44 pm
content: Quote from: ejd.pol on May 13, 2023, 01:29:25 pmHello,The metal can package is usually less sensitive to lead stress than SMD or DIP packages.(Keep the leads long).I usually make a test by bending the PCB. If the bending creates more than +/- 1 ppm shift I use some kind of stress relief.with best regardsAndreas

author: randymatt444
author_rank: Contributor
timestamp: May 13, 2023, 09:42:41 pm
content: Quote from: ejd.pol on May 13, 2023, 01:29:25 pmThis is averygood question.The first step is to measure the tempco with no compensation diodes to see it's "natural" tempco. Then, I'm just guessing where to go next, (i.g., try 1, 2, 3 ...6 diodes).There are folks here much more intune than myself that can probably answer this and calculate the numbers of diodes and the values of the associated resistors.

author: Andreas
author_rank: Super Contributor
timestamp: May 14, 2023, 04:59:56 am
content: Quote from: randymatt444 on May 13, 2023, 09:42:41 pmHello,the influence of the trim pin on output voltage is usually given as "trim range" in the data sheet.the T.C. of one diode is about -2mV/K.All other is simple math. (can also be done by EXCEL or some LTSPICE model)see for a different reference (with a thermistor and pot to trim T.C. according to Lars Walenius) also here:https://www.eevblog.com/forum/metrology/ad587lw-10v-precision-travel-standard/msg1547132/#msg1547132And of course you need many temperature sweeps to verify results.Quote from: randymatt444 on May 11, 2023, 07:00:46 amDepends on the temperature range.in 10-40 deg C range I usually use either a 33K NTC (cheap) or some kind of digital (calibrated) temperature sensor. (TMP112, SHT25, MS5611)for larger temperature ranges a PT1000 measured ratiometric with one of my 24 Bit ADCs.https://www.eevblog.com/forum/metrology/lowest-drift-lowest-noise-voltage-reference/msg3758498/#msg3758498with best regardsAndreas

author: iMo
author_rank: Super Contributor
timestamp: May 14, 2023, 07:28:51 am
content: I spent months playing with 1021 (epoxy) and MAC-01 (metal) TC compensation - ntc thermistors as well as diodes. I would not do the exercise again. It is a difficult process (as Andreas has shown above) with many degrees of freedom (trim voltage ratio/trim range/trim input impedance/TC of trim input/TC of diodes/TC of thermistors/TC of trimmers/TC of resistors/long term stability of all above).. Better to jump straight into the world of 399/1399/1000/1001 references.

author: iMo
author_rank: Super Contributor
timestamp: May 14, 2023, 09:18:05 am
content: In case you want to play with TC compensation (w/ diodes) of the 1021 in LTSpice - here is an example..Mind the level of the compensation depends on the current flowing via the diodes too, as well as of the TC of the resistors..The trim voltage should be around 5V.As there is none TC in the model of the 1021 you may add the TC via the B source - see below.Also - the TC21 is in ppm/C (ie. 3*1e-6/C in the example), the result in the graph - the "Slope" is in uV/C (ie. 2uV/C in the example, what is 0.2ppm/C).The actual TC21 is an "U" or "S" shaped curve, thus to get better results you have to find the higher order TC21 equation (ie. like Andreas did for TC21_2*Temp_diff^2 and TC21_3*Temp_diff^3) and do use it in the B1 voltage source (see below an example).

author: ejd.pol
author_rank: Contributor
timestamp: May 14, 2023, 01:05:56 pm
content: Hello,Many thanks for all replies! I would have guessed that it is only to be expected that the TC of the LT1021 is device dependent,because if there were a systematic TC, LT would have undoubtedly compensated that to make their product less temperature dependent.So, perhaps (at best) there might be a correlation between devices of the same batch,but in general, the TC measured over devices should be noise with average zero. Right?That means indeed that the TC needs to be determined per device.So, before (slowly) becoming a volt-nut, I first need to become a temp-nut! (Resistance is futile....)[Side note: I have designed and constructed a 0.1 ppb GPSDO, so I am also on my way to becoming a time-nut!]What follows shows my current thinking (which is undoubtedly flawed), and I would be very grateful to receive your comments.It so happens that I have a small metal box that used to be the enclosure of an OCXO, with two ground pins (connected to the box itself), and four signal pins.I also have an LM35A, would that have sufficient accuracy to serve as the sensor in the measurements needed to assess the TC of the LT1021?The initial idea is to put some isolation in the box, and in the isolated space put the LT1021, the temp sensor, and a heating element of some kind.Then hook the sensor up to an ADS1115 (also available in my spare parts box), connect the ADC and the heating element to a controller (Arduino Uno or such),and that should result in an oven that I can preset to an arbitrary (within reason) temperature. I need to take care to make a sufficiently stable supply for the sensor and ADC, of course.(How about using an LT3042 for that purpose?) The build should be such that I can measure all four of my LT1021 (indeed in a metal can, not the plastic version).Then there is the question of how to measure small changes in voltage as produced by the LT1021.First of all, a stable reference is needed. This is almost a chicken and egg issue, but the most important thing is that the new reference is stable.I do have an LM399H. With an opamp (LT1012 is ok here?) I could make a stable (but not very accurate) 10 Volt reference.In fact, I could trim the LM399-based reference to the untrimmed LT1021.Then I would need a difference amplifier to track the differences between the LM399 and the LT1021 references (another LT1012?). Gain set to 100 or so?Come to think of it, I have two LM399... Hmm... If I would build two such references, I could also measure how the difference develops between these two.Just to get a better view on the drift of the LM399 references themselves. Would that make sense?Anyway, after measuring all four LT1021, I could make temperature-compensation circuits for all four, and measure them again.Then the best could be selected to be built into the oven permanently. So all trimming, compensation, etc. should be done at the target temperature. Say, 40 C?Then, finally, the reference-in-the-oven can be trimmed. For trimming of my egg, I will need access to some professional reference chicken.Or are there any other fun things to do with 6(!) references? (Bob Pease had a setup with 4x4 LM399, measuring drift within each group of 4, and across the four groups.)The main purpose of all this is to learn, and also to see how far stability and accuracy can be pushed with moderate means.Any comments and ideas are highly appreciated!

author: Andreas
author_rank: Super Contributor
timestamp: May 14, 2023, 03:04:32 pm
content: Quote from: ejd.pol on May 14, 2023, 01:05:56 pmHello,no.Modern high end references like LT1021 or LT1236 are already individually factory trimmed for the temperature range intended.Trimming is done at 3 temperatures only (e.g. 0 deg C, 25 deg C and 70 deg C for a comercial device).a T.C. of 5 ppm / K means that the 3 temperature points lie in a window of +/- 350 ppm.There is no guarantee being within the window for temperatures between the points and the curvature in between is also not defined.So external trimming well below 1 ppm/K is always only possible for a limited (+/- 5 .. 10 deg C) temperature range.For larger temperature ranges (10-40 deg C) I have already thought of a microprocessor with D/A converter or PWM trimming with a 3rd order temperature compensation.Measuring T.C. is relative simple when measuring voltage differences.One reference at stable temperature and the other in a temperature controlled environment.A LM35 may be sufficient when the temperature swing is large enough. (well above 10 deg C).Having only 2 references for regarding differences over time is not enough.You never know if there is a slow drift in between how the absolute drift over time is.So you need at least 3 references to sort out the bad one.with best regardsAndreas

author: branadic
author_rank: Super Contributor
timestamp: May 14, 2023, 07:06:46 pm
content: QuoteLet me add here, that these three references should in theory be completely uncorrelated. Having three references from the very same batch adds some potential correlation to them. You wouldn't notice if all three references simultaneously jump by the same amount into the same direction, when only observing their differences.-branadic-

author: ejd.pol
author_rank: Contributor
timestamp: May 16, 2023, 09:25:04 am
content: @Andreas:Thank you for the explanation of the trimming process. Indeed, if trimming is done only for three fixed temperatures,the TC for other temperature points may have a systematic deviation from zero.Would you agree that, in theory, and for a large enough population of devices, the TC at these three trimming temperatures should have zero average?(The average is never exactly zero of course, but presumably it should be significantly smaller than the datasheet value valid across the entire temperature range.)Using a DAC to generate the trim voltage sounds like a neat idea! A processor can easily interpolate (polynomially to any order) between a number of predetermined table entries.If the trim value is determined at, say, 5 C intervals from 0 to, say, 80 C, the table contains only 17 values. Assuming that the TC is a relatively smooth function of temperature,third order interpolation of four table entries should suffice (hopefully!). That is a lot less work than determining the optimal 3rd order polynomial per interval.But I guess one runs into the question of how much the DAC output drifts? I guess the DAC resistor chain be set by the reference to be trimmed?? (Similar to bootstrapping a zener...)

author: iMo
author_rank: Super Contributor
timestamp: May 16, 2023, 10:45:47 am
content: A "disciplining" of the LT1021 has a sense only when the DAC plus its own Vref_DAC reference is "much more" long term stable than the LT1021 itself.. It works nice with OCXO discipling where the GPS is indeed "much more" long term stable.. In order to discipline the LT1021 you would need at least a 399/1000 based Vref_DAC, imho..

author: alm
author_rank: Super Contributor
timestamp: May 16, 2023, 06:14:57 pm
content: Quote from: ejd.pol on May 16, 2023, 09:25:04 amI don't see why. Why would the population mean be expected to be close to zero? Why not +10 ppm/K? Or -5 ppm/K?

author: Andreas
author_rank: Super Contributor
timestamp: May 16, 2023, 06:53:11 pm
content: Quote from: ejd.pol on May 16, 2023, 09:25:04 amI would not make any assumptions about the curvature (local temperature gradient) of a trimmed device.You can only expect that by trimming you get more higher order curvatures than untrimmed devices.(compare a simple TL431 datasheet with a LT1019 or LTC6655 datasheet for "typical" response over temperature).with best regardsAndreas

author: iMo
author_rank: Super Contributor
timestamp: May 17, 2023, 09:20:19 am
content: Out of curiosity I put into my sim above the TC coefficients from Andreas' posthttps://www.eevblog.com/forum/metrology/ad587lw-10v-precision-travel-standard/msg1547132/#msg1547132The no-comp shape identical with that from Andreas, and the compensation with diodes (an example only), see below.