MAX6226 voltage reference - Page 1
Forum section: electronics / general

author: exe
author_rank: Unknown
timestamp: May 14, 2021, 07:58:58 pm
content: Hi there,It seems there is not much info about MAX6226 voltage reference. So, I decided to start a thread about it.I bought two pieces of this reference, and today I finally powered it up.I attach the plot of the very first start. If I read the plot correctly, the vertical scale is 10uV, or 4ppm (still learning how to do plots). The disturbance at around 21:40 is when I put the reference into a plastic jar.Measurements are done by my (un)trusty Keithley K2000 which I bought on this forum. It has unknown calibration, might be recapped and has some repairs, but to the best of my knowledge it is stableI warmed it up for 2h before starting measurements. Current room temperature is 22.5C. I set NLPC to 10 ("slow" update mode).Video of the first start:https://www.youtube.com/channel/UCvTzuQaA_jxHFvox4VnlLnQ("reaction" video, not much is happening there).

author: Kleinstein
author_rank: Super Contributor
timestamp: May 14, 2021, 08:29:44 pm
content: 10 µV per tic sound reasonable. The plot program and raw data should tell. There should not be guesswork with the scale.The time is so short than the warm up from self heating of the reference can be a significan part of the curve. So a large part of the observed drift could be just a temperature effect.Ideally one would also record the actual temperature at the refrence and measure the temperature dependence as a seprate experiment.

author: exe
author_rank: Unknown
timestamp: May 15, 2021, 08:08:56 am
content: Here is data over night, from 21:15 yesterday till 9:15 today. The voltage didn't change from yesterday, it still shows rock-solid 2.49994V. The input voltage might dropped by ~80-100mV over night (it measures now 3.27). That's because my DIY power supply is not precise and that's why I'm looking for a better voltage reference.The temperature dropped by 2C from yesterday. Unfortunately, I don't have any logging thermometer. I'll try to resolve that. In the worst case I'll record my thermometer on webcamera and manually add data.I said that my K2000, but I actually don't have any evidence to support this. I mean, it's stable up to some 100uV because it agrees with another reference, but that's not enough for this experiment. I also didn't power my K2000 since a long time ago.I plan to do two more experiments:1. Change input voltage and see how this affects output voltage. Say, in 2V increments from 3V up to 12V. Probably I should let it set for, say, 30minutes after each change?2. Do thermal cycling to see hysteresis. Say, up to some 70C and back to room temperature.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 15, 2021, 12:42:55 pm
content: The K2000 has a LM399 reference inside, that by now is probably well aged. The LM399 refrence can still show some random effects (e.g. jumps) comparable or slightly smaller than the max6226. So the meter can still contribute.For testing a few supply voltage there is no need to wait that long. Some 2-5 min should be good enough.

author: exe
author_rank: Unknown
timestamp: May 15, 2021, 01:00:43 pm
content: I tested line regulation. The settle time is surprisingly long. Between 3.2V and 5V it settled much faster than between other voltages. Finally, at 13V (that is absolute max for the IC) it was doing a weird thing: see the plot. I'm a bit surprised. Also, the regulation is exceeding 25uV/V specification, unless we use the "box" plot. Might it be I damaged the ic?PS vertical scale is volts, one tick is 100uV.

author: Andreas
author_rank: Super Contributor
timestamp: May 15, 2021, 01:39:51 pm
content: Quote from: exe on May 15, 2021, 01:00:43 pmIt is a mix of line regulation and tempco due to self heating.The manufacturers test the line regulation with short pulses only(leaving the chip temperature constant at 25 deg C)but of course this does not help in a "real application" where you have the self heating.with best regardsAndreas

author: Andreas
author_rank: Super Contributor
timestamp: May 15, 2021, 01:51:22 pm
content: Quote from: exe on May 15, 2021, 01:00:43 pmHello,read the datasheet carefully: the maximum recommended operating voltage is 12.6 V.So the behaviour above 12.6V is not specified.Did you check for oscillations of the output in this state?I would also recommend to use only one connection to the DMM(and solder the other end to the reference instead of using crocodile clips).with best regardsAndreas

author: exe
author_rank: Unknown
timestamp: May 16, 2021, 08:43:57 am
content: Current voltage is 2.499930, which is 10uV below of what was yesterday (I kept DMM and the reference powered the whole time). I attribute this to testing the line regulation yesterday. I'm a bit disappointed.Today I want to do two things:1) clean the flux to see if there is any effect (I used quality flux, but there was also flux in the solder, so not sure how the two mix).2) Heat it up with a hotairgun to, say, 61C for a few minutes, then let cool down and let's see how this affect the voltage.3) re-arrange DMM crocodile clips to see if this makes a difference on the voltage. That's because ground lead carries some 0.4mA of quiscent current and I wonder if longer/shorter ground lead would affect the reading.I wish I had a thermal chamber. I have a bottom heater, but it has up to 20C overshoot and poor regulation, so, not exactly suitable for our purposesQuote from: Andreas on May 15, 2021, 01:51:22 pm13V is just 0.4V above recommended operating voltage, I decided to try it. I have no intention to power the reference with voltages above 3.3V in a real circuit. I didn't check for oscillation with a 'scope, but looking at the readings, I think we'd see that on the plot if it was oscillating. There is only one DMM connected to the reference, and I know why you recommend this.As of soldering, I don't get it. Could you please elaborate more? The power and DMM are connected with crocodile clips. I don't have DMM leads that I could solder.

author: exe
author_rank: Unknown
timestamp: May 16, 2021, 09:45:33 am
content: Hey, I reattached negative lead of my DMM and got my 10uV back! Wow, I need to be careful with wiring. Yesterday I re-arranged wires a little bit, may be that's why I got that 10uV drop. That's because I tried to put negative lead as close to the IC as possible. I'll try to reduce length of wires. The downside is, it will be some thermal shock for the IC.So far the results are not too bad. Next I'll clean the flux and do some very crude thermal cycling (more like thermal shock).

author: Kleinstein
author_rank: Super Contributor
timestamp: May 16, 2021, 09:48:50 am
content: The crocodile clips are not ideal for precision measurements. They may cause some thermal EMF if the is a thermal gradient and the wires may cause mechanical stress to the reference. The air-wired connection to the capacitors is good in keeping the mechanical stress low - it may be better than practical use on a PCB.I would suggset soldering wires for the supply and meter to the caps, so to get even even less disturbance and possible bad contracts. Just simple banana plugs can cause a few µV of thermal EMF from not that much temperature difference (e.g. after touching them).

author: exe
author_rank: Unknown
timestamp: May 16, 2021, 04:22:17 pm
content: I cleaned the IC from flux. It recovered to previous voltage pretty quickly. However, then it gradually dropped by 10uV againI thought it might be due to mechanical stress, so I knocked on the chip with a guitar pick just in case, but it seems this didn't affect it. So, I had another creative idea how to relief the stress, I used a freezing spray to do a thermal cycling to -55C and back to room temperature. I've got a 10mV jump, which in 5 mins leveled off. But the thing is, the voltage keeps going down. So, it's now at about 2.499918 V, which is 22uV below what I had yesterday. I'll probably leave it alone for the next 12h to see how it goes.Next I'll try to make a crude temperature logger with a raspberry pi and ds18b20.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 16, 2021, 04:27:13 pm
content: cooling to -50 C can be quite some stress and there may be some hysteresist from this.

author: exe
author_rank: Unknown
timestamp: May 17, 2021, 06:28:59 am
content: This is data over night. However, the power supply sagged by ~50mV. Considering previous data showed line regulation 100uV/V, this perfectly maps to 5uV change that we see on the plot. So, at least I need to fix that before making any conclusions. I guess all the previous data was affected by that too.I have a question: is power supply noise important in this case? Asking because I consider using lm317n, which has noise, according to datasheet, about 8uV in 10kHz BW (it's unspecified if it is p-p or rms).

author: Kleinstein
author_rank: Super Contributor
timestamp: May 17, 2021, 06:55:04 am
content: A LM317 regulator for the supply should be good enough. It is not very low noise, but the power supply noise should be attenuated similar to the line regulation. There can be slight differences because of the thermal effects, but the order of magnitude is usually similar for the lower frequency part one usually cares about.

author: exe
author_rank: Unknown
timestamp: May 18, 2021, 07:52:55 pm
content: The reference (let's call it IC1) settled at around 2.499901, which is like 40uV difference to initial voltage. I guess thermal shocking wasn't the best idea.For the experiment with a stable supply, I deployed my second MAX6226 (let's call it IC2). The initial voltage is 2.5002.This time I tried to get the wiring "right". I also increased output capacitance to 1uF. I used a polyester cap because it was lying on my bench (kemet R82DC4100DQ60K). I hope it's fine (I mean ESR and ESL). I didn't install cap on NR pin (noise reduction) because I'm lazy. According to the datasheet, it should lower the reference noise from 75uV/sqrt(Hz) to 45uV or so, and hope my K2000 doesn't care about it.I didn't find a raspberry pi to do 24x7 voltage and temperature monitoring, so I ordered a new one, raspberry pi with four cores and 8 gigs of ram. A bit pricey and has more power than I need, but I wanted to get the best for my lab. It will arrive tomorrow. When it comes I'll try to solder the output wires to crocodile clips.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 18, 2021, 08:57:48 pm
content: A capacitor at the NR pin is filtering some of the higher frequency ( e.g. > 10 Hz) noise. A slow meter, like the K2000 does not care much about the higher frequency noise. So for the test there is no real need for the cap at the NR pin.The ouput capacitance is also mainly about the higher frequenies. So it should not make that much difference here.One may still need some capacitance to avoid oscillation of the reference by itself.The curve from the 2nd chip looks quite a bit better.

author: exe
author_rank: Unknown
timestamp: May 19, 2021, 05:45:57 am
content: The voltage dropped more than 15uV over night, which is about -6ppm. I wonder if this is due to different wires I used to connect the DMM. I think I'll replace them just in case. Temperature logging would probably help with the investigation.

author: exe
author_rank: Unknown
timestamp: May 20, 2021, 06:55:09 am
content: On ic2 the output voltage keeps going down. It lost ~24uV since first report.During next month I won't be able to access my lab and I think I'll shutdown the equipment while I'm absent.

author: d-smes
author_rank: Regular Contributor
timestamp: May 20, 2021, 08:20:57 am
content: If you can do so safely, I'd leave the reference powered while you are gone. It will allow the reference to 'burn in' and hopefully stabilize it a bit.

author: exe
author_rank: Unknown
timestamp: May 22, 2021, 10:30:25 pm
content: Unfortunately, didn't have time to arrange that. On a positive side, this gives me time to make a proto-board with multiple references. So I can test, say, four references instead of one. Also I'll add a temperature sensor. Also, now max6226 is again available on digikey and will be soon in mouser.PS any ideas how could I make a simple multiplexer for, say, four references? Would CD4051B work? I now have raspberry pi, so I have digital io that I can integrate into my measurement script.

author: Andreas
author_rank: Super Contributor
timestamp: May 23, 2021, 04:32:09 am
content: Quote from: exe on May 22, 2021, 10:30:25 pmHello,Could work if the supply + digital input levels are 3.3 V and input voltage is max 3.3V.And the input impedance of your Instrument is >> 10 Meg.For my ageing box I use CD4067 (16-fold) multiplexers.One for the Reference output and one for the Gnd-Signal(which differs by up to 30 uV depending on position of the reference).But since in my case I have a 14V positive supply (since the references are up to 10V) i need level shifters from the 5V digital level to the 14V input level of the multiplexers.For my LM399 ageing box I prefer MAX4051A (A-Grade).These have lower leakage current and also level shifters included.In any case I have 100 nF bypass capacitors at the output of the Multiplexers to avoid EMI effects.with best regardsAndreas

author: Kleinstein
author_rank: Super Contributor
timestamp: May 23, 2021, 10:32:45 am
content: The CMOS MUX work for signals within the supply range. So the cheap CD4051 , 4052 and so one work only to the logic supply. For higher voltages I would consider DG408 with seprate supply (up to some +-20 V) for the analog part.With an isolated meter and having the ground level near the negative supply the CD4051 could also work for 5 V or 7 V references. Just have a -5 to -8 V and +3.3 V supply and the ground level near the negative supply. So the Raspberry ground would not be the reference/analog ground.

author: exe
author_rank: Unknown
timestamp: May 23, 2021, 10:52:00 am
content: Thank you very much. Since my references are only ~2.5V, I think I'll try to use 74LV4051/74LV4052 with a signgle 3.3V supply, same rail that I'm using to supply the references.How important it is how have a separate switch for ground terminal? I'm not looking for absolute precision, I'm only interested in stability (initial drift is some tens of uV anyway). I was hoping to get away with a solid ground poor and not to have an additional multiplexer. Although, probably, it won't be a big problem to do kelvin connection.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 23, 2021, 11:38:08 am
content: For precision circuit a ground pour is poor. One has little control over the actual curren paths and variable supply current at some places can cause extra offsets.So one should better have dedicated ground traces, ideally to a star ground. Also EMI wise, the left over parts are usually not a valid ground plane. It only needs a few slits to destroy a ground plane.Depending on the circuit, if there is a good ground, one may get away without separate mutiplexers for the low side. It still depends. If the ADC is single sided, there would be no seprate MUX, but only some mux channels used for ground.

author: Andreas
author_rank: Super Contributor
timestamp: May 23, 2021, 12:52:39 pm
content: Quote from: exe on May 23, 2021, 10:52:00 amHello,as long as the power supply current (and the load) are constant it is simply a position dependant offset.On the other side: when having a 8 channel multiplexer and only 4 references you can also use the spare channels to sense the gnd pin levels. (Quasi-differential with 2 sequential measurements).with best regardsAndreas

author: hgl
author_rank: Regular Contributor
timestamp: May 23, 2021, 05:10:31 pm
content: This is a short paper from Maxim about the long-term stability of the MAX6126AASA25

author: dietert1
author_rank: Super Contributor
timestamp: May 23, 2021, 08:44:29 pm
content: The Maxim paper is interesting in that they confirm aging may take up to 10 000 h. Maybe this is the proper time scale for references running at room temperature, while for hot references like the LM399 and LTZ1000 the time scale seems much shorter, more like 2000 h.Regards, Dieter

author: Andreas
author_rank: Super Contributor
timestamp: May 24, 2021, 05:33:04 am
content: Hello,I (dont) like these "artificial" tests "with sockets (non soldered)" at constant temperature and humidity keeping all stress from the references away.What has this to do with a "real life" application in industrial environment?Even a short period (perhaps 2 days at the week end) with different humidity which has a time constant of several days (more like 5-7 days) has more influence than all other effects during the whole 3kHrs.with best regardsAndreas

author: 2N3055
author_rank: Super Contributor
timestamp: May 24, 2021, 05:54:58 am
content: Quote from: dietert1 on May 23, 2021, 08:44:29 pmAD588/688 also settled mostly after 2 months (cca 2kh) and was fully settled after 3kh(cca 3 months).. It is a CERDIP case.It has quite high internal dissipation, so you could consider it heated reference from a standpoint of thermal stress though...I believe it is mostly mounting stress of the crystal, i.e. interaction between crystal and mounting material relaxation..

author: kleiner Rainer
author_rank: Regular Contributor
timestamp: May 24, 2021, 07:57:53 am
content: Quote from: Andreas on May 24, 2021, 05:33:04 amHobbyists are not the market they aim for. In industrial applications, you want stability. Removing a sensor unit from a production line for calibration is expensive in downtime and handling costs. The aim is to calibrate it once (after burn-in), install it and forget it. The sensor is sealed, and if necessary fitted with special membranes to allow for pressure relief and to keep humidity out, combined with humidity absorbing material. Anything that needs extra precision lives in a controlled environment (temperature, humidity, EMC). My employer has such an environment for the precision measurements toolmaking (diecast and plastic molds) needs.I suspect the measurement regime Maxim chose is based on some MIL Std. and as such it is easily reproducible by anyone with similar equipment. Changing temperature and humidity (simultaneously? one after the other?) requires more detailed specifications: rate of change for example. How comparable are the measurements if your climate chamber is slower in changing temperature and/or humidity?Standards often also specify the number of specimens you must test to get statistically relevant results - this gets expensive for hobbyists if precision references are involvedBTW "artificial" tests are the norm in industry standards work: take the LISN used in EMC work. The typical line impedance specified in the sixties is as artificial as it gets, but it is useful because everyone measures to the same standard, so results are comparable.Sockets are not a problem, if you use the special type that is designed for burn-in: Teflon body with precision gold plated contacts - Fischer Elektronik still manufactures them:https://www.fischerelektronik.de/web_fischer/de_DE/Steckverbinder/F03/Fassungen%20f%C3%BCr%20TO%20...-Geh%C3%A4use/search.xhtmlTo sum it up: this reference is NOT designed for metrology grade applications. It is nice if it works for our voltnut aims, but there is no guarantee. It works for applications that need a "set and forget" approach as in industrial applications, where calibration after burn-in is done once. We are not the market. Period.Greetings,Rainer

author: exe
author_rank: Unknown
timestamp: June 04, 2021, 01:15:30 pm
content: I ordered ten max6226 . I plan to make a pcb with eight ICs: four mounted normally (well, kinda, I don't plan to solder pins that are not internally connected or not supposed to be connected), and four to be mounted as a "dead bug".Also ordered several humidity and temperature sensors from aliexpress.

author: Andreas
author_rank: Super Contributor
timestamp: June 04, 2021, 09:27:33 pm
content: Hello,I ususally bend the PCB to see if the reference is influenced by PCB stress.If the bending ends with <5 uV influence on the 5V reference (1 ppm) I usually stop.But this ends usually that I solder only the GND pin of the reference to the PCB (near the NTC for the temperature sensing) and the rest of the pins with thin VERO-wire.with best regardsAndreas

author: exe
author_rank: Unknown
timestamp: June 25, 2021, 06:39:04 pm
content: I'm back. I got the ICs, now designing a pcb. The board will have two 74hc4051 to switch between eight references.

author: Andreas
author_rank: Super Contributor
timestamp: June 25, 2021, 06:53:21 pm
content: Quote from: exe on June 25, 2021, 06:39:04 pmWhy not MAX4051A?And where are the bypass capacitors from Pin 3 of the Muxes to Gnd?With best regardsAndreas

author: exe
author_rank: Unknown
timestamp: June 25, 2021, 07:42:29 pm
content: Quote from: Andreas on June 25, 2021, 06:53:21 pmI didn't use MAX4051A because I have doubts it will make a difference. The real leakage is probably below 1nA. I'll check on breadboard if I don't forget.As of bypass caps, it's not on any schematic in datasheets, I checked several datasheets. I concluded the mux is too simple to need a bypass cap. I also used 4052 switch on breadboard without bypassing, it worked just fine. If it had an internal clock, or brown-out detector or something, I'd include include it.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 25, 2021, 08:46:02 pm
content: Bypassing is a good idea. In the moment the CMOS muxes are switching they take quite some supply current. For the PCB it is easy to include.I would consider to also have optional resistors to create an average over groups of some 2 or 4 reference for later use.It makes not much sense to first burn in the reference and than later unsolder. A more realistic case would be cutting the PCB in parts and use smaller parts, like 2 references in parallel.Usually the leakage currents should be OK, but one never knows for sure. The chips are not tested for low leakage and 74HC... is more like made in a low cost process / fab.A place for a temperature sensor would be also nice for the PCB.

author: exe
author_rank: Unknown
timestamp: June 25, 2021, 09:30:11 pm
content: As of temperature monitoring, I bought two temperature and humidity sensors, Si7021 and SHT31. I quickly tested the first one, humidity seems to be working, but temperature resolution is too low (like ~1C or so). Perhaps, need to fix initialization code.About averaging. I think I can just do it in software. This way I can mix arbitrary references on the board and see their combined performance (but that's assuming my K2000 is stable).Making a snap-off board sounds like an interesting idea, because that allows selecting references with smallest drift. Currently I don't have much plans to use the references other than just discover their properties. I'm also a bit skeptical about performance of these references. May be in the future, if max6226 proves it's worth it.

author: exe
author_rank: Unknown
timestamp: June 26, 2021, 06:58:02 pm
content: Pre-final version of the board. Changes:1. Added a small decoupling cap shared by both switches just in case. Routing is getting a mess...2. Finished routing, reduced board size, etc.3. Added reverse polarity protection (diode)4. Added an output capacitor after switch for filtering output.5. Top copper pour connected to +3.3V.I also include schematic of the reference. The header allows kelvin sensing, and also allows activating (with a jumper) a loading resistor. I plan measure the voltage changed with 1mA sinking current. The loading resistor is quite close to the reference, hope its heating won't affect the reference much.What I want to try now is to make a dead-bug mount footprint for four references out of eight to see if this makes any difference.PS Can 74HC4051 switch signals slightly below Vee? Like, a millivolt or two? Datasheets say no, but I'd expect it to handle some 0.2-0.3V beyond rails.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 26, 2021, 07:51:02 pm
content: I would also expect the 4051 to also handle up to some 200 mV beyound the supplies. There would be some some additional leakage though from "diodes" slightly forward biased. At least the HC4053 / LV4053 / CD4053 do work that way.The voltage regulator is quite close to the 4051 - this may cause some gradient there.

author: Andreas
author_rank: Super Contributor
timestamp: June 26, 2021, 08:05:26 pm
content: Quote from: exe on June 26, 2021, 06:58:02 pmHello,Yes some millivolts should be no problem.As long as the input protection diodes do not get conductive.Above 100mV the leakage will increase.with best regardsAndreas

author: exe
author_rank: Unknown
timestamp: June 27, 2021, 06:37:23 pm
content: Quote from: Kleinstein on June 26, 2021, 07:51:02 pmYou guys have sharp eyes, and I didn't even post the schematic. Moved the reg a little bit.This is the board I'm going to order. There is a room for improvement, but as long as there are no critical bugs it's fine.PS board thickness will be 1.0 mm so it's easier to bend in bending test.PPS I think text needs to be on silkscreen layer, trap for young players

author: exe
author_rank: Unknown
timestamp: August 15, 2021, 09:18:02 pm
content: The board is fully assembled and all channels seem to be working. Supply voltage after lm317 is 3.32. Now I'm writing a script to do automatic measurements.

author: exe
author_rank: Unknown
timestamp: August 16, 2021, 03:23:43 pm
content: Very first plot of all references. Vertical scale is 50uV. I guess we all want to see deviation in ppms from a mean value, right?.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 16, 2021, 04:10:15 pm
content: There is quite some correlation between the curves, especially around 60% of the x axis. There seem to be a common effect of some effect of the measurement system.How are the voltages measured ?Except for the yellow and brown curve the drift so far looks acceptable, but hard to really tell.

author: Andreas
author_rank: Super Contributor
timestamp: August 16, 2021, 05:29:45 pm
content: Quote from: exe on August 16, 2021, 03:23:43 pmAnd the time scale? I interpret 2 seconds per axis-tick (but this would mean many hundreds of measurements per second?)with best regardsAndreas

author: iMo
author_rank: Super Contributor
timestamp: August 17, 2021, 08:00:24 am
content: Quote from: Kleinstein on August 16, 2021, 04:10:15 pm@exe: You have to plot the temperature of the pcb as well. The correlation there comes from a pcb/env temperature change, I guess..

author: exe
author_rank: Unknown
timestamp: August 19, 2021, 06:23:07 pm
content: Last couple of days I was busy with trying to make meaningful plots out of the data I've collected. I'm attaching the test output, which is smoothed by moving average of 50 points. Sample rate is 6 samples/min (i.e., every 10s all references are scanned and measured). So far I'm very happy with results. I wanted to have some +-5ppm stability and it seems I got it. The setup is quite poor, I'll describe it below. So, probably, real performance is even better than on the plots.As of temperature and humidity collection: working on it. I have issues with it, it gives very limited resolution. Figuring out why.For debugging reasons the board is just laying on my table, and cats walk on it from time to time, causing disturbance in measurements. I plan to make an enclosure for it. As of connection to DMM, I use very cheap leads for now. I plan to replace that once I make enclosure and setup temperature/humidity sensor properly.

author: exe
author_rank: Unknown
timestamp: August 22, 2021, 12:18:33 pm
content: I added temperature and humidity logging yesterday. Now making an enclosure. What surprises me is how stable voltage was. It seems tempco is about +1ppm/C. The disturbance after 7am is when I switched on fan, it was creating air disturbance. Temperature variation over night is much smaller than I expected, even with windows open.Current connection is quite crappy (those red and black crocodile on the photo). Unfortunately, I cannot find banana plugs in my lab to make a good connection. I wonder if I can take some better DMM leads, cut off tips, and add a female 2.54mm connector so I can plug it in. Or those 2.54mm headers (commonly used for breadboarding) are not suitable for measurements? I can make them tight fit...

author: exe
author_rank: Unknown
timestamp: August 23, 2021, 05:47:10 pm
content: I've made an enclosure, and also test leads (on photo).There is +-0.5ppm or so variation from measurement to measurement. To remove that, I apply heavy averaging (moving average of 100 measurements), otherwise it's hard to see eight plots simultaneously. I attach what a plot looks like without averaging.Some facts about max6226:*. Price raised from 7.83 to 8.32 euro (before VAT) on Mouser.*. Average voltage today across all measurements is 2.500032252142955*. It looks like ic3 has some popcorn noise

author: dietert1
author_rank: Super Contributor
timestamp: August 24, 2021, 06:59:09 am
content: Precision work at 2.5 V! Point to point RMS noise should be a fraction of a ppm, lets say 1 uV. I think that is a very good result when using banana plugs. I am also surprised you get that with those standard CMOS MUXes. Good news! Unit 1 seems to drift in relation to the others, so it may be the second one to reject, but that's a bit early to tell.Regards, Dieter

author: Kleinstein
author_rank: Super Contributor
timestamp: August 24, 2021, 07:56:19 am
content: There is quite some correlation between the references. Unless there is a prblem with the supply, chances are this could be an effect of the meter or maybe thermal EMF from the cables. What meter is used for the measurement ? Looks like it has a good ref. with low popcorn noise.The dead bug mounted refs look good.

author: exe
author_rank: Unknown
timestamp: August 24, 2021, 07:49:52 pm
content: The meter is K2000 which was probably re-capped by previous owner(s). K2000's manual says it has 1ppm/C coefficient on 10V .I too noticed correlation, and I don't know if it because of the meter, because all references exhibit similar tempco. Or may be leads are not very good.As of IC1 and IC2, those were subject to freezing well below zero (I used a freeze spray), then hotair, then I physically knocked it with quite some force (I wanted to see how sturdy is ceramic carrier), they were also resoldered at least once... May be that's why IC2 is not doing well.I kept the lab with closed window over night, so there is not much temperature and humidity variation. Tomorrow I'll probably leave the window open to see how that will affect measurements. The downside is, this can create drafts.After about a week or so I plan to load one or two references with a 2.4K resistor (~1.04mA current) to see load regulation.

author: Andreas
author_rank: Super Contributor
timestamp: August 24, 2021, 08:01:58 pm
content: Quote from: exe on August 24, 2021, 07:49:52 pmHello,where exactly do you read this?I read:for 18-28 deg C the tempco is within uncertainity specs.outside of 18-28 deg C the TC is 2 ppm of reading/K (2,5V in your case = 5 uV/K) + 1 ppm of range/K (10V = 10uV/K) or a total of up to 15 uV/K for this measurement.I fear you will have to do some T.C. measurements of the instrument first.See:https://www.eevblog.com/forum/metrology/project-pimp-a-keithley-2000/msg1106829/#msg1106829with best regardsAndreas

author: exe
author_rank: Unknown
timestamp: August 25, 2021, 07:07:08 pm
content: 2Andreas: oh, right. There is no spec for it. Well, then I can't really say anything about precision of this instrument. Even if I find a cal lab (probably I'll have to send it to Germany as local labs turned me away because I'm not a company), I guess they charge me extra for climate chamber.

author: exe
author_rank: Unknown
timestamp: August 30, 2021, 07:58:16 pm
content: Current state of references.

author: exe
author_rank: Unknown
timestamp: September 17, 2021, 09:55:41 am
content: Currently logged data. I averaged all values with a sliding window of 1000 measurements. It's still hard to see individual voltages, so I'll re-plot each reference individually.On Sept 6 I loaded ic1 ic3 ic6 and ic8 with 2.4k resistors (~1.04mA). Vertical black dashed line indicates that moment.On Sep 1 05:37:45 PM (UTC) there was a 30s power outage in my lab, but I don't see any impact on measurements.IC2 is a wreck.

author: exe
author_rank: Unknown
timestamp: October 17, 2021, 09:01:55 am
content: I'm attaching latest data (extremely averaged so the trend is clear). There is a 5-day gap because script crashed.From what I see, long-term drift is in spec ("max 11ppm/1000hr Long-Term Drift"). Tempco looks fine too.I'd say it's good-enough for 4.5digit or may be even 5.5digit portable instruments if environment conditions are stable.What should I do with it next? Shall I switch it off for one day and then on again to see if there is any hysteresis?

author: Andreas
author_rank: Super Contributor
timestamp: October 17, 2021, 06:45:07 pm
content: Quote from: exe on October 17, 2021, 09:01:55 amHello,if it is related to humidity one day might be not sufficient.(Its more like 1-2 weeks off time needed here).I had one reference (ISL21009) which had after a short power interruption (minutes) a nearly 300 uV offset which needed several days to settle again.with best regardsAndreas

author: exe
author_rank: Unknown
timestamp: November 10, 2021, 08:36:40 pm
content: I unplugged my board for two weeks, then plugged again. The DMM was off as well, but I let it warm up for two days before starting measurements.I don't see any significant drift except ic2.One thing I was curious is how bad my cables are. So, when the board was unpowered, it was showing 0.47-0.48mV voltage on output. Touching/moving cables would cause -100uV change, with recovery to within 10uV or so within some tens of second. Not sure if this observation means anything other than my cables suck. 10uV is pretty large value comparing to drift of references themselves.

author: exe
author_rank: Unknown
timestamp: November 24, 2021, 06:30:54 pm
content: Latest data. Periods with less disturbance are when I was absent. I don't know what happened to ic2. It seems it recovered and doesn't suffer excessive drift anymore.

author: dietert1
author_rank: Super Contributor
timestamp: November 24, 2021, 07:12:18 pm
content: After three months of operation one could try correlation plots of each channel agains temperature in order to derive a TC, e.g. for the last data taking period. The resulting TC values will contain a common contribution from the DVM you use. If you then plot the residuals after numerically subtracting the average TC effect for each reference, unexpected changes will be much easier to pin down. I'd guess all your references together can make a sub-ppm voltage standard.Regards, Dieter

author: exe
author_rank: Unknown
timestamp: November 24, 2021, 09:42:06 pm
content: My friends, on the latest plot I missed that ic1 went nuts. This is weird. I only see the jump correlates with drop of humidity. May be hermetic sealing is broken or something... Idk, 8 ppm sudden change is very suspicions, considering other references don't behave like that.

author: b_force
author_rank: Super Contributor
timestamp: April 13, 2022, 12:22:12 pm
content: Bit of an older topic, but I was just looking for some kind of affordable voltage reference myself.There doesn't seem to be a whole lot of information shared about these things unfortunately.I would still like to add some important points to the discussion, especially on the first couple of comments.It all depends if you're looking for thebest possiblevsworst casescenario.Unless I can really specify something, I would go for the last one.Looking at the long term stability, this will eventually lead to say something like 15-20ppm or so.Which is an equivalent of 0.002%.Some devices seem to have an asymptote here, while others just seem to drift forever?So instead of trying to bring that number down, one can also just use that number as a starting point.Everything less will be just better in performance.Obviously it depends on your application, but 0.022% total error is pretty good or very good in most cases.Although there are two questions from my side;1 - It seems a little unclear what they sometimes mean by the max accuracy?According to this article;https://e2e.ti.com/blogs_/archives/b/precisionhub/posts/ic-long-term-stability-the-only-constant-is-changeThey see it the rated accuracy as the max accuracy after x many years (see end of the article). While others sometimes seem to use this term as the initial value?So in the case of the MAX6226, this 0.02% is worst case after many years.2 - I assume that the long-term stability rating counts for when the device is powered on? Or is there also some natural drifting, even when the device is not being used at all? Certain epoxies and other bonds age over time as well for example.

author: exe
author_rank: Unknown
timestamp: April 18, 2022, 09:47:06 am
content: Hi b_force!Designing around worst-case scenario is a safe bet, but more often than not leads to unsatisfying guaranteed performance. This is why I made this experiment - to check what I can get in practice. As of ageing, its a good question. I contacted their support for that.

author: Andreas
author_rank: Super Contributor
timestamp: April 18, 2022, 01:50:03 pm
content: Quote from: exe on April 18, 2022, 09:47:06 amHello,please let us know if you get a better/more detailed answer than the typical 11 ppm/1000hrs from the data sheet.But keep in mind: this ageing drift is done under lab conditions (25 deg C constant over time).So practically you might have different conditions.An in my ageing experiment I see sometimes sudden drift steps which I cannot explain.with best regardsAndreas

author: b_force
author_rank: Super Contributor
timestamp: April 20, 2022, 01:22:03 pm
content: Quote from: exe on April 18, 2022, 09:47:06 amExtremely curious about this, please keep us updated!

author: b_force
author_rank: Super Contributor
timestamp: April 20, 2022, 01:24:30 pm
content: Quote from: Andreas on April 18, 2022, 01:50:03 pmI was also thinking more in the practical sense of aging.What happens when those IC's are in storage for a long time, do they need to be kept at constant temperature/humidity level etc etc?I can't find any information about it with actual data, only anecdotal and personal experiences.

author: b_force
author_rank: Super Contributor
timestamp: April 25, 2022, 11:47:11 am
content: Well, an update from my side, since I had to ask some other things anyway'Quote

author: Svgeesus
author_rank: Regular Contributor
timestamp: October 16, 2023, 04:10:49 pm
content: Sorry for the necro-bump but this thread seemed the best place to put this.Context:I have built a small voltage reference board using the ceramic package MAX6226_50 together with a dual low-drift, low-offset op-amp, OPA2186D. One half of this is used as a unity gain buffer (voltage follower) while the other half provides unity gain inversion. This board will interface with two other DAC boards; the AD5781ARUZ DAC requires both a positive and a negative voltage reference. This is a "fit it and forget it" application; after burn-in and measurement, the circuit is to be used without further measurement or adjustment for some years.Circuit:±9.5V power from linear regulators. MAX6226_50 has 1μF and 100nF input caps, 100nF noise reduction cap, 1μF and 100nF output caps. The 100nF are C0G and the 1μF are X7R.Finding:The reference has been powered on for four days now and is settling nicely. Measured with Keysight 34465A at 100PLC4.999727Vsd 6µV. However, it has twice made sudden jumps in output: one of about 30µV jump when it became less noisy, then hours later -30µV jump and became more noisy again. Measurements are at the output of the voltage follower.(The photo was taken when I was monitoring the inverting output, but this post is regarding the positive output)

author: Kleinstein
author_rank: Super Contributor
timestamp: October 16, 2023, 04:45:47 pm
content: In the times when noisy the reference may be oscillating. The ground part of the layout is not visible an may not be that great. E.g. adding inductance to the supply decoupling or the load side capacitance.The AZ type OP amp has some current spikes that can cause ringing at the reference. So ideally one would have a little filtering between the reference and the voltage follower.The OP amps is also noisy on the supply side and ideally has good decoupling with series impedance in the supplies.It is not in the DS, but usually a voltage regulator or similar output wants a mix of a relative low ESR capacitor and some larger lossy capacitor, e.g. with series resistance of a few ohms.The DS example curves with 10 µF load capacitor already show some ringing on the transisents. Some 1-10 ohm of series resistance to the 1 µF cap may help. For a test one could also use an electrolytic instead of the X7R.The resistors for the inverter don't look right: 100 K is too high and add quite some noise and the OPA2186 may not be that happy with such a high impedance. The right choice would be more like a 10K/10K or maybe 5K/5K resistor pair as an array to get good matching (e.g. as div23). At least the resistor should be good thin film ones, possible as 2-4 in parallel each (some types, like Susumu RR are cheap in quantety) for better averaging / matching by numbers.

author: iMo
author_rank: Super Contributor
timestamp: October 16, 2023, 04:57:17 pm
content: Also do not use the solder-less breadboard (and jump wires as well) when chasing uVolts..A slight knocking on your desk may cause your issues easily..

author: Svgeesus
author_rank: Regular Contributor
timestamp: October 16, 2023, 08:25:32 pm
content: I was worried that the op-amp was oscillating; I had not considered that the reference could also be oscillating.Not visible from the photo, but the entire back side of the board is a ground plane (minus a couple of power tracks, as this is a two-layer board).Thanks for the hints regarding series impedance. I haven't designed with inductors, what sort of ballpark values am I looking to use here?Regarding the resistors for the inverter: I had planned to use a pair of 10k resistors here, Susumu RG2012V-103-P-T1 10k 0.02% 5ppm/C 0805 which are $2.57 each if you get 25; I had some left over from another project.When I built up this board I couldn't find them, however, so I substituted Susumu RG2012N-104-W-T1 100k 0.05% 10ppm/C which I had on hand. As you say, the value is too high.I had also looked into the Vishay MPM divider resistor pairs, but Mouser and Digikey don't stock the best F and Z grades (0.025% and 0.01% ratio matching, respectively) and they have to be special ordered in lots of 1000. So the individual P grade Susumu RG2012N was better specified.Thanks again for the rapid and helpful feedback.

author: Svgeesus
author_rank: Regular Contributor
timestamp: October 16, 2023, 08:29:17 pm
content: I can try soldering the wires that go to the meter; but I suspect the oscillations are the big culprit here.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 17, 2023, 08:30:00 am
content: Oscillation of the OP-amps can be an issue with capacitive loading of the output. For a test some 100 ohm in series, close to the PCB could be a good idea.The PCB takes care with drive and sense at the output side, but the ground part is not ideal. For the given circuit it is not too bad as there is no other DC GND current than the reference chip. The GND side for the reference should be ideally taken from close to the ref. chip, e.g. from the via.

author: Svgeesus
author_rank: Regular Contributor
timestamp: October 18, 2023, 12:33:39 am
content: I made another test with a second sample of the voltage reference. The differences to the first experiment:no op-amp is fittedmeasurement wire soldered directly to the junction of the force and sense tracesno breadboardHere are the measurements of the first few minutes after power-on, and then a two-hour measurement.As there is no AZ op-amp here and the results are similar to the other board:5.00014Vsd 10µV it seems that the reference is oscillating?(Sadly my oscilloscope, Owon SDS 7102 which has been in storage a couple of years, appears to be busted. It switches on, the screen lights up, there is no trace and none of the menu buttons do anything. I plan to get a Siglent SDS1104X-E once I have the money. So not possible to verify oscillation directly.)

author: Kleinstein
author_rank: Super Contributor
timestamp: October 18, 2023, 06:31:55 am
content: The readings are unstable and there is thus something wrong. Oscillation of the reference or maybe at the supply is one possibilty, though not the only one.Some additional RC loading (possibly just a 10 µF electrolytic cap) could be a try to fight osicillation.

author: dietert1
author_rank: Super Contributor
timestamp: October 18, 2023, 07:18:18 am
content: Yes, damping at the output should help.Something else one can do without a scope is insert a resistor (maybe 10 or 100 Ohm) into the reference supply and measure current consumption. I would expect much higher consumption when the device is oscillationg, let's say more than 10 mA instead of the 380 to 550 uA spec.Regards, Dieter

author: iMo
author_rank: Super Contributor
timestamp: October 18, 2023, 01:32:41 pm
content: Another point is the thermal stability there - you have to put the pcb into a box, thermally isolated from the ambient air flow. The large part of the p-p voltage may come from the open air situation. The 30uVpp looks pretty high, indeed, I would expect something like 5-10x smaller value with no air drafts.Would be interesting to observe the chip current Icc - whether it is higher than those 380uA, you may try to load the output with different capacitancies and the Icc should be changing then when oscillating, imho.

author: Svgeesus
author_rank: Regular Contributor
timestamp: October 21, 2023, 06:44:01 pm
content: I added a 10R (Dale CMF60) and 100µF (Nichicon electrolytic) filter between the power supply and the voltage reference board (the second one, with no op-amp fitted).Initial results indicate much better stability.I had measured the exact resistor value (4 wire measurement) to calculate the current draw. But since the oscillation is removed by the resistor, happily that is no longer needed.I will leave this running for a while, then try the first board (with op-amp) with a pair of RC filters on the positive and negative rails (and soldered test wires). If that goes well I will put it in an insulated box and leave it running for a while.Thanks to those who suggested improvements and experiments; much appreciated!

author: thermistor-guy
author_rank: Frequent Contributor
timestamp: October 23, 2023, 01:10:20 am
content: Quote from: iMo on October 18, 2023, 01:32:41 pmFor maximum benefit, also consider suppressing convection currents inside the box, if it's easy to do:https://www.eevblog.com/forum/metrology/thermal-fluctuations-on-pcb/msg4705313/#msg4705313

author: Svgeesus
author_rank: Regular Contributor
timestamp: October 24, 2023, 03:37:18 am
content: Quote from: thermistor-guy on October 23, 2023, 01:10:20 amThanks for the advice. My current setup sandwiches the (small) PCB in a folded hand towel - four thicknesses below, four above. So air convection should be minimal.

author: Svgeesus
author_rank: Regular Contributor
timestamp: October 24, 2023, 03:38:08 am
content: My main problem is the AZ (chopper-stabilized) OPA2186 op-amp. On one board, without it, and with 10R 100µF RC filtering between the power supply and the board, everything is nice and stable. On my other board, with the same RC filtering and the op-amp fitted, it can be stable for some hours then jump into unstable mode for some more hours. A 22R output resistor after the op-amp and before the DMM did not improve things at all.

author: dietert1
author_rank: Super Contributor
timestamp: October 24, 2023, 06:36:16 am
content: The OPAx186 is a very low power device and from its datasheet Figs. 6.25 and 6.26 you can see that a 50 Ohm isolation resistor doesn't help with a capacitive load. You could try an OPA2189 instead.Another mod to try would be a low leakage capacitor parallel to R2, e.g. 1 or 10 nF.Regards, Dieter

author: iMo
author_rank: Super Contributor
timestamp: October 24, 2023, 06:40:23 am
content: Btw showing us your actual schematics would help..

author: Svgeesus
author_rank: Regular Contributor
timestamp: October 24, 2023, 11:52:14 am
content: Here is the schematic.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: October 26, 2023, 04:15:45 am
content: Just wondering, why do it need C4 , C5 ?

author: RandallMcRee
author_rank: Frequent Contributor
timestamp: October 26, 2023, 02:25:50 pm
content: Quote from: GigaJoe on October 26, 2023, 04:15:45 amIt is specifically mentioned in the data sheet. As with most refs!

author: Svgeesus
author_rank: Regular Contributor
timestamp: October 26, 2023, 03:54:58 pm
content: Quote from: GigaJoe on October 26, 2023, 04:15:45 amFrom the MAX6226 data sheet:QuoteHowever, I do wonder if that capacitive load on the input to the voltage follower op-amp upsets it.

author: iMo
author_rank: Super Contributor
timestamp: October 26, 2023, 07:23:30 pm
content: That has been a topic here recently. I put an 1k resistor in front of my OPA189 therefore (as recommended with the choppers)..Quote from: Svgeesus on October 26, 2023, 03:54:58 pm

author: exe
author_rank: Unknown
timestamp: October 26, 2023, 09:32:32 pm
content: Quote from: iMo on October 26, 2023, 07:23:30 pmWhy is this recommended? I think it increases noise as choppers have current noise due to switching inputs. I'd suggest an RC-filter instead of just a resistor if bandwidth is not a concern.

author: iMo
author_rank: Super Contributor
timestamp: October 27, 2023, 07:30:19 am
content: https://www.eevblog.com/forum/metrology/zero-drift-amplifier-input-bias-current/FYI - the voltage peaks at the inputs of the OPA188 in the TI's presentation (in the above link) were 35mVpp and 13ns width.

author: Svgeesus
author_rank: Regular Contributor
timestamp: October 27, 2023, 09:39:42 pm
content: While a series 10k resistor at the op-amp input makes sense in the typical 7V-to-10V reference buffer, I don't see how to do that with a Vref that has force and sense connections. A resistor in the force line (which is supposed to be broad and low resistance) seems odd and would just drop the voltage a little, although the feedback via the sense line would presumably correct that. But would you also need a resistor in the sense line as well.I'm thinking that the next iteration of this board will use a low-drift, e-trim opamp such as OPA2192. The input bias is way higher (4.8nA instead of 5pA) but comparable to the 4.5nA of the AD8676B that I saw recommended by Egan. The OPA2192 is fine driving modest capacitative load and doesn't have the charge injection issue which this MAX Vref seems sensitive to. I'm also considering using the in-the-loop isolation resistor configuration (two resistors and a capacitor) to avoid any ringing or instability there.Egan, M.The 20-Bit DAC Is the Easiest Part of a 1-ppm-Accurate Precision Voltage Source

author: ivo
author_rank: Regular Contributor
timestamp: March 09, 2024, 06:32:27 am
content: Quote from: Svgeesus on October 27, 2023, 09:39:42 pmThis is true while the REF is trying to drive a tricky/heavy load, but falls apart if that's not the case. The problem is the chopper's charge switching will enter in tiny amounts on the inputs, and you don't want the REF's own error amp reacting resonantly with that to create instability. In many cases excessive direct capacitance going into the chopper's inputs can also mess with its chopping.When the only draw for the REF is less than nA of current from an opamp, loading down the force line with resistance shouldn't be much of a problem because it's not like it has to provide current anyway, and the actual error is correct through the sense line. The poster child for the opposite case is of course an ADC which is swinging around Vref trying to charge capacitors and switching currents.The primary critical thing for DC accuracy is the sense line can accurately sense the the voltage where you want the REF's output. So I would put a larger resistance on the force and a much smaller resistance on the sense.Compromise of the OPA192 is much larger voltage noise (186: ~600nV, 192: ~1uV, 189: 100nV, all peak-peak) although there's trade-offs in all of them, depends which thing you want don't mind sacrificingI've played with a 186 as well (was most attracted to 1uV offset and <10pA bias) and yeah it doesn't like capacitance at all, I found on its inputs or outputs. Just connecting it to a DVM I found it needed more loading to stabilise (practical workaround), I'm presuming cables and only 10M made it wobble.