Ultra low-noise, short-term stable references - Page 1
Forum section: electronics / general

author: branadic
author_rank: Super Contributor
timestamp: August 28, 2024, 07:16:27 am
content: Hello everyone,at and after MM2024, there is/was quite some discussion going on due to the Z10 presented by chuckb. While we have no clue what chuck is using as his short-term reference, I thought it is worth having a dedicated thread concerning that topic and maybe some of you have some ideas how to put together something similar.Having a low-noise reference combined with a noisy long-term stable reference is something we have seen in some multimeters before (1N829 + LM399 IIRC).So here is my first take on that. I remembered MM2022 and theSPC1550I got from Rainer:https://www.eevblog.com/forum/metrology/volt-nut-meeting-2019-in-stuttgartgermany/msg4409104/#msg4409104While noise already looked good, it was time to test stability of two SPC1550 connected in series and measured with a 3458A. I started a first measurement and I quickly noticed a huge sensitivity to ambient temperature changes (AC in the lab turning on and off more or less frequently), so the measurement was stopped and the SPC1550s put into a styrofoam box with TEC-controlled, temperature-stabilized conditions inside.Attached are the data for that measurement as well as a zoomed image of the temperature stable area (roughly 4 hours) at the very end of the first diagram. While the short-term noise looks rather good and most likely limited by the 3458A, there is a very low frequency change visible.So the question arrises, where is this coming from? To me it seems like a sensitivity to ambient pressure, but with a certain time constant. Could be the SPC1550s or partially the 3458A, which would mean the cells have to go into a pressure tight box to surpress the effect on them.What is your take on that experiment?-branadic-

author: Kleinstein
author_rank: Super Contributor
timestamp: August 28, 2024, 08:27:49 am
content: I would expect a super cap to behave a little similar to a battery. So there is a temperature dependence, depending on the chemistry and possibly charge history.Another point is a possible effect of mechanical stress, e.g. from air pressure or via the leads and longer time humidity effecting plastic insulation.AFAIR there are reports about the pressure effect of 10 V reference (more like LFLU based).For the test one may have to measure the difference to a more stable 7 V reference, or use 2 such references and look at the difference.

author: dietert1
author_rank: Super Contributor
timestamp: August 28, 2024, 12:31:07 pm
content: I'd guess it is about making a constant temperature, constant humidity and constant pressure oven. I think that was the claim in the Z10 info.Making a constant humidity oven is quite difficult. Most ovens have some tiny leakage. So that results in a low pass filter behavior. For the IP66 enclosures with rubber seals i am using i have seen time constants up to 5 years. Of course there are much better and much more expensive hermetic enclosures and feedthroughs.After reading the Z10 info i started to prepare a study including pressure measurement, using a Weidmüller K41 enclosure that we had. Results can be expected after a year or so. Probably there will be a low pass behavior also for pressure.I know it sounds strange to write about years in this thread, but at "short" time scales low pass filters work very well. During my LTFLU studies i learned that for precision references what you see as noise at time scales of minutes to days are ambient variations attenuated by some low pass.Regards, Dieter

author: branadic
author_rank: Super Contributor
timestamp: August 28, 2024, 06:42:45 pm
content: The two SPC1550 I have today made their way into a sealed die cast aluminum case with sealed connector. Not fully pressure thight though, but the best I had at hand to see if this approach is worth to follow. The case went into the temperature stabilized styrofoam box. Tomorrow I have more data to show.-branadic-

author: chekhov
author_rank: Regular Contributor
timestamp: August 29, 2024, 09:29:43 am
content: IMHO, whatever makes Z10 reference wonderful is not the ultra low noise but the way how this low noise ref is adjusted or kept long term stable.Let's imagine we have low noise source already, short time stable. And we have our array of LTZ/whatever long-term reference.How to compare them, quickly or during sufficient time period, keeping in mind all 1/f noise produced by it, different fluctuations, popcorn, ...How we adjust our low noise ref ... so that this does not affect the process of watching it as well

author: iMo
author_rank: Super Contributor
timestamp: August 29, 2024, 09:51:45 am
content: The Z10 box weights 50pounds (based on the preliminary DS), so there is a lot of metal inside - like a larger chamber or system of chambers, perhaps evacuated and ovenized (or cooled) of course..The box could be 5kg, transformer and batteries 8kg, capacitors + electronics 5kg, so 5kg is the chamber(s).."14Ah 24V for 10hours of normal operation" may mean aprox 1.4Amp current, say 1.1A/24V (25W) for heating or cooling..PS: my bet he is using large banks of low leakage capacitors (like 100 pcs. each) for signal filtering (or processing when using some servo loops), perhaps ovenized and shielded as well.PPS: "4days transport power" - it could be he is switching off the ovenizing of the capacitor banks during the transport, leaving only the reference chamber ovenized (like 2-3W).

author: branadic
author_rank: Super Contributor
timestamp: September 02, 2024, 08:06:40 am
content: As promissed, here are results with the SPC1550s sitting inside a sealed Rolec case with sealed D-sub connector, while the Rolec case itself was sitting inside a TEC stabilized styrofoam box.Obviously, things haven't really improved, but hey, that's why I made this experiement in the first place.-branadic-

author: Kleinstein
author_rank: Super Contributor
timestamp: September 02, 2024, 08:35:22 am
content: A low pass filter with some suitebale capacitors (e.g. supercapacitor) could indeed be a solution. The capacitor seems to have quite some temperature effect though. To get an idea on how stable the temperature needs to be it may make sense to measure the sensitivity with some intentional temperature excursions. It can still be tricky with capacitor internal DA relatived effects that react to the rate of temperature change.Another point that can make sense is a really sensitive temperature sensor (e.g. 10 K NTC) inside the stabilized box to see how good the stabilization works.The way such a filter would likely work is to no expect DC precision from the fitler, but more use an extra comparison of the filter output (can include the buffer) and the long term stable references. The filter part is more like to slow down and filter the correction signal. The feedback part could in theory be digital, e.g. to incluce corrections (e.g. for long term drift or pressure).

author: branadic
author_rank: Super Contributor
timestamp: September 02, 2024, 09:39:41 am
content: Just for completeness, the TEC controller already uses a 10k thermistor to stabilize the temperature inside the styrofoam box to 23 °C.-branadic-

author: dietert1
author_rank: Super Contributor
timestamp: September 02, 2024, 01:11:31 pm
content: I think the logs in SPC1550-5.png exhibit a clear correlation of battery voltage with the sensor logs outside of the chamber and besides that very low noise. So the batteries with the 3458A are a good enough sensor to detect variations inside the chamber. In other words that chamber isn't good enough.If the 3458A was running outside of the chamber, one could try and put it inside.Regards, Dieter

author: Kleinstein
author_rank: Super Contributor
timestamp: September 02, 2024, 02:49:44 pm
content: For really judgeing the temperature stabilty a separate sensor, not the one used for the regulation should be used. Ideally there would be not much temperature gradient, but one never knows before measuring it and the sense circuit to the NTC may add noise / drift that the regulator could cancel.One way to get temperature fluctuations, especially with relatively large air volume is from pressure changes that convert to heat sources in air.

author: branadic
author_rank: Super Contributor
timestamp: September 02, 2024, 04:52:57 pm
content: The styrofoam isn't large and the TEC controller with 60 W not strong enough to handle the heat produced by a 3458A.Our large climate chamber is currently in use, so I have to either stop that pathway at the moment or test the reference at home, with way lower temperature excursions over a day, but larger overall temperatures at the moment.-branadic-

author: branadic
author_rank: Super Contributor
timestamp: September 03, 2024, 08:41:57 pm
content: I today hooked up the SPC1550s to my meter at home (higher temps in the room, but not corrupted by any AC). After an initial run down from equalizing to the new environment and some settling region I can now see the cells slowly discharge, but other than that it is low noise.Attached are the data (about 5 hours of data) in case you want to analyze them yourself.-branadic-

author: tszaboo
author_rank: Super Contributor
timestamp: September 03, 2024, 08:51:06 pm
content: Before spending a lot of time investigating and compensating all the physical effects, I would suggest to replace the supercap with one made by Tadiran.

author: miro123
author_rank: Frequent Contributor
timestamp: September 03, 2024, 09:12:50 pm
content: As already mentioned - used high quality supercaps.Select the one from low current rangeWhy not place two caps good thermally couples and with tick mechanical cushoning. Log temperature atmospherics pressure and vibration/accleration sensors./Measure the voltage difference. Os such way you avoid many factors. Measure the temerature at least at to corners of capacitors. Temperature probing in non moving air is an issue. Alternatively you can use oil bath. I bought 1l. silicon oil for this purpose.1. The two capacitors has similar conditions they must drift on the same way and difference will less vary from outside conditions2. Difference should be in mV range. It will lessen the DMM/Nanovoltmeter or acq system requirements.

author: dietert1
author_rank: Super Contributor
timestamp: September 04, 2024, 05:24:37 am
content: branadics last log demonstrates a useful time constant of about an hour, e.g. between 19:30 and 20:30. Probably the diagram in the Z10 info isn't 80 minutes by accident.The test also demonstrates that a battery can't be a reference but a filter for a reference.With the usual 0.1 to 10 Hz spec of reference parts, we are looking at time constants of about 3 seconds. If a battery filter can extend that about 1000 times, then the filtered reference noise should be down about a factor 30. Let's say from 200 nV rms to 7 nV rms - approaching the Z10 1E-9 claim. I think branadic is on the right track.Regards, Dieter

author: branadic
author_rank: Super Contributor
timestamp: September 04, 2024, 07:08:17 am
content: Rainer presented "supercapacitors as voltage source" at MM2022. In there he presented that EVE SPC1550 is similar to Tadiran HLC-1550A. So without further information of insights or experience a statement like "Before spending a lot of time investigating and compensating all the physical effects, I would suggest to replace the supercap with one made by Tadiran." is just like saying, a rocket to go to Mars has to be red.-branadic-

author: Kleinstein
author_rank: Super Contributor
timestamp: September 04, 2024, 07:34:16 am
content: The different bands likely use a very similar chemistry (if not even comming from the same source) and will thus have similar charactristics.It can still make sense to use a major brand, as the chances may be a bit higher to sill get that type in a few years. So not so much because of the performance, but more a supply issue.There is a chance that the properties depend on the actually used voltage and maybe the history. The 3.5 V performance may be different from 3.7 V. Also the history may make a different. I would be a bit careful with charging to a much higher voltage, especially close to the maximum, as this can alter the cell. There is a chance to get Li trapped deep inside the electrode and overcharging is suspected to cause cracks.

author: iMo
author_rank: Super Contributor
timestamp: September 04, 2024, 03:00:32 pm
content: Quote from: dietert1 on September 04, 2024, 05:24:37 amThe power consumption of aprox 2-3W in the "transport mode" against aprox 25W in the "operational mode" supports that as well (see my post above). The reference itself is a small box of size of an larger OCXO perhaps (2-3W always ovenized).. The banks/chambers with the capacitors are much larger, ovenizing of which is switched off during the transport..

author: miro123
author_rank: Frequent Contributor
timestamp: September 04, 2024, 04:05:21 pm
content: Quote from: iMo on September 04, 2024, 03:00:32 pmSmall remarks. I see mixture of different thinks together. Battery Capacitor chargeble or not. Fanso Spc1550 isnon-rechargebleLiSOCl₂ (Lithium Thionyl Chloride) battery, There dozen of different super caps and chargeble anon chargeble batteries. All has different properties and application area. moer info over SPC 1550 form manufacture website -https://www.fansobattery.com/?list_46/516.html@ branadic - Your setup shows limited capability. Electrochemical noise in batteries must be visible. It is not ultra-low noise either. I term of low electrochemical noise the Weston cell remains by far the best is this class.

author: tszaboo
author_rank: Super Contributor
timestamp: September 04, 2024, 04:16:20 pm
content: Quote from: branadic on September 04, 2024, 07:08:17 amOr maybe it's because Tadiran has decades of experience making batteries, and they don't cheap out on the assembly processes. They might look the same, the chemistry might be the same. And one uses laser welding to close, the other sticking the empty hole with some random resin, that changes every 6 months of production.There are a lot tiny differences in batteries that you cannot tell from the outside.

author: iMo
author_rank: Super Contributor
timestamp: September 05, 2024, 06:55:38 am
content: Quote from: miro123 on September 04, 2024, 04:05:21 pmAfaik all small size "super.." capacitors with XXX Farad "capacity" are built as batteries with chemicals inside. I bought one of the first Seiko Kinetics in '96 in Schiphol airport because it contained a supercapacitor thus no danger of leaking and no replacement of it needed. A big BS..I would never have considered to use those supercaps in a voltage reference which shall work fine for say105 years..

author: Kleinstein
author_rank: Super Contributor
timestamp: September 05, 2024, 09:25:55 am
content: The limited lifetime of the chemical cell type capacitors could be an issue. If made right the cell can still work for quite some time. Using them at a stable and not so high temperature could prolong the life. So peltier cooling instead of just a simple oven could be a good idea.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: September 05, 2024, 12:41:12 pm
content: On the milli volt level, battery voltage is very sensitive to temperature changes. And the part you have chosen is not a capacitor, but a "Li-Ion battery with integrated capacitor". And I suspect that even standard electrolytic capacitors are temperature sensitive Materials expand when heated, and this changes distances, and thus capacitance. Same electric charge but with a different capacitance results in a different voltage.Just like ESD. When you rub some materials together, some electrons jump to the other side, and when you increase the distance, the voltage goes up into the kV range.Marco Reps has shown the temperature dependence of a regular 9V battery by just holding his hand near one. It's quite impressive, and enlightening too.

author: tszaboo
author_rank: Super Contributor
timestamp: September 05, 2024, 01:41:48 pm
content: Quote from: Doctorandus_P on September 05, 2024, 12:41:12 pmIf you have a 8.5 digit meter, you can change the reading by squeezing the battery, or holding it upside down.I don't have that now, but a 34465A and a LiSOCL4 that I had handy, jumped 100uV just by me holding it in my hand for few seconds.

author: iMo
author_rank: Super Contributor
timestamp: September 05, 2024, 04:07:59 pm
content: Those supercaps may work in RC filters for short-term measurements - when thermostatized and kept mechanically stable (like an at least partially evacuated chamber). With time their capacity (in Farad) will drop down, but that may not have large impact on the noise characteristic provided filter's sufficient low time constant will be planned ahead (to account for the expected capacity drop)..

author: Kleinstein
author_rank: Super Contributor
timestamp: September 05, 2024, 04:25:55 pm
content: The temperature dependence is definitely a thing with battery cells. How much depends on the chemistry. With some (e.g. non saturated mercury ref. cells) one can be quite good, but other may need a really good oven. Measuring the effect may be a first thing to see if there is a chance to reach a stable enough temperature.Electrolytic capacitors also show a temperaure effect - touch a capacitor and the voltage tends to change. It can be more tricky than just constant charge and variable capacitance, as there is a high level of DA (5-10% range) and the history and not just the current voltage can have an effect. Similar for the battery cells it can make a difference whether it is fresh charges or coming from a high voltage before.Because of the relatively large leakage one would likely not use a simple classic RC filter, but more like an active fitler of higher order. Here the higher order part could be implemented digital to a large part. This way a stable leakage or required charging current would not cause an offset. The question is only on where to put the cross over from the long term stable (e.g. LTZ1000) reference to the shorter time filter. This may have to be a rather long time constant, already in the region where the Allan variance curve of the final circuit will go up from slow drift parts. It would still need a rather good long term stable reference and thus very good design (e.g. stabile temperature also for the resistors at the LTZ ref.) there. With the filter I don't see an improvement much faster than ~ T maybe even only ~ sqrt(T) towards shorter times.

author: moerm
author_rank: Regular Contributor
timestamp: September 05, 2024, 08:23:50 pm
content: First: kudos to branadic, Kleinstein and some other gurus here (I mean that sincerely) for their work, knowledge and experience!I'm a modest and pragmatic guy and am not even considering going beyond 1 ppm (with meters) and hence I'm not really interested in anything beyondhonest and true6.5 digits, which just so happen to be 1 ppm - but still I'm reading with attention and interest almost any and everything certain users (like the above mentioned) do and write about ... and learn from.Here is my personal take on this topic:I studied the ADR. Carefully. But I stick with the LTZ. Yes, in a few parameters the ADR is more attractive, noise being one of them. But for me, and presumably most hobbyists and people outside of cal labs, the "problems" the ADR solves are, excuse me, in between luxury and irrelevant. Why?What I (I mean 'we' but I do not want to arrogate to speak for others) really need is long-term stability with acceptable to good precision and accuracy. Simple as that. And in that the LTZ still is unsurpassed. In addition it is well known and understood and "battle-proven" over decades.Being in that department let me address another issue: I quite like the diverse long-term studies some of you made. But I don't care. At all. For two main reasons, a) Fluke, Datron, etc. make/made a living building the very best possible/feasible, and I guess trying to do better than them is quite futile (for mere mortals without a very serious budget), and b) how tomeasure(as opposed to guesstimate) and verify it? With an '8.5 digits' multimeter? Haha, good joke.Btw, just look at that "new" (well, not really old) brit company "time" (or similar) who seems to be somewhat of a wannabe Datron "new edition" - and who seemed to fail and still be failing to achieve what Datron achieved decades ago.Plus, again, IMO our problem isn't a few uV more or less noise. Our problem is 1 ppm per year stability.Where I see the ADR is in places, for which the LTZ was overkill but LM399 isn't quite up to par that is, 7.5 and better 6.5 digits meters and not-quite-top reference boxes.Again, thanks for your work. At the very least it helps us to better reflect those matters.

author: Echo88
author_rank: Frequent Contributor
timestamp: September 05, 2024, 09:52:55 pm
content: Short term stability is irrelevant guys, lets end this thread, the 9.5 digit DMM thread and our hobby.I wouldnt have guessed that youre modest, but after claiming it in the second sentence it must be true.Anyway: Attached is a proposition for the supercap/ref/buffer.The chamber parts are on the way.At the moment i would stuff two ref/supercap/buffer circuits in the chamber and then intercompare the achieved noise via a 34420A with and without buffer.The ADR1399 is just used as a cheaper alternative to the LTZ that would be used later, when this experiment actually works.Any ideas what to improve/any found errors?I assume the AZ-OP averaging will pose no problems with the supercaps, but i could be wrong here and i assume the AZ-OPs have a central clock thats used.The hybrid supercaps were chosen for max capacity in the given chamber size at acceptable price point.Mouser and Digikey stock no rechargeable Tadiran batteries, so they werent yet considered.Since the chamber is very sturdy i didnt yet include a barometer, desiccant will keep the humidity constant and the chamber temp will be regulated by a peltier element, with the chamber in a dewar vessel.

author: moerm
author_rank: Regular Contributor
timestamp: September 05, 2024, 11:00:37 pm
content: Quote from: Echo88 on September 05, 2024, 09:52:55 pmThat's not what I said. Of course short-time stability, or more precisely, short-term accuracy and precisionisimportant, too, but neither the ADR nor the LTZ nor the LMx99 mainly are designed for and serve for that. They clearly are meant for long-term stability and actually come with ridiculous initial accuracy.As for ending "this thread, the 9.5 digit DMM thread and our hobby", how about the alternative of accepting views one doesn't agree with or like?Quote from: Echo88 on September 05, 2024, 09:52:55 pmThat statement was madein a context("only" 1 ppm) and looking at discussions striving for single digit ppbdesiring no more than 1 ppm indeed is modest.Also, more of a side note, kindly note that neither did I attack or ridicule you ("ad hominem"), nor did I suggest that everyone should follow my view. In fact, I expressly thanked some users who seem to have a quite different point of view.

author: dietert1
author_rank: Super Contributor
timestamp: September 05, 2024, 11:17:25 pm
content: A good bootstrap resistor for a ADR1399 from 10 V is 910R for about 3 mA current.We have a set of 500F 2.7V supercaps mounted on a voltage distribution board. I measured the discharge current of that circuit to be about 1 uA at 27 °C ambient temperature, so that can't be used. I think between the voltage divider R2..R4 in your schematic and the capacitor nodes could be pairs of antiparallel diodes. Of course the divider should be fed from +10V_Ref (in front of R14).Also the circuit needs some active control of total capacitor voltage to eliminate the voltage drop along R14 caused by capacitor leakage. I think there was a circuit proposal how to do it in this forum, but don't remember any details.Regards, Dieter

author: Echo88
author_rank: Frequent Contributor
timestamp: September 06, 2024, 12:08:33 am
content: Indeed the modest comment wasnt seen in context, for that i apologize.Lets agree to disagree, because our views differ here in many ways.

author: Echo88
author_rank: Frequent Contributor
timestamp: September 06, 2024, 12:19:24 am
content: Thanks for the suggestions.R11/12/13 havent yet been fully determined; i still need to see if i have fitting resistors for the task in the drawer.The antiparallel diodes between the cap voltage equalizing resistors and the caps is a nice idea.Indeed the voltage drop over R14 could cause issues, so far i hope that the leakage remains stable, so as not to influence the voltage stability.

author: moerm
author_rank: Regular Contributor
timestamp: September 06, 2024, 02:29:54 am
content: Quote from: Echo88 on September 06, 2024, 12:08:33 amNo problem, Echo88. That's what forum are for; to see, and potentially learn from, different views and angles.That you apologize honours you but I didn't even expect it (but, of course accept it with a friendly smile).

author: dietert1
author_rank: Super Contributor
timestamp: September 06, 2024, 04:24:48 am
content: With 3x 220F the resulting capacitance is 73 F and for a time constant of one hour R14 isn't 10K but 50 Ohm. A different beast!

author: iMo
author_rank: Super Contributor
timestamp: September 06, 2024, 05:23:56 am
content: ..and the 1399 wants the RC snubber..

author: Andreas
author_rank: Super Contributor
timestamp: September 06, 2024, 05:26:16 am
content: Hello,the fuse F1 has a considerable resistance.I would want to have it outside the filter cirquit. (at least not as additional ESR to the capacitor)with best regardsAndreas

author: iMo
author_rank: Super Contributor
timestamp: September 06, 2024, 05:43:48 am
content: Quote from: dietert1 on September 06, 2024, 04:24:48 amWhat should be the time constant actually?ChuckB in his opamp thread indicated some opamps have good ADEV (below 1nV, ADA4528 and OPA189, the 4523 might be even better) till 1 day, afaik..https://www.eevblog.com/forum/metrology/low-frequency-noise-of-zero-drift-amplifiers/msg2294556/#msg2294556

author: Echo88
author_rank: Frequent Contributor
timestamp: September 06, 2024, 04:35:56 pm
content: Added the suggestions.Thanks for the reminder of the OP-tests iMo, i forgot to look into it before going with the OPA4189.Lucky that it turns out to be very good regarding stability and allows for 36V max supply, unlike the 5,5Vmax ADA4528.Pdf and Kicad 8 Project are attached.Maybe its beneficial to add an analog switch/OptoFET to change the R14 lowpass resistor inside the chamber, to check for different Allan Deviation?Edit:Added layouts: main-pcb, stacked on supercap-pcb and thermal sensor alucore-pcb which is attached to the chamber.

author: dietert1
author_rank: Super Contributor
timestamp: September 07, 2024, 10:34:58 am
content: These Lithium ultracaps have roughly 100x lower or even better leakage than the standard 2.7 V ultracap. Looking at the AHC-S04R0S datasheet the leakage spec doesn't scale with capacitance but with square root of capacitance. If that's true one should use the 850 F instead of the 220 F part.By the way i read some serious warnings about handling these capacitors as their fire/explosion risk is similar to Li-Ion batteries.At 10 V the 3x 220 F cap takes 730 Cb of charge, that is 730 seconds of 1 A constant current charging. With a resistor it will take forever to approach 1 ppb.Regards, Dieter

author: Echo88
author_rank: Frequent Contributor
timestamp: September 07, 2024, 09:44:31 pm
content: Yeah, i was sceptical about lithium hybrid supercaps at first and had fears about degradation, the battery like chemistry and the limited discharge voltage (opposed to no minimum dicharge voltage will nonhybrid supercaps).But the about 10-fold capacity increase and the very low leakage currents are great, so i will try them.The chosen 220F caps are used because of chamber size limits, otherwise i would have gone for the available 850F versions.I also have a bit of fear about accidentally shorting those babies, thats why i tried to limit/fuse all outgoing cap connections, while allowing for lowest noise measurements.Youre completely right about the charging taking forever even with the low resistance fast charging path. Would spoil the fun when it takes some years to stabilize.Added the new schematic: current limited source instead of simple resistor and protection measures against the charged supercaps when no +18V supply is applied.Dielectric absorption is another worry:https://www.doeeet.com/content/wp-content/uploads/2022/12/Table-4.-dielectric-absorption-of-most-common-dielectrics.pngAm i correct in assuming that R14 / Q3 Beta = Effective R14 for the lowpass consideration?-> 1k / ~100 = 10R low pass resistor equivalent?

author: Kleinstein
author_rank: Super Contributor
timestamp: September 08, 2024, 05:38:16 am
content: For the output driver there is more than R14 / beta. One thing is that R5 adds to the resistance. The other part is the effective emitter resistance for the transistor. Here 26 mV divided by the current for Q3 is the usual approximation. This could become an issue, as the current finally will go very low and thus high effective resistance.The chain for balancing the capacitors would act as a rather missive leakage path. From the logic R2,R3,R4 should be more connected to the output buffer. However this would cause problems with the off case and would need extra care (e.g. extra switches or similar).For a proof of concept there is no need to use 4 OP-amps in parallel for the output buffer - the extra current and power consumption there can also cause problems.For U3 one would more look at an OP07 or OPA189 instead of the OPA140.The ADR1399 inside the same thermal chamber would be quite some heat source. Normally one would want to avoid heat around the capacitors, as leakage goes up and it would take a rather long (e.g. maybe a day) time for stabilization.

author: Echo88
author_rank: Frequent Contributor
timestamp: September 08, 2024, 05:42:00 pm
content: I hope i understood your statements about emitter resistance and the balancing resistors R2/3/4 correctly.The emitter output resistance can be kept low by allowing a constant 1mA (roughly 26mV/1mA = 26R?) to flow through R2/3/4 which i decreased in value and changed their groundpath, to omit this gnd-current from the supercap star gnd.Is that okay or does this 1mA need to flow into a separate negative supply instead of gnd, like in this example, see figure 22?:https://www.analog.com/en/resources/technical-articles/simulation-shows-how-real-op-amps-can-drive-capacitive-loads.htmlSeems to be lowest part count solution to this problem.Another solution might be a different limited current source or the source (R14,Q2,Q3,R5) could be bypassed by a resistor via switch after the supercaps are sufficiently charged.I changed the OPA4189 to a OPA189.U3 OPA140 being a SOT23-5 footprint can be directly swapped with a OPA189 or OPA207 for example.I plan to have little thermal baffles around the ADR1399 and since the whole chamber is thermally stabilized im not really worried about temp fluctuations inside the chamber once the supercaps are charged.The changed schematic is attached.Thanks for your contributions

author: Kleinstein
author_rank: Super Contributor
timestamp: September 08, 2024, 07:54:39 pm
content: Driving the super cap with a low impedance is a strange way, but some 50 ohm resistance is needed to get a time constant of around 1 hour.Still the 10 K resistance from the balancing part looks odd and give quite some "leakage" compared to the supercap.With the capacitive drive circuit the capacitor C7 may also need to get quite large. So C7 gets nearly as critical as the main capacitor, with problems from DA and so on.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 08, 2024, 08:56:07 pm
content: A possible solution could be replacing the driving OP-amp and filter part (U5, C7,R17) with a digital solution. So amplifier, ADC, µC, DAC and likely a current source. The digital solution can be really good in integrating and handling the slow part. For a digital filter the rather long time constant is not a problem: The demand on the ADC and DAC should not be that large, as this is mainly for a tiny difference (possibly even just some 10 µV) and thus not much resolution needed to reach the limit of the reference noise. One could possibly even get away with a µC internal ADC, though likely want a better DAC.Chances are one may want some range switching for coarse and fine approach.

author: iMo
author_rank: Super Contributor
timestamp: September 09, 2024, 09:42:55 am
content: The R14 (50ohm) looks to me too small. When you short the output of the Star+ to ground the output of the opamp goes full high and the opamp's output current (XXXmA) will flow through the 50ohm, and BE of the output transistor (or CE of the current limiter) and/or R5 to ground. I would put there 1k..

author: dietert1
author_rank: Super Contributor
timestamp: September 13, 2024, 11:25:19 am
content: Eight days ago i connected our 4x 500 F ultracapacitor set to an adjustable 10 V supply and its leakage (< 1,1 mA spec) has been decreasing continuously. I found that the voltage balancing circuit is "digital". It has a 2.7 V reset controller per cell that switches a mosfet with 5 Ohm load resistor to generate 0.54 A discharge current until cell voltage dropped.Meanwhile the set arrived at 110 uA leakage at about 20 °C room temperature. The leakage spec of a LiC ultracap of similar size is about 10x lower.I am using a 120R resistor for the filter and four 1K resistors for voltage balancing. So the time constant is about 4 hours as 125F x 120R = 15 000 secs. Meanwhile the cell voltages are only 2 or 3 mV off from ideal. I am observing myself adjusting the input voltage down in steps of 1 mV about once per day in order to keep filter output at 10.00x V. The time constant of dielectric absorption appears to be several days, somewhat longer than the filter time constant.I will probably try Kleinsteins proposal of a digital controller to automate the adjustments.Regards, Dieter

author: Kleinstein
author_rank: Super Contributor
timestamp: September 13, 2024, 01:07:07 pm
content: The dielectric absorbtion is often not just a simple exponential decay with a fixed time constant, but a mix of many time scales from short to very slow.There may still be a dominant time constant - I have not checked a supercap or seen data.The fast time scale part will settle and is no problem.The very long time scale part will look a lot like leakage and would be compensated by the long term reference. The current from the DA part natrally gets smaller the longer the time scale.The tricky part is from about the RC time constant to maybe 10 x longer - so unpleasant long for waiting and still not a fixed current.To get this part settled it may take quite some time. Also the long term ref. part can have a difficulty compensating the faster part if it.During use as a reference the balancing part should not be active. For the rare cases (e.g. after a few years maybe) chances are one would want a smaller current and may still have to wait for DA to settle again after this.

author: dietert1
author_rank: Super Contributor
timestamp: September 13, 2024, 03:00:38 pm
content: Yes, certainly one needs mosfet switches (e.g. BSS123) between the voltage balancer and the cells, in order to avoid losing cell charge in case of power down. For the filter resistor the connection requires two switches for separate force and sense.The ADC for sensing capacitor leakage along the filter resistor could be one of the isolated ADS1263 modules i made in 2021. As far as i remember as a nullmeter it measures to 11 nVrms in 5 seconds. The DAC can be PWM type, using the same isolated supply as the ADC. It only needs to supply a small current, like 100 uA. Maybe it can be inside the oven for better stability. It's easy to make the current source bipolar if necessary.Maybe one can use the capacitor without output buffer. I mean a voltage loss of 1 ppb = 10 nV in a 120R filter resistor would allow 80 pA meter input current. Of course there needs to be some fuse in the sense line. One could then use the output terminals for charging the capacitor in case it needs higher currents.Regards, Dieter

author: Kleinstein
author_rank: Super Contributor
timestamp: September 13, 2024, 05:02:55 pm
content: For the digital long term correction part, there is no need for a high performance ADC. The main point is to be well better (e.g. 5 x) than the reference (e.g. LTZ1000 based) over a longer time and the small difference (e.g. 1 mV, maybe 50 mV range to also do fine trim). If at all it is about the amplifier, not so much the ADC. The ADC for the correction should use the output of the buffer used for the main output. This way the correction would include long term drift of that buffer. A kind of nullmeter would be a good starting point.A PWM DAC for the digital correction part output could be an option. One does not need good linearity or much extra filtering. The main point is getting a monotonic response with not even a really large resolution or long term stability needed. An auxiliary buffer at the DAC output would also not really hurt. An off the shelf DAC chip could be good enough too if the range is small. It is only for the leakage current and small longer term fluctuations to compensate, possibly not even a static difference for trim. If needed the initial settling phase could go for a smaller resistor (and different digital filter part to keep it stable) to allow for a faster initial settling. The DAC would be for X in 10 V + X, not for the full reference voltage at the input side. With gain in the amplifier+ADC+digital filter+DAC part the actual resistor used in the feedback will be much higher than 120 ohms used now. The voltage gain would reduce the time constant and still allow less noise in the current flowing to the capacitor. So no worries about noise when seeing a 100 K (or maybe even larger) range resistor for the current.I am afraid the settling time is not just the RC time constant, but more waiting for DA and similar relaxation effects inside the super-cap. So I would expect quite some settling time on the order of days, at least a few time constants of the analog (super cap) to correction part (LTZ1000 ref.) cross over.

author: bsdphk
author_rank: Regular Contributor
timestamp: September 13, 2024, 09:15:57 pm
content: Apologies for not spotting this sooner and sharing my own related experience.At one point I tried to implement digital control of the Vadj on an OCXO with an integrator instead of a DAC, to avoid the oven around the DAC+Vref.That is basically the same thing you are trying to do, only I had an input which could be be brought high or low for some amount of time to adjust the voltage as necessary.My direct inspiration was the integrator circuit in the HP5061 and HP5065 atom frequency standards, which use a 5µF Polyethylene capacitor.My target time constant was much longer than HP's, hours and days rather than milliseconds, so I spent (too much) time trying to find a better capacitor.My findings were roughly:I didn't try vacuum capacitors - too bulky for their small capacitance, but I suppose they would be close to perfect (if kept in darkness!)Silver-mica was very stable, but since they come only in very low capacitance, even fA leakage dooms the idea.Tiny ceramics have the same problem, but worse stability.High capacitance ceramics are sensitive to everything, up to and including what you ate the day before.Anything with liquids in it is hideously sensitive to temperatureandair-pressure.Stabilizing the temperature only got me to a nonlinear barometric sensor.(I only tried ultracaps up to about 5F in the usual cylindrical housing.)That more or less left film capacitors, and they were all over the place, even for what is supposed to be the same kind of film from the same manufacturer.Dont eventhinkabout paper.Teflon is supposed to be the superior plastic, but impossible to get hold of in relevant capacitance. (Probably because of Wassenaar 3.A.1.e.2)Back then eBay sellers from Russia sold what they claimed was teflon capacitors, but at prices and with appraisals which literally screamed audiohomoeopathy, so I figured they didn't have any teflon in them in to begin with.Taking a clue from HP's use of PE, I spent a fair bit of time on plastic film capacitors.Around this time AoE III came out and said basically the same thing: kV plastic films.The good news is that those are cheap, so you can afford to experiment.With µF sized capacitors rated for 1kV voltage, and everything washed in iso-propanol after soldering, leakage current is a non-issue, my HP3458 had a hard time measuring the self-discharge on my timescale of hours and days.They were heavy enough that I saw no signs of microphonics until I literally struck them with a pencil.I could not attribute anything to humidity, but could also not yet rule it out.Some of them were light sensitive, but a good box fixes that.Air pressure mattered, not much, but relevant. I saw both signs, so I figured I could probably compensate that out with a set of handpicked capacitors.But temperature mattered a lot, and with the selection of dielectrics I tried, the sign was always the same, so ovenizing would be required.Since that was exactly what I had tried to avoid in the first place, I sort of gave up around that point.I should note that I saw "pop-corn" changes in the voltage, and since their rate seemed to be roughly proportional to the physical volume of the capacitors I made a note to find out if cosmic rays or background radiation were relevant. Never got to it.As I understand it, you are targeting a timescale of "a day in the lab"-ish.You can probably ignore the air-pressure, provided you check the barometer before and after to see that it didn't change too much.I'm less convinced you can avoid an oven, or at least thermal insulation, so I'll conclude with a cheap trick for high stability work in both frequency and voltage:Find an old fridge, ignore the compressor.Convenient door. Good thermal insulation. Faraday shielded. Easy to drill holes for wires.Preinstalled tubing to run water through, if you want to do active temperature controlAdd a couple of bricks on the top and bottom shelves for increased thermal impedance.Have fun :-)/phk My thoughts go out to Kleinstein and PTB: Looks like some really bad weather coming your way. Please get a qualified cooling technician to evacuate the gas so you don't contribute to atmospheric pollution.

author: dietert1
author_rank: Super Contributor
timestamp: September 13, 2024, 10:41:27 pm
content: We also made an oven with two 380 uF film caps (seehttps://www.eevblog.com/forum/metrology/making-an-array-of-ltz1000/msg4772546/#msg4772546). The advantage of the film cap filter: It might be used in a calibrator, i mean a source with variable output voltage.The advantage of the ultra-cap solution: It can be used without a buffer amplifier. I mean any reasonable voltmeter comes with a good input buffer anyway, some with leakage currents in the low pA.After branadics test i thought i should look into this, as i made TEC ovens before that are short-term stable to some microK. The TC of my ultracap set is roughly 1 mV / 10 V /K = 100 ppm/K. So a 10 uK oven should be sufficient for 1 ppb voltage stability. For this application we neither need to calibrate temperature to that level nor guarantee long-term stability to that level.Regards, Dieter

author: Echo88
author_rank: Frequent Contributor
timestamp: September 14, 2024, 12:23:36 am
content: Took me a few days to rethink the idea. I stepped away from the idea of OP -In feedback connecting directly to the supercap, due to the ridiculous size the compensation cap (in the single Farad range) would need to have according to the rule of thumb from the mentioned article "To start, the value of C1 is chosen such that R3C2 = C1R2"https://www.analog.com/en/resources/technical-articles/simulation-shows-how-real-op-amps-can-drive-capacitive-loads.htmlI havent yet done a phase margin analysis on the circuit to see how it could be properly compensated, quite frankly because im not familiar with it.Now im trying to use a simple Riso between the OP and the supercaps and doing what Dieter suggested: compensating the leakage current. When going the analog way one can for example inject a suitable current to compensate the Riso voltage drop or go with the old trick of using two caps in series with a suitable middle connection bias voltage.Regarding the current injection method: Still unsure about the necessary feedback look for near perfect current compensation, LTSpice indicates a high voltage gain leads to better compensation?Regarding the series cap method: Hybrid caps cant be discharged to 0V, so the top caps have to be standard supercaps with higher leakage/lower capacitance with regards to physical size, meanwhile the bottom caps can be of the hybrid type.Both variants are attached.Please ignore the supercap capacitance values in the schematics/simulations, they are relatively small. Now i got a new chamber and can use way bigger supercaps and am no longer so limited in the physical size regard.@ iMo: The +Star is never allowed to connect to ground as that would short the supercaps and lead to mayhem.The digital solution might be really the way to go to properly design a complete solution in the end. But for now ill stay with the analog design as a prototype to try to get familiar with the supercapacitor behavior itself.The DA of those caps is enormous as expected and since all supercap datasheets spec the leakage spec after 72h at nominal voltage the true leakage will be buried under the DA.Some relevant papers dealing with supercaps and one with low noise diodes:https://www.mdpi.com/2079-9292/9/8/1245#B28-electronics-09-01245https://iopscience.iop.org/article/10.1088/1742-6596/1015/5/052011/pdfhttps://www.researchgate.net/publication/3950664_Very_low_noise_high_accuracy_programmable_voltage_referencehttps://www.wellesu.com/10.1063/1.4870248https://www.wellesu.com/10.1142/s021947750700388xA diode based low noise voltage source variant, no supercaps.

author: MiDi
author_rank: Frequent Contributor
timestamp: September 14, 2024, 07:35:15 am
content: Noise tests for NCR18650A & BR2032 Panasonic gave very low noise, NCR18650A is even below NF of ULF-ULNA (see attachments).3xNCR18650A in series (11.8V) have a TC of ~280µV/K (~24ppm/K) at 22°C and self-discharge of ~2.6nV/s (~82mV/a).The TC is in the range of unheated LTZ/ADR1000.

author: guenthert
author_rank: Frequent Contributor
timestamp: September 14, 2024, 07:59:42 am
content: Quote from: Echo88 on September 14, 2024, 12:23:36 amIs this actually needed here? The voltage drop over Riso would be before the star point and hence irrelevant, no?

author: Kleinstein
author_rank: Super Contributor
timestamp: September 14, 2024, 08:05:22 am
content: A TC of only 24 ppm/K for the super cap / LI cell looks promissing. This should be better than most film capacitors. With essentially no power loss in the capacitor a thermal stabilization could be relatively easy at least for the shorter times.Also just the 100 ppm/k reported by Dietert look OK and managable.With the Li based "super-caps" there is a chance that the TC depends on the actual voltage per cell and not just the capacitor type.The main advantage of a film capacitor would be less DA and other relaxation effects (e.g. after mechanical stress). The lower capacitance still would need really low leakage currents.

author: dietert1
author_rank: Super Contributor
timestamp: September 14, 2024, 08:14:24 am
content: Quote from: MiDi on September 14, 2024, 07:35:15 amWith our ultra-caps the discharge rate would be 120 uA from 125 F, about -1 uV/sec, about a factor 400x higher than your result. If i compare to the LiC cap specs, that would still be about 40x. Seems like those lithium batteries are a good choice. Are the Allen diagrams TC compensated? What kind of oven was used for this?Regards, Dieter

author: iMo
author_rank: Super Contributor
timestamp: September 14, 2024, 09:46:37 am
content: Quote from: MiDi on September 14, 2024, 07:35:15 amWhat about to "trickle charge" the battery with a couple of nA such you compensate for the voltage_drop/sec??

author: MiDi
author_rank: Frequent Contributor
timestamp: September 14, 2024, 10:21:17 am
content: Quote from: dietert1 on September 14, 2024, 08:14:24 amThere is no TC-compensation for the NSD plots, the cells were just inside an alu case on the bench in my basement with quite stable temperature (around ±0.1°C over a day during those measurements).Allan deviation for 18650 and NF ULF-ULNA attached (as 18650 is below NF, it mainly shows ULF-ULNA NF)Quote from: iMo on September 14, 2024, 09:46:37 amRoughly compensate self-discharge with a low noise constant current and let the LS reference take over the fine adjustment, main question is where to set the transfer point?As already stated, it needs to sit in an ovenized, sealed and sturdy case that rejects influence of temperature, pressure and humidity enough.

author: iMo
author_rank: Super Contributor
timestamp: September 14, 2024, 10:37:58 am
content: I added above a naive sim, with 251nA you may compensate the 2.6mV/Megsec drop at the 100F battery.. Not sure about the 1/f noise impact of a trickle charging current source though..

author: dietert1
author_rank: Super Contributor
timestamp: September 14, 2024, 01:05:32 pm
content: The backup battery pack of our ADR1399 has 5 Li-Ion cells of 2 AH and would need a 960 F capacitor to simulate. It has been running for a year or so at a stable 20 V voltage, directly controlled by the reference voltage. It has a 200R safety resistor and i see about 35 uV across that resistor, in other words 175 nA leakage current. This includes some leakage of the voltage balancer and is almost 700x less than the 120 uA i have with our ultra-cap set. Seems to confirm MiDis numbers. I started a log to determine the average over one day/night temperature cycle.Regards, DieterEdit:Looking at a complete day/night temperature cycle of 0.4 °C i got an average leakage current of 460 nA, a factor 260x less than the 120 uA of the ultra-caps. Estimated TC is about -20 ppm/K, also similar to what MiDi reported. The time constant of the setup should be about 2000 F * 200R = 400 000 seconds. Part of the voltage change gets compensated by the charger.

author: MiDi
author_rank: Frequent Contributor
timestamp: September 14, 2024, 02:38:09 pm
content: The current noise & drift of the trickle charger should not be a problem.Self-discharge for those 18650 is ~40µA and differential capacity is ~14.000F/cell (dV=0.5V, I=1.45A, t=80minfrom DS).With worst case 3x1Ω ESR (typ. 0.3Ω/cell) the noise attenuation factor at 10V is ~83k (typ. ~250k).For 10µHz (1.2 days) the noise attenuation factor at 10V is ~55k (typ. ~70k) - 100ppm current noise/drift gives <0.002ppm voltage noise/drift.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 14, 2024, 05:45:03 pm
content: The leakage compensation can likely be combined with the loop that links the capacitor part to the long term stable ref. (e.g. LTZ1000). I see no real need for extra current source. Chances are this part would be some kind of PID regulator.

author: iMo
author_rank: Super Contributor
timestamp: September 14, 2024, 08:29:20 pm
content: One issue I see with the 18650 cells is you cannot create 10V easily out of them (nor filter the 10V)..3 cells at 3.33333V each in series - that is well below their voltage minimum..Even filtering 7V with 2 cells in series is not feasible, imho.With those 2.7V max supercaps you may use 4 in series and you are at 10V (and with 3 at 7V and with 2 at 5V)..

author: KT88
author_rank: Frequent Contributor
timestamp: September 14, 2024, 08:56:25 pm
content: LFP cells would work. they also have a fairly flat discharge curve.

author: MiDi
author_rank: Frequent Contributor
timestamp: September 15, 2024, 11:25:16 am
content: Quote from: iMo on September 14, 2024, 08:29:20 pmThe safe minimum voltage for NMC is 2.7V .. 3V.So 3.33V should be fine, but at the low end of SOC (could decrease lifetime).Could you please give a bit more info on your statement?What do you mean with "filter"?

author: iMo
author_rank: Super Contributor
timestamp: September 15, 2024, 11:48:54 am
content: To use it as a simple RC filtering capacitor with 10V (when there is 10V at the 3x18650 in series)..

author: dietert1
author_rank: Super Contributor
timestamp: October 24, 2024, 07:48:25 pm
content: Meanwhile i got some basic data concerning LiFePo4 batteries for a voltage reference low pass filter. I have been looking at no-name AA size cells IFR14500 (600 mAh), two sets of 3 cells each.Leakage current/self discharge is known to be 2-3 % per month. For a 600 mAh cell that amounts to about 20 uA. Measured values of my two battery sets are 3 and 8 uA, with little dependence on temperature (tested at 22 and 28 °C). TCs of the two sets were found to be 14 ppm/K and 7 ppm/K. One could try to select cells from a larger set in order to improve those results.Series resistance of the cells is specified as < 80 mOhn. That means each 10 V set will be < 0.24 Ohm. A filter with a 100 Ohm resistor can certainly suppress 1 uV zener noise to about 2 nV or better.Today i tested a 0603 PTC fuse, Digikey part number 507-0ZCM0001FF2GCT-ND. Cold resistance is 20 Ohm and steady short circuit current is about 50 mA (about 200 Ohm). Should be good enough to protect the cells from a short. Don't know yet how much thermal EMF the part causes.Also there is some progress building the constant humidity, pressure and temperature oven. Pressure leakage time constant increased to about 400 days. Currently i am trying to embed the cells into a piece of copper in order to improve temperature control.Regards, Dieter

author: Kleinstein
author_rank: Super Contributor
timestamp: October 25, 2024, 08:03:07 am
content: A TC of some 7 and 14 ppm/K looks very promissing. That is better than the saturated Hg ref. cells (AFAIR some 40 ppm/K). I don't think the internal resistance is that critical. With some active feedback to compensate the leakage current one would likely not have a simple low resistor from the reference side, but more a controlled current source with a much higher resistance. So not simple RC time constant by a larger resistor and some electronic / digital gain.The PTC fuse looks good. If really needed one could use a high voltage one, or possibly 2 in parallel to average thermal EMF. Testing the thermal EMF would make sense. I have tested a THT part (more suitable to protect a DMM input) and found surprosingly low thermal EMF. So the changes good to find a suitable version. AFAIK there are 2 types: ceramic and polymer ones.In a stable temperature case there should also be small gradients.One could consider 2 temperature stabilized parts:1) the Li-cells and some parts with essentially zero (<< 1mW) power consumption: this needs a really stable temperature, but low power makes it easy2) the amplifiers and maybe the LTZ1000 (or similar) reference part: may not need that stable a temperature, but has significant power consumption

author: Korchahin
author_rank: Regular Contributor
timestamp: October 25, 2024, 09:56:57 am
content: I would like to know the thermal stability of LTO Li Akku. Theoretically, it should be better than LiFePO4. It is also interesting to use it in parallel with a zener diode as a buffer, as in the current experiment.Usually everything tantalum is many times more stable and surprisingly higher. I looked at Moser prices for some Vishay capacitors (>300usd) and involuntarily wondered why and where are these used?

author: Kleinstein
author_rank: Super Contributor
timestamp: October 25, 2024, 11:01:33 am
content: More expensive does not necessary mean better. It is hard to predict the TC of different battery chemistries. So hard to tell up front if the LTO would be even better than LiFePO4, that turned out surprisingly good.The wet tantalum capacitors are an old technology used for high reliabilty / low temperature. So expect space and avionics or militry - so areas where component costs is not that relevant and numbers are small.Tantalum is not necessary more stable. The old style tag tantalums from the 1970/1980s have a pretty bad reputation. There is a little sense behind the idea, as tantalum is relatively expensive. So they would only use it if the products of some advantage over cheaper alternative products. For the tantalum capacitors the main point is however more low ESR also at low temperature.

author: dietert1
author_rank: Super Contributor
timestamp: October 28, 2024, 11:52:33 am
content: Meanwhile i have some measurements of thermal EMF of the 20 Ohm PTC mentioned above. In a typical setup with low thermal gradients it adds 8.2 nV of thermal EMF with about 2 nVrms extra noise. In comparison a current limiter made with two back-to-back JFETs (R = 400 Ohm) adds 10.4 nV with about 1 nVrms extra noise. The PTC is a bit more sensitive with temperature changes.Regards, Dieter

author: Echo88
author_rank: Frequent Contributor
timestamp: October 29, 2024, 05:24:50 pm
content: Here are some preliminary pictures from my setup and chamber stability results.I chose to go with stainless steel chambers with standard vacuum conflat flanges, held at temperature via an analog heater PI-controller + kanthalwire, while thermally insulated by about 5cm styrofoam surrounding it.While more expensive i wanted to have a very sturdy, shielding and virtually zero pressure leakage chamber for these longterm refstability tests.To keep the chamber at constant temp kanthalwire was applied to the mantle and bottom, with a standard OP-Amp PI-Controller keeping the walltemp at 30°C.Normally this heater controller would sit inside the chamber, but for preliminary tests it was sitting outside of the chamber as visible on the photo.Chamber humidity was kept at 0° RH by using fresh dried silica gel.That shouldnt be necessary for the reference though, as i have no idea how 0° RH will influence the capacitor stability.In the pictures the unpopulated stacked rev 0 pcbs can be seen:Heater Controller/Sensors BME280 + TMP117/Signal feedthrough - OPA206 + PTF56 + NTC + BJT-Driver, thermally coupled to the chamber by a thermal matADR1399-Ref - LTZ1000 or ADR1000 would be better, but to test the feasibility of this project i chose to go with the simpler/cheaper ADR1399Supercap-pcb - series connected standard and lithium hybrid supercaps, the hybrid supercaps compensate the standard supercap leakage currentThermal shielding/baffle-pcbs that surround the Ref-pcbThe visible radial fan in the chamber is necessary to circulate the air for uniformly temp distribution (thanks TiN).Without a fan the BME280 inside the chamber indicated way worse thermal stability (about factor 30 thermal attenuation vs roomtemp change) than measured by a NTC + HP34465A connected directly to the outside chamber wall.PCBs that support a version which uses the big visible PP-cap + tantalum caps for leakage compensation are also on the way.The PP-cap should provide way less headache/time constant waiting due to way lower DA.Preliminary stability results explanation:BME280 inside and outside of the chamber were sampled by a Raspberry Pi 4 for about a week.A standard rubber gasket was used, the two Swagelok signal feedthrough ports were closed with fittings, which were sealed with "Uhu Endfest" epoxy.The fan was running inside the chamber and the heater controller sat outside the chamber, which gives slightly worse thermal attenuation values compared to it sitting at constant temp inside the chamber.The visible curves start after the heater has stabilized the chamber temperature and therefore pressure.The silicagel had time to absorb nearly all chamber humidity.The environmental temperature and also the chamber wall temperature were measured via a HP34465A and HP34461A + PT100 running in parallel to the BME280 logging, to ensure proper measurement values.As can be seen the roomtemp changed by about 8K (i intentionally disabled the room radiator for the test), meanwhile the chamber temp changed 0.05K -> Thermal attenuation is about 160, which was confirmed by the DMMs.The chamber pressure stability is about 0,3hPa, while the environmental pressure changed by about 43hPa.The chamber relative humidity keeps on playing dryness limbo, while the environmental humidity changed by about 12%.A chamber can house either two independent supercap filtered references or one PP-cap filtered reference.Two opposition mode connected filtered references allow for easier noise measurement with a nV-meter.The new improved heater controller design should yield slightly better thermal attenuation values of about 200, limited by the styrofoam insulation - i dream of cheap, large and millable aerogel-cubes

author: dietert1
author_rank: Super Contributor
timestamp: October 29, 2024, 08:51:15 pm
content: That looks interesting.When i tried to get a quantitative model of chamber pressure measurements i found three components:- Sensor drift. In my case the test chamber runs at 37.4 °C and drift was determined as -1.7 Pa/day. Drift is caused by the over pressure of the reference volume inside the sensor caused by running the sensor at higher temperatures. The membrane slowly gives in and makes the chamber pressure appear less. For low sensor drift the chamber better operates near ambient temperature.- In a closed gas volume temperature changes will be visible as pressure changes at about 3 hPa/K. That means a temperature variation of 20 mK is expected to show up as a 6 Pa pressure variation and it does in my BMP388 data. The gas thermometer measures average temperature inside the chamber.- Chamber leakage depends on pressure difference between chamber and ambient. One needs to understand the other factors before one can check leakage and make claims about it.Regards, Dieter

author: dietert1
author_rank: Super Contributor
timestamp: November 01, 2024, 10:19:06 am
content: Meanwhile i tested a current limiter with 2x BSP135 and a 100R resistor, as proposed in another thread.Resistance as measured by multimeter is 160 Ohm. Current limit is about 8.3 mA at 30 V.Diagram shows a 24h log. Thermal EMF is about 23 nV where temperature is flat. Maybe it can be built different in order to further reduce temperature gradients.This time the 20R PTC appears with about 5 nV.Regards, Dieter

author: Echo88
author_rank: Frequent Contributor
timestamp: November 01, 2024, 10:33:50 am
content: I havent yet tested my idea that thermal bridges (e.g. Vishay THJP) may be able to reduce the thermal gradients across components like your current limiter or relays. If youre interested to test it i can send you some for free.

author: dietert1
author_rank: Super Contributor
timestamp: November 01, 2024, 04:54:14 pm
content: Thanks for the thermal bridge hint.Yes, i will definitely try other geometries. This kind of current limiter (600V, some mA, low thermal EMF) can be a very useful part. It could replace the input protection resistors in vintage multimeters like the Advantest R6581T or HP 3457A, thus reducing noise. I would also use it to protect the multiplexer inputs of ADS1256 or ADS1263 setups.By the way, similar circuits are used in power electronics, e.g.https://www.electronicdesign.com/technologies/power/article/21808402/a-better-way-to-build-current-limiters-and-circuit-breakers.Image shows a first 10 V LiFePo4 set with its thermistor embedded in ETP copper. The copper part was pretty difficult but probably serves the purpose. Maybe the second one will be nice. Meanwhile leakage currents dropped to 2 uA.Regards, Dieter

author: Echo88
author_rank: Frequent Contributor
timestamp: November 01, 2024, 08:28:49 pm
content: While waiting for the new pcbs i thought about the mentioned paperhttps://www.wellesu.com/10.1142/s021947750700388xand tried to see whether it could potentially provide an interesting path.However, some questions came up reading it:Taking the square root of the used Power spectral density gives the usual Noise Spectral density, right? V²/Hz -> nV/sqrt(Hz)They only mention/calculate the white noise contribution of the used voltage reference, current setting resistor and the dynamic diode resistances.Yet their calculations seem to correspond to their measured curves in Figure 7, while they are working clearly inside the 1/f noise region. Cant make sense of that.How did they arrive at the mentioned 8 x 10^-17 V²/Hz spec for the used AD586?Attached is a general first idea how it could be implemented.ADR1399 7.05V *2 gain stage provides the necessary >10V reference voltage.The transistor stage Q1/Q4 provides the current to the diodes, without loading the OPA189.The original SSM2220 are obsolete, but THAT320 arrays or similar should also be suitable.Then again i just opted to go for 12x ZTX951 transistors (lowest noise according to AoE3, about 1€/piece) attached to a temp controlled aluminium core pcb, which would sit inside the already temp controlled chamber mentioned a few posts earlier.The 12x diode connected transistors provide the wanted 10Vout by adjusting the flowing current through them, the heater stage keeps the 10V stable by minimizing the diode TC.The paper only mentioned the equivalent diode resistance and doesnt go into depth about 1/f noise contribution of diode connected transistors, so i just thought its best to use low noise (low rbb) transistors for this job.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 01, 2024, 08:54:56 pm
content: The heated diodes system is somewhat related to the idea we had here. In the article the temperaure control works as a kind of slow thermal low pass filter. I am not so sure if exactly this way is a good idea, as it may not scale so well to lower frequencies and it is easy to get temperature variations from the outside to add low frequency noise. On the upside regulation of a thermal system is well known. The case of Li cap (battery) and regulator for the voltage may actually be a bit on the slow side - especially for testing.For the initial tests and learning one may think about a more classic super cap or even a large normal electrolytic capacitor. This won't work perfect, but could test of the correction loop works.

author: dietert1
author_rank: Super Contributor
timestamp: November 02, 2024, 11:44:49 pm
content: The diode array comes with a TC of about -2 mV/K / 600 mV = -3300 ppm/K, while the LiFePo4 batteries i tested have around 12 ppm/K, a 278x advantage. In the paper they are looking above 0.1 Hz. Certainly ambient temperature variations are easier to suppress at short time scales.I think the LiFePo4 averager can work at time scales of hours or days. And yes, the cells are terribly slow for development and tests. While i glued the cells into the copper block they already lost some charge. When i connected them again, i made a minor mistake that changed the state of the cells even more. One definitely needs an automatic controller to charge and balance the cells as fast as possible and make them finally track the zener reference. Otherwise the idea is useless.The schematic is a possible starting point. It still needs some level shifters. As Kleinstein wrote above, resistor R13 won't be necessary, so optional or switchable. I have been using it to measure leakage current.Regards, DieterEdit: The schematic isn't more than a sketch. The amplifier can't feed the 10R charger resistor. And there should be a hardware protection when reaching 3x 3.6V = 11 V total voltage for whatever reason. And the nullmeter needs its own battery tap in order to measure battery voltage independent of charger current and during output off.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 03, 2024, 09:03:31 am
content: I think the bancing part should have a way to turn it off all the way (e.g. via the FETs that are there for the protection anyway). Just switching the OP-amps to nominal zero current may not be good enough. Once balanced and not using much charge / discharge it should be OK for the nex years, possibly the life of the cells.The diode array has the high TC, but this high TC is also the way to do the adjustment / regulation. So they kind of need at least some TC to get regulation. The speed of the regulation part sets the lower limit for the useful frequency. This means that slow regulaton would be wanted to also get to lower frequencies. The analog regulator used with the diode stack works OK for relatively fast regulation, but gets more tricky with even lower speed. One kind of gets the RC leakage problem at the regulator instead of directly using it for the voltage. At some frequency there should be a cross over where digital regulation can be better. Digital has no issue with integrator drift from leaking / bias.

author: dietert1
author_rank: Super Contributor
timestamp: November 03, 2024, 06:36:08 pm
content: Meanwhile i made another current limiter with 2x BSP135, this time with a 150R resistor to get about 5 instead of 8 mA. Total resistance is about 200R. Also i turned off an external hard disk that had been running about 10 cm away from the thermal EMF test location. This time thermal EMF came out to be 0.5 nV for the low thermal short, 2.0 nV for the PTC 20R and 5.2 nV for the new BSP135 limiter. Its geometry is "folded" to make it more compact. Both mosfets are glued to a small piece of fiber isolation sheet. The JFET limiter is in a short piece of aluminum tube.Regards, Dieter

author: branadic
author_rank: Super Contributor
timestamp: November 03, 2024, 07:39:27 pm
content: If you were located in Germany, I could have sent you a brass tube with glass feedthroughs and some parafine to make a hermetically sealed, oil-filled version of it, which should reduce thermal gradients between all components.-branadic-

author: Echo88
author_rank: Frequent Contributor
timestamp: November 03, 2024, 10:31:20 pm
content: Yeah, i missed the giant TC of the diode stack when posting and only recognized it later.They dont mention the achieved oven stability or even the big TC problem at all, shame.With about -3000ppm/K i dont think theres a single heater solution that would get it sufficiently close to useable, without an additional PTC component (which gives the standard Z-diode + diode ref topology).I have two ideas apart from using Z-diodes for PTC compensation that ill test to see if they have merit.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 04, 2024, 09:36:29 am
content: The diode stack solution would likely want a 2nd oven layer, to keep out external temperature variations and so that the critical inner regulator does not have to work that hard. The oven stability is not such an issue, as the regulation is from the target voltage, not actually the temperature.It is still an interesting article to compare. One could be free to replace the diode stack with a chemical battery - though I don't think the batteries would like temperature variations that much.

author: dietert1
author_rank: Super Contributor
timestamp: November 14, 2024, 10:26:39 am
content: Meanwhile i got the setup in the image, with 2x AD587 references, 2x chargers similar to my sketch above and 2x copper-embedded LiFePo4 10 V battery packs, everything in a card box. The setup is wired to a K2700 with a 7706 MUX in order to log six charger currents and the two thermistors inside the copper.Charger circuit is based on OPA4140, MUX4053 and NOMCT 8x 5K resistor array. For battery protection there are BS170 mosfets with Rdson about 1 or 2 Ohm. They limit the gain of constant voltage charging to about 0.67 uA for each uV of deviation. Separate mosfets for force/sense could speed that up somewhat as the battery ESR is less than 80 mOhm. Until now the batteries exhibit a time constant of about a day near the desired state, so it may take a week to descend to the desired precision.Until now i can see the expected noise hierarchy of the three batteries in each pack. That means the noise in the charger currents is already dominated by the AD587 noise.Next step is the constant humidity, temperature and pressure oven. Later one may want separate ovens for the two filters.Regards, Dieter

author: dietert1
author_rank: Super Contributor
timestamp: November 20, 2024, 11:19:16 am
content: Here i have a log of charger operation for battery pack A. During the night, when room heaters are off and temperature is decreasing slowly, charger currents are in the low uA range. Heater turn on in the morning causes discharge due to positive battery voltage TC.Meanwhile there is a design for the oven(s). I printed two brackets to put the battery packs inside a Weidmüller K41 enclosure that i have been using for hermetic oven tests. One or two 9x7 cm boards will also fit inside. That enclosure will run at 22 °C, using TEC temperature control.Each battery pack received a small heater to use the copper as an inner oven, running at 23 °C, 1 °C above outer oven temperature. See Kleinsteins remark above about double ovenizing the diode array. Yesterday i made a linear temperature control for oven B. I got a log of battery pack temperature, still exposed to ambient temperature. Heater power is 15 V times the measured current, so about 45 mW on average. Without the heater, the temperature curve would look like the one of pack A, so the heater adds about 1 or 2 °C. Once the battery pack goes inside the outer oven, temperature variations will be much smaller. One may need a digital controller though, as the large integrator cap won't fit inside the outer oven.While working on the temperature controller i screwed up the charging state of battery pack B. So i don't yet have a charger log. Some more work until things can go inside the hermetic enclosure...Regards, Dieter

author: dietert1
author_rank: Super Contributor
timestamp: November 22, 2024, 09:58:31 am
content: Meanwhile i tuned the temperature control loop. The PID is now much faster, with the integrator cap reduced from 100 uF to 2,2 uF. The OPA140 now runs at a gain of 86 dB, retaining about 40 dB for negative feedback.To my surprise the loop is still stable - probably due to the fact that there is about 1 cm in ETP copper from heater to thermistor. Temperature noise came down to about 100 uKpp, still exposed to ambient temperature change. During the night the controller runs into it's 6 mA current limit, when ambient temperature cools down more than 2.5 K below "oven" temperature.This is to demonstrate the importance of PID controller tuning. A stable loop doesn't mean the controller is well designed.The analog PID test wasn't a waste of time, although: 2x OPA140 at 15 V consume about 54 mW, while a ADS1256 + some PWM will be more like 40 mW and the ADS1256 does eight channels.Regards, Dieter

author: dietert1
author_rank: Super Contributor
timestamp: May 07, 2025, 09:59:06 am
content: Meanwhile i made two prototypes of the outer oven. The hermetic enclosure shown above (Weidmüller K41) resides inside a box with 30 mm PUR sheets for thermal isolation. As the enclosure has two parts it got two TEC12706, one at the top, one at the bottom. Those are chinese TEC cooler kits including heat sinks and fans. The setup has four I2C sensors: A BME680 and two SHT45 inside the oven and another BME 680 for ambient monitoring. The two SHT45 sit next to the Peltier elements. The whole setup runs at TEC currents of about 100 to 200 mA total at an oven temperature of 22 °C.Image shows the TEC driver. It implements two independent PID controllers based on the SHT45 readings and supports heating and cooling. The diagram shows an 11 hour log with an ambient temperature change of about 1.7 K. Oven temperature deviations are about 1 milliKelvin (standard deviation of 1 minute averages). The internal BME680 sensor not used for temperature control exhibits about 4 mK deviations.One disadvantage of using an enclosure with rubber seal is that after closing the lid one gets a little higher pressure inside, due to the movement of the lid. One needs to choose a day with average or a little less than average air pressure.Regards, Dieter

author: dietert1
author_rank: Super Contributor
timestamp: May 07, 2025, 08:33:16 pm
content: @iMo: When i tried to use those bluepill modules from ebay, the USB CDC link did not work with the drivers provided by STM. I knew it works with STM32L433 MCUs. They have the same pinout as the F103, except one of the boot mode pins. I also modded the USB re-enumeration pull-up to be controlled by a GPIO, in order to get automatic re-enumeration during boot. That helps with debugging. The 48-pin MCU of the bluepill has the clock battery pin and i get a time tagged log data stream via USB to be recorded with a general purpose terminal program (no special application required).By the way those ribbon cables i am using with epoxy and some additional aluminum parts to make a hermetic wire feed-through aren't air tight as their wire strands let air pass. One needs to seal all strands with epoxy to get constant pressure inside the enclosure. Those ribbon cables are copper and provide low EMF connections.Regards, Dieter

author: iMo
author_rank: Super Contributor
timestamp: May 07, 2025, 09:14:39 pm
content: @dietert1: ..thanks, I removed my post on the BP not to distract you.. I did the same with stm32F303CB (found several in my junkbox) and the upload of binaries and communication via usb works fine as well (and it is the CC silicon so twice the flash for free).

author: miro123
author_rank: Frequent Contributor
timestamp: May 15, 2025, 09:43:58 am
content: Hi Dieter.have one question. As far as I understood you are using SHT45 input to control temperature. You measurements shows deviation in mK and sub-mK range.How do you achieve it since SHT45 resolution is 10mK?I have build small oven in the past and I have used two NTCs in bridge configuration for resolution and PT1000 for accuracy. The more resolution I have the more easy control was. I my case I have dual control loops. Inner controlled the Peltier plate temperature and outer the DUT

author: dietert1
author_rank: Super Contributor
timestamp: May 15, 2025, 10:54:28 am
content: I tried using those digital sensors as i wanted to have all analog circuitry inside the oven. I have those nice glas NTCs inside the battery copper block used as internal ovens, using them with an analog PID. To use them for the outer oven, i need to put an ADC inside the oven, like the ADS1256 + ADG736 circuit i showed elsewhere.The SHT45s worked better than expected. The image shows a random sample of the raw log data. For the PID i am using a median of three filter to suppress some of the sensor noise. The oven has a lot of thermal mass, so it doesn't matter that the noise filter introduces a timing jitter of 2 seconds. The D gain of the controller is null.The diagram i showed above had 1 minute averages of those raw data. I think averaging is justified as the sensor noise dithers enough.The device will have more temperature sensors, anyway, for example the heater currents of the two inner ovens. The device also has two LTZ1000 references to discipline the LiFePo4 batteries. Their temperature control part can be used as well. The control board has some space left, so one could even implement the two LTZ temperature controllers in the standard fashion. First i am trying to just tune the LTZs for zero TC near the 22 °C outer oven temperature.Regards, Dieter

author: miro123
author_rank: Frequent Contributor
timestamp: June 02, 2025, 02:47:41 pm
content: Quote from: dietert1 on November 01, 2024, 04:54:14 pmInspired by idea of Dieter I've made setup of two 18650 LiFePO4 One NTC and one PT1000 on single AL block. I have access to copper blocks but for easiness to drill I have chosen AL. The hole are firmly fitted with thermal paste. The block is thermaly insulated . The battery cells ware charged and balanced. I left the setup for 5 months - I was 100% busy with my regular job. I measured the voltage difference toady . It was 38uV. The results are promising. I consider to make noise measurements. of the LiFePO4 cells . I consider using OPA2189 powered from 2S cells Gain 1000= 100* 10 BW limited to 1KHz - output wil be measured by DMM6500 digitazer on 100mV range. At the same time AL block temerature Humidity atmosfphere pressure and acceleration will be monitored via stm32H7 nucleo board.I still have doubt about the amplifier stage. Is the opamp good choise? I preffere this one because I heve OPA2189 and 2182, 140, 2205 in my drawer. What type of gain resistor to choose? Is the BW of 1KHz wide enough? Comments and suggestions are welcome.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 02, 2025, 03:43:17 pm
content: The interesting part is the moderately low frequencies. So 1 kHz BW would already be plenty and for this frequency range classic RC filtering is possible. Chances are one could get away with get some 50/60 SPS readings with mains frequency suppression.The really interesting frequency band would be something like some 1 mHz to some 10 or 25 Hz.Getting the noise density at 1 mHz down to some 100 nV/sqrt(Hz) would aready be a very good value.The OPA2189 is not an ideal choice, but also not so bad. To reduce the heat effect I would prefer using a single OP-amps, and not a dual. As the voltage is relatively low one could also use a OPA388. As the source is rather low impedance one could consider 2 parallel amplifiers and independent paths to have some check on noise from the amplifiers. It is not so much the normal white noise, but thermal fluctuations causing extra low frequency noise.So even higher noise AZ amplifiers like OPA387, LTC2057 or even AD8628 should be good enough. Because of the low frequency noise AZ amplifiers would be preferred.With significant amplification upfront the ADC would not be that critical - it could be as little as an MCP3561.The resistor choice can still be a bit tricky - not because of resistance dirft, but for getting low thermal EMF to keep thermal moise low. I would consider having some 2-4 resistors in parallel for the lower value resistor. So maybe 2x100 ohms in parallel and some 50 K for a gain of 1000 with the option to add more resistors for higher gain.Hard to tell which resistors have low thermal EMF - not often specified and one needs to define a sensible measurement procedure, especially for different form factors.

author: miro123
author_rank: Frequent Contributor
timestamp: June 02, 2025, 08:19:47 pm
content: Thanks for quick and helpful tips.My previous post was too cryptic.I have opa388 and opa2388 to but they just not fit. I power the amplifiers with 2S LiIon cells from 6Ah. something in 6....8.5V range.I have good experience of using opa2182 at single 5V PSU.I connect the output to my Keithley DMM6500 in digitizing mode. If gain and voltage difference is OK I can perform continuously streaming. I have been using the multimeter of this mode for a while - works ok up until 33Ksample/sec. So I have software ready to use at any sample frequemcy between 0..33KHz.In theory I can stream up until 6Ah battery is empty - few weeks or even mounts. :-) To get a clear picture I plan to use welch method. SO one day clean data is needed to obtain good NSD from 1mHzYour idea with dual channel amplifier is also interesting.I was not aware over resistors thermal EMF issues - thanks for the hint.See simplified drawing of noise amplifier section - V1 & V2 are two cells that power the circuit.V3 and V3 are the two DUTS buried in AL block.I have added filtering capacitors to prevent some electrochemical processes caused by chopper injection. Maybe those capacitors are not needed but I know how the chemical engineers looks at electrochemical impedance in wide frequency range. I have no idea does the chopper injection makes in lifepo4 cells.Next steps if results are positive.I will upgrade my exisiting nanovoltmeter from single channel ADS127H02 to ADS127H02 + AD7177-2 - AD will monitor the difference between the exisiting ADR1399 Vref and LeFePo cells. I dont expect to be a easy way. Many obstacle will be on the road. e.g. dealing with trilling, air pressure difference, temperature waves. etc. etc.

author: dietert1
author_rank: Super Contributor
timestamp: June 02, 2025, 10:53:29 pm
content: Yes, it's a complex project, but i'd expect much lower noise than a zener standard as the physical dimension and mass of the 10 V LiFePo4 battery kit is so much larger and it's a near equilibrium process. Battery self discharge current is about 1000 times smaller than the 4 mA zener current of a LTZ1000 and it does not emit light! The battery TC of 7 ppm/K times the oven temperature standard deviation estimates the expected voltage standard deviation. I think the result is almost guaranteed with a good setup, optimizing the hermetic double oven and the battery control circuit.Currently my in-oven control circuit uses seven opamps per device:1x for 15 V power supply1x for inner oven temperature control2x for LTZ1000 and 7.2 V to 10 V gain stage3x for battery chargers 10 V, 6.6 V and 3.3 VRecently i got the LTZ1000 references optimized to < 0.05 ppm/K near the outer oven temperature, i mean without using their heaters. The in-oven control circuit for two devices consumes about 540 mW, that is 2 * 15 V * 18 mA.The schematic/sketch i showed above had a ADS1263 and i want to use it with some relays to log voltage differences - between different disciplined batteries and also their zener references to determine the trickle charge current of each battery set with long enough time constant (digital). I want to discipline the battery set by control of its trickle charge current as proposed by Kleinstein before:https://www.eevblog.com/forum/metrology/ultra-low-noise-short-term-stable-references/msg5631293/#msg5631293.Regards, Dieter

author: Kleinstein
author_rank: Super Contributor
timestamp: June 03, 2025, 06:22:03 am
content: I would consider the first buffer amplifier as part of the low noise reference. If needed this could also include a gain step (e.g. to get to 10 V).The link to the long term stable part would also compensate for long term drift of the gain part.I have not measured myseld, but from the reports I found, it looks a bit like that the TC of the LiIon batteries depends on the charge state. So a cell at 3.3 V can be different from a cell at 3.6 V.When measuring only the difference with rather low voltage, the ADC is no longer critical. It would still help to have an essentially contineous sampling to keep the noise BW low.

author: dietert1
author_rank: Super Contributor
timestamp: June 03, 2025, 09:43:13 am
content: I found some 50 VA Bürklin mains transformers with builtin mains shield. Their two "native" 15 Vac secondaries serve for the two voltage references and it had some empty space, so i could insert one layer of thicker wire for an additional secondary of 7.2 Vac. That supplies the oven TEC drivers and the digital control. All supplies have a shield that can be connected to Guard, with a stray coupling of a few pF.The mains transformer also got a 100 mH common mode choke for its primary.For the ADS1263 i will try TI ISOW7841 SPI isolators with built-in isolated supply. It yields about 100 mA isolated supply and the ADS1263 needs up to 37 mA. Don't know yet whether that will work well.Regards, Dieter

author: MiDi
author_rank: Frequent Contributor
timestamp: June 03, 2025, 05:17:14 pm
content: Quote from: miro123 on June 02, 2025, 02:47:41 pmOPA2189 will dominate the noise and to measure down to very low frequencies you need to switch to paralleled JFET op amps e.g. OPAx140.To get a decent LPF you want low current noise in order to increase the resistor and use a low leakage PP capacitor.A single 18650 LiIon cell measured below NF ofULF-ULNA (~0.7nV/Hz^0.5) - see attachment.Quote from: miro123 on June 02, 2025, 08:19:47 pmSome options to get a smooth NSD plot:Python LPSDMatlab LTPDAWindows/Linux NSDRemark: For a correct NSD you need to switch off AZ, otherwise the noise is increased and 1/f corner is off.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 03, 2025, 07:08:20 pm
content: For testing the noise of the L cells one could look at the difference between 2 cells or 2 stacks. So only a small DC voltage and thus the possibility for DC coupled amplification. DC couples and thus with low resistors the OPA189 and most other AZ amplifiers would be OK, as the Li cells are low impedance.For AC coupling the AZ amplifiers like OPA189 would need rather large coupling capacitors, which may be more sensitive to temperature and mechanical stress than LI cells. I don't think one would need a serious low pass filter - the LI cells should be very low noise over quite some frequency range. It would be really hard to amplifiy with AC couling. A moderate LPF may be wanted to filter out AZ amplifier artifacts.

author: miro123
author_rank: Frequent Contributor
timestamp: June 04, 2025, 03:47:52 pm
content: One stupid question arise when I start playing with those LiFePO4 cells.How do you deal with power off state of supported electronics. Imagine power goes of and all clamping diodes acivates

author: dietert1
author_rank: Super Contributor
timestamp: June 04, 2025, 04:18:03 pm
content: In my tests i used some BS170 mosfets to disconnect the electronics automatically, as shown in the schematic i posted above. They are driven by a very simple circuit with a pnp transistor, a zener and some resistors to avoid any glitches during power on or off. Later three PC817 optocouplers were added per device so the MCU can select the charger operation mode, disconnect the chargers and disconnect the device output. The fourth opto drives the trickle current PWM.The schematic doesn't show the necessary output protection circuit.Regards, Dieter

author: mzzj
author_rank: Super Contributor
timestamp: September 26, 2025, 10:19:35 pm
content: Quote from: dietert1 on November 02, 2024, 11:44:49 pmCame across your post when I was looking for LiFePo4 voltage tempco.Some LiFepo4 batteries seem to have considerably lower self discharge rate than others. 100Ah cell I'm currently measuring seem to have about 0.4% per month self discharge rate.Coupled with the flattest portion of the LiFepo4 discharge curve (stage 1 lithiation between 80-90% SOC) we would get -0.000184 volts per month or 2ppm per day.Didn't see that coming but explains why self-discharge measurement can be a bugger.

author: dietert1
author_rank: Super Contributor
timestamp: September 28, 2025, 09:20:29 am
content: Yes, a good disciplined LiFePo4 standard requires a continuous charger current to compensate self discharge. In my case that circuit is digital in order to achieve a large time constant for adjustments of the charger current. There is one DAC that controls charger current for all three cells.Those larger cells used for energy storage often incude safety circuits that are incompatible with this application. E.g. the power mosfets of a charge balancing circuit will certainly increase discharge current.Regards, Dieter

author: Kleinstein
author_rank: Super Contributor
timestamp: September 28, 2025, 09:44:47 am
content: With the low load there is likely no need to really worry about balaning between the cells. Chances are one could leave that part to maintenance, maybe every 10 years or so.A digitally controlled charge current does make sense. It could be the same current source / DAC as used to link the Li cells to the long term reference (e.g. Zener based).A still open point may be the question on how to handle voltage scaling.It could make sense to have 3 Li cells and 10 V directly and than scale from 10 to 7.x (or 6.6 V with an ADR1000) for the long term corrections.I consider this better than 2 Li cells to directly follow the 7.x V and scale the 7.x V to 10 V as a separate step.In both cases the buffer after the Li cells would be included in the long term corrections.

author: mzzj
author_rank: Super Contributor
timestamp: October 01, 2025, 01:53:03 pm
content: 30 minute stability of 3S LiFePo4 pack sitting idle at somewhere 70...85% SOCWas not certainly in my books that cheapest LiFePo4batteryis indistinquishable from 3458A 0.1ppm 10 minute DC transfer stability.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 01, 2025, 02:41:04 pm
content: Chances are that much of the noise is from the meter. To really see the noise / stability one could chave 2 battery packs connecting to directly looking at the difference.It is only over a long time (like hours or days - depending on the temperature stability and stress effects) that the zener reference will be more stable.