A discrete bandgap reference - Page 1
Forum section: electronics / general

author: branadic
author_rank: Super Contributor
timestamp: November 08, 2020, 12:40:06 pm
content: So I came across two articleshttps://www.ama-science.org/proceedings/getFile/Zmp5BN==https://www.ptb.de/cms/ptb/fachabteilungen/abt2/aktuelleforschung2/forschungsnachrichten.html?tx_news_pi1%5Bnews%5D=9994&tx_news_pi1%5Bcontroller%5D=News&tx_news_pi1%5Baction%5D=detail&tx_news_pi1%5Bday%5D=9&tx_news_pi1%5Bmonth%5D=12&tx_news_pi1%5Byear%5D=2019&cHash=c365fa74e9be84ae491d1c7da5dab802about a discrete bandgap reference and thought it would be fun to reproduce the results, which is science, right? Spoiler alert, I failed or did I?In the second link we can see, that BC548C and BC328 were used and that they state: "... Because of the high temperature sensitivity of the p-n-junction the temporally stability is insufficient. Work is focussed on improving temperature stability by an appropriate set-up..."Okay, well if that's the case, let's address that too. So I designed a small round board with BC848 and BC808, the SMD versions of the transistors mentioned, some 10 ppm/K resistors, a ceramic capacitor and had some copper bar being made the board was fitting into with some heat-conductive paste between the copper and the transistors. The back of it was potted. The raw circuit was assembled on a perfboard, which was later extended with the shown 7805 voltage regulator.To be continued...-branadic-

author: Gyro
author_rank: Super Contributor
timestamp: November 08, 2020, 01:03:53 pm
content: It would be good to see that implemented with hermetic transistors.

author: kosine
author_rank: Regular Contributor
timestamp: November 08, 2020, 02:09:46 pm
content: You should check out Bob Widlar's patents: US3320439, US3617859 and US4249122Very clever circuit he came up with, just a modification of his current mirror at it's simplest. That guy really knew how to make a transistor dance!Here's an article by him on the subject as well:https://pdfs.semanticscholar.org/ca1e/a52a2e846749fc960fa911f23329ba2ef3f0.pdf

author: chuckb
author_rank: Frequent Contributor
timestamp: November 08, 2020, 02:21:16 pm
content: Great experiment!Do you know how much bias current was used in the PTB bandgap experiment?I wonder if an LTC6655-5 would be a better power source for your tests. Those 7805 regulators are pretty noisy.Good luck.

author: Andreas
author_rank: Super Contributor
timestamp: November 08, 2020, 02:50:35 pm
content: Hello branadic,I also experimented with a "discrete" bandgap some years ago.Actually I needed a 3.3V low power current regulator and had one unused half of a TL062:This is the result. But since I had +/- 5% voltage tolerance I never checked within PPM range or noise.Nowadays I would use a dual BC846 in SOT23-6 or something like that for the bandgap.with best regards

author: kleiner Rainer
author_rank: Regular Contributor
timestamp: November 08, 2020, 02:58:35 pm
content: BC548 and Co. are not especially low noise in my experience. Better to use low saturation types with low base spreading resistance made by Ferranti/Zetex, now DIODES, FMMT617/717:https://www.diodes.com/assets/Datasheets/FMMT617.pdfBeing SOT23, they should fit your layout without changes.Application note here:https://www.diodes.com/assets/App-Note-Files/dn11.pdfMore low noise stuff:https://beis.de/Elektronik/LNPreAmp/LNPreAmp.htmlAll about the Brokaw bandgap cell by the inventor:https://www.idt.com/us/en/document/whp/how-make-bandgap-voltage-reference-one-easy-lesson-paul-brokawIf I had to tackle a discrete bandgap reference, I would design a hybrid circuit to ensure equal temperature distribution and freedom from humidity induced stresses of the circuit board. Resistors would be thin film MicroMELF, the only printed resistor would be used for ovenizing the hybrid circuit to minimise temperature drift. The power supply would be an uA723 based low-noise solution, the best you can get for a few cent.Put it into a styrofoam insulation, package it in a milled, thick walled aluminium box and use feedthrough filters to keep noise and RFI out.Greetings,Rainer

author: David Hess
author_rank: Super Contributor
timestamp: November 09, 2020, 01:20:26 am
content: Does it have to be a discrete bandgap reference or is a dual transistor or transistor array acceptable? In the later case, a transistor array can temperature stabilize itself.

author: KT88
author_rank: Frequent Contributor
timestamp: November 09, 2020, 02:41:56 pm
content: I guess the purpose of using discretes is to minimize stress across the die. With arrays the dies get larger and more prone to mechanichal (thermal) stress.CheersAndreas

author: silverback
author_rank: Contributor
timestamp: November 09, 2020, 03:12:15 pm
content: Flying above you are 100's of discrete bandgap references using discrete die configured in ceramic LCC 18 & 20 packages. These used the Widlar configuration initially, later using the Brokaw configuration as it was easier to set up. Using individual transistor die in ceramic packages over came single event problems in space. Some have over 20 years in space and still OK.

author: tszaboo
author_rank: Super Contributor
timestamp: November 09, 2020, 04:45:48 pm
content: Are these supposed to be temperature controlled? Or are they only looking for a transfer standard?Otherwise I dont really see the reason for doing this, because all they prove is that "diodes work".That RCRC filter at the end has 5.6Hz -3dB frequency, so above ~50Hz, you can just do the same with the zener. And then you have a high impedance source, that you likely need to buffer. And your buffer makes noise.So you connected a bunch of SOT23 transistor in parallel to get more die area. So why not a SOT223, that is bigger to start it. Or why not a TO247? There are just so many design decisions that are not described...

author: silverback
author_rank: Contributor
timestamp: November 09, 2020, 05:13:34 pm
content: A band gap reference can provide a stable more predictable voltage reference at lower voltage than a Zener can and operate over a large dynamic input voltage range. Also an important point its used to provide IPTAT currents.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 09, 2020, 05:29:32 pm
content: With discrete transistors one can choose to run it would a high current and thus relatively low noise. However one should be careful with thermal coupling and the current mirror drawn at the positive side. With discrete transistors I would add additional emitter resistors.For a larger transistor I would no look for TO220 or TO247, but more like TO3 - at least this one is metal sealed.The interesting point with the transistors is getting low 1/f noise - some of the slightly larger transistors are supposed to be relatively good in this respect. However I have not seen noise specs for an 2N3055.The point I don't understand is why they usually only use a single junction and not some 3 or 4 in series to start with a high voltage. For me this would be a logical way to get lower noise. It may be less practical on a chip, but no problem with discrete.There are already a few band-gap references with relatively low noise as single chips (e.g. LTC6655 and similar). Not sure if there is a real advantage for a discrete solution.

author: magic
author_rank: Super Contributor
timestamp: November 10, 2020, 10:52:30 pm
content: I was thinking about such thing a few months ago but lacked time to build and play with it and then kinda forgot about the project.Regarding noise, the most sensitive part is the transistor pair which generates PTAT and its associated resistor (Johnson is not negligible) because this tiny voltage is amplified many times just to get 1.2V.Stacking a bunch of junctions would perhaps help, that should be easy enough to sim. There are commercial references which use single-level PTAT generator to thermally compensate a series connection of multiple junctions (TL431, LM336 come to mind) but I'm not aware of any that uses a stacked-junctions PTAT generator. Dunno why.I think low noise audio transistors like 2SA1312/2SC3324 may work well for 1/f noise. Ultra low rbb transistors with good LF characteristics seem to be unobtainum these days, but Zetex would do if ludicrously low noise only at audio frequencies is desired.I came up with the attached circuit, based on simplified TL431 topology, which uses only two transistors (or common centroid pairs if you wanna be fancy) and three resistors for the reference cell plus a discrete opamp to implement a 1.2V shunt reference. The opamp could likely be replaced with an IC. Nice thing about SPICE is you can easily experiment with adding offset voltage to transistors, tempco to resistors, etc and see what happens. IIRC, satisfactory performance should already be achievable with 15ppm metal film Rs.One gotcha of Brokaw cells: SPICE says they have terrible PSRR because the two transistors operate at different current densities and apparently it affects their Early effect behavior differently. The TL431 cell solves this problem by always having the regulated reference voltage across the cell.editQuote from: kosine on November 08, 2020, 02:09:46 pmThe first seems to be a current source. The second is kinda meh but I don't remember what I didn't like about it. It surely is sensitive to Q3 bias current. The fourth is the circuit used in LM385, and LM336 is almost the same but with two diodes in series and hence 2.5V.

author: razvan784
author_rank: Regular Contributor
timestamp: November 11, 2020, 10:58:43 am
content: Here is a concept for a stacked-junction bandgap. The current sources of course have to be designed, and their ratio should be stable. The concept is pretty old. One can use less transistors in the stack but with a lower current in the right branch. I chose 16 because it's a nice round number, it's about the maximum for 12V operation, and it gives a reasonably high current in the right branch, which should be good for noise.

author: dietert1
author_rank: Super Contributor
timestamp: November 12, 2020, 02:46:17 pm
content: As far as i understand what is needed for a 10 V reference are two 128 long chains. At Digikey $ 20 buys you 128 dual npn transistors BCM847DS,135. Those are matched pairs and you can put one each into the two chains to simplify temperature compensation. Board area can be about 50 x 50 mm, so that still fits into a small TEC oven. This method will give you a 10 V reference without using a gain stage.It will resemble a Josephson array. You need a little more than 100 V to drive it, but no cryo system, no microwave RF and no magnetic fields...Regards, Dieter

author: Kleinstein
author_rank: Super Contributor
timestamp: November 12, 2020, 03:43:18 pm
content: I don't think one would need so many bandgap voltages in series. AFAIK the simple version gives some 1.24 V when adjusted to zero TC. So it would be 8 junctions in series to get close to 10 V (some 9.9 V).For me the big question is, if with a discrete setup the temperature differences are small enough and whether the noise level and stability is competitive with ready made chips. Naturally the PAT part has quite some gain.

author: dietert1
author_rank: Super Contributor
timestamp: November 12, 2020, 04:39:54 pm
content: I was responding to the previous post and it was obvious that without gain stage a single bandgap pair will give a small fraction of 0.7 V, in razvan784s simulation 1.24 V / 16 = 0.0775 V. Can you understand that?The remaining question for me is the current division resistors. How precise and small those should be to get something interesting.Regards, Dieter

author: magic
author_rank: Super Contributor
timestamp: November 12, 2020, 05:34:38 pm
content: There are two amplifications:A tiny ΔVbe is amplified 10x or more* and added to one Vbe for a stable 1.24V.The 1.24V is amplified 8x to 10V.*depending on its initial magnitude, which is log of current density ratio and about 60mV at 25°C for a somewhat practical 10x ratioReplacing either amplification with multiplication of the circuit, noise should go down by sqrt of gain, it seems.There is a tradeoff between stacking more ΔVbe units or paralleling transistors in each ΔVbe unit to increase current density ratio.There is a tradeoff between paralleling transistors or reducing current in one branch, the latter causing increase in noise.There is a tradeoff between shot noise and flicker noise (low Ic / high Ic) and I have zero experience with that and LTspice doesn't model it.There is a question of power dissipation (and temperature uniformity) if something is to be powered from 100V.

author: dietert1
author_rank: Super Contributor
timestamp: November 12, 2020, 07:39:14 pm
content: What i liked about razvan784s simulation is the idea to build a "pure" bandgap reference without gain stage. As far as i understand this will result in the lowest possible noise. Concerning power consumption: When using dual transistors temperature differences will already compensate to some degree, and if we stay with the 10 mA simulated by razvan784, times 100 V the circuit will output about 1 W of heat, evenly distributed on an area of about 50 x 50 mm. Each transistor pair will generate 7 mW or so, which is nothing for those SMD parts.Regards, Dieter

author: Kleinstein
author_rank: Super Contributor
timestamp: November 12, 2020, 07:51:04 pm
content: With discrete parts one could mix different parts. Especially low leakage diodes with a relatively high forward voltage (e.g. in the 0.8 V range) could be interesting. This would requite considerably less of the PAT part to compensate the TC.An extreme version is the combination of a orange LED with some 1.5 V and around -2 mV/K and a single transistor to get some 1 V with a low TC at the right current levels. However I know the long time stability is not that good for LEDs in a epoxy case.

author: dietert1
author_rank: Super Contributor
timestamp: November 12, 2020, 11:38:08 pm
content: Even the most simple prototype reveals that a bandgap reference from discrete components will be useless unless you can build something with tight thermal coupling. The bandgap voltage differences aren't that large in comparison to the 2 mV/K TC of each contribution.I just tried a transistor pair BC847S we had in the drawer and it gives a pretty stable 66.65 mV at about 20 °C and 10 mA/1mA.I also tried to reproduce the above simulation with LTSpice and couldn't confirm the results shown. At a current ratio of 1 - 0.239 to 0.239 the output voltage of 16 cells was much smaller, about 0.6 V. And the temperature curve was rather linear and did not exhibit a maximum at 50 °C. Maybe LTSpice isn't the proper tool for this. Anyway at 20 °C it predicts the cell voltage as 66.25 mV so that seems to be about correct.Regards, DIeter

author: magic
author_rank: Super Contributor
timestamp: November 13, 2020, 12:02:25 am
content: The ΔVbe of course is PTAT (proportional to abs temp). Observe that the schematic has one extra diode on the left, which adds another 600mV to the PTAT stack and cancels its tempco.The rest is precise tuning of current ratios. Simulations with small voltages added in series with only one of the bases or even simply different type of transistor throughout the whole circuit suggested that getting the compensation perfect "by design" is highly unlikely in the real world.Similarly, I don't think we have yet seen a bandgap IC which doesn't include at least a few fuses for production trim. Even LM317.

author: dietert1
author_rank: Super Contributor
timestamp: November 13, 2020, 09:28:22 am
content: Yes, with the extra diode my LTSpice confirms the simulation of razvan784. That extra diode for TC compensation will be difficult again with thermal coupling as it needs to have the average temperature of the others.Also the simulation says that the current ratio needs to be very precise. That 0,239 number in razvan784s simulation isn't random. With 0.238 the flat top of the TC curve moves from 50 °C to 60 °C.Regards, Dieter

author: razvan784
author_rank: Regular Contributor
timestamp: November 13, 2020, 01:59:35 pm
content: Thank you for your interest and remarks regarding the stacked bandgap concept.First, the concept is quite old, as I've said. See for examplehttps://www.tayloredge.com/reference/Ganssle-Pease/bobpease-bandgap.pdfFigure 2, orhttps://patents.google.com/patent/US3271660.I'm not expecting the simulation to translate as such in practice, it was given as a proof-of-concept only.The current ratio is indeed hand-tuned to give a zero TC around 50 °C. It also varies quite a lot with current.As magic has stated, it has to be optimized for noise, which I'm not an expert on at the transistor level. Regarding flicker noise especially, which is important for metrology applications, the 10 mA current is probably way too high for BC847s, but may be suitable for some power transistors. For BC847s, probably around 100 uA would be a better choice - the current ratio would then be 0.184 according to the LTSpice models. It's clear that a prototype, if built, needs to have the ratio adjustable (and highly stable), and probably the main current as well.Regarding thermal matching, I had in mind something like a 4x4 common-centroid layout. If pairs are to be used, the extra transistor in the left string can be paired with the one regulating the current for the right string, which should have similar dissipation to the others. I expect additional measures to be necessary, such as a high thermal conductivity PCB wrapped in thermal insulation and placed in a thick metal box. The whole thing would probably need to be temperature-controlled to achieve very low TC, which is why I chose 50 °C for the top of those curves.Regarding the 10V version, I would be happy to see at least the 1.2V version designed and workingAfter that, up-scaling should be relatively easy.

author: dietert1
author_rank: Super Contributor
timestamp: November 13, 2020, 02:42:21 pm
content: Since we are in the metrology section, here another proposal for a 10 V bandgap reference. It looks somewhat similar to the usual zener reference. This design used only npn pairs, so it should be easier to tame. The diagram has a vertical scale of 400 uV = 40 ppm.Regards, Dieter

author: babysitter
author_rank: Frequent Contributor
timestamp: November 15, 2020, 09:50:12 am
content: You could furble a trace along a space-filling curve into free spaces of your PCB and using it as either a temperature sensor, heater or both.

author: dietert1
author_rank: Super Contributor
timestamp: November 17, 2020, 07:19:49 pm
content: Now i made a prototype with old parts from the drawer. I found some CA3039 6x diode arrays. They make a nice and simple band gap reference. Please note that the circuit works with low gain. The image shows the six foil resistors. R1 is for tuning output voltage and TC compensation (hidden under board).To my complete surprise the TC curve exhibits a minimum in my first tests, need to examine that. Anyway, while it exhibited a drift of about 24 ppm during the first hours of operation, now it seems to be stable in the low ppms, when operating near 35 °C. Due to initial drift i separated the data collected yesterday and today into two curves and line fits.Some may not consider this a discrete solution anymore. In my discrete opinion that PTB paper mentioned by the OP is a joke. The web has nice papers by B. Pease, K.E. Kuijk and A. Paul Brokaw.Regards, Dieter

author: Kleinstein
author_rank: Super Contributor
timestamp: November 17, 2020, 07:33:30 pm
content: The circuit looks nice simple and the number of diodes is still manageable. Any idea how much noise is expected ? I know the spice models on noise are not very accurate, but it would be a starting point, at least for the white noise part.

author: dietert1
author_rank: Super Contributor
timestamp: November 18, 2020, 12:53:17 pm
content: Currently i can't yet answer about the noise. I think one needs to make two prototypes and put them into metal enclosures in order to evaluate noise, so it's some more work to do.Before that i need to understand why i have a minimum in the TC curve where everybody else seems to have a maximum. Maybe the difference is that in my circuit the diode chains aren't running at constant currents, but with resistors as current source.When you read the papers, they mention "curvature correction" - add-on circuits meant to compensate the T² term over some temperature range. While this may be desirable for "utility references", for a metrology application it may be better to run the diode array in a thermostat. Anyway the circuit itself includes a nice temperature sensor.For the measurements above i used a modified NH2 incubator. The modification lets the inner fan run continuously. That way it works a lot better, because its temperature sensor sits on the inner heatsink. In the default factory configuration both fans stop running when the TEC is off. Then the air temperature in the incubator decouples from the heatsink temperature being measured.Regards, Dieter

author: Kleinstein
author_rank: Super Contributor
timestamp: November 18, 2020, 01:16:28 pm
content: The opposite curvature is indeed odd, but it also is a chance, as this means there are 2 contributions to the curvature that could compensate as a kind of curvature compensation.For highest stability a stable temperature is often easier than compensating high order effects. It still helps if the linear TC part is low. Against higher order parts even a relatively small oven is quite effective.

author: tszaboo
author_rank: Super Contributor
timestamp: November 18, 2020, 02:23:55 pm
content: Quote from: dietert1 on November 18, 2020, 12:53:17 pmhttps://upload.wikimedia.org/wikipedia/commons/thumb/6/6d/Bandgap-charakteristic.svg/440px-Bandgap-charakteristic.svg.pngSince you cannot get the exact point, the shape of your curviture will depend on your resistor. Others might be just a bit above the poitn, you might be below. I guess, correct me if I'm wrong.BTW you just had these components lying around at home, including a bunch of VPG resistors?

author: dietert1
author_rank: Super Contributor
timestamp: November 18, 2020, 03:44:28 pm
content: Yes, i bought a bag of those 1K foil resistors some years ago for another project, they were sold as "S102K".And i recovered 9x CA3039 from a 1982 surplus device (Hellige Servomed). When testing the six diode chains with 1K from a 10V supply, the six forward voltages were 6.899 +/- 0.0009 V (stdev) at 19.7 °C. They are monolithic arrays and still fine after all those years. Today i found FSC FSA2720p diode arrays on ebay that appear to be similar.Yes, the resistors are suspects for the TC curve shape. Need to try different ones to learn how it depends. Normally, if the tuning resistor is off, there will not be a flat TC curve at all, but a steep rising or falling slope. In the 40 °C temperature variation i measured, LTSPICE predicts a 10 % change of forward voltage.Regards, Dieter

author: magic
author_rank: Super Contributor
timestamp: November 18, 2020, 05:29:38 pm
content: Quote from: dietert1 on November 18, 2020, 12:53:17 pmOn paper your circuit is perfectly normal. Right side operating current is 8·ΔVbe/200Ω which is proportional to thermal voltage and to absolute temperature - bandgap ICs do the same. Well, assuming that your diodes behave similarly to transistors.Quote from: dietert1 on November 18, 2020, 12:53:17 pmPeter Brokaw's paper states that the simplest method is to use appropriately drifty resistors. What did you use for R1 by the way, a combination of the same foil resistors or something different?

author: dietert1
author_rank: Super Contributor
timestamp: November 18, 2020, 06:34:09 pm
content: R1 is a 1K52 SFernice foil resistor ( < 5 ppm/K), plus a little fine tuning with PTF56 thin film resistors in parallel, all low TC. Will try and wire three of those 1K0000 resistors as a 1K5 and tune it from there. And try another OpAmp, it's in a socket.Regards, Dieter

author: Kleinstein
author_rank: Super Contributor
timestamp: November 18, 2020, 07:41:41 pm
content: I did a quick simulation on the noise, though in version with only 2 of the bandgaps in series.One point is that the 1K Ohms resistors in series with the lower current "diodes" are a major noise source, about half the noise power. So there is quite some gain and having multiple of the "diodes" in series can actually help.Another point is that there is quite some gain also for the OPs noise, it's noise is amplified about 9.6 times.To reduce the OPs noise, there is the option to use one of the transistors in the high current string to actually do some amplification. This way the OPs noise is essentially substituted by the transistor noise that is already there anyway to a large part.The modified circuit seems to also work, though it needs a slightly different trim (probably due to the base current).Attached is the modified circuit with gain from a transistor.The resistor R4 in front of the OP is a way to add some noise to the OP and this way see the noise gain from the noise coming from R4.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 18, 2020, 08:15:46 pm
content: I did the simulation also for the 8 in series case, and it looks promising: The simulated output noise is at some 21 nV/sqrt(Hz) and largest noise source are still the resistors. So chances are there would not be very much 1/f noise from the transistors.However the simulation is with the default NPN transistor model and this gives a rather high VBE. This way there is a lower TC to start with and thus less contribution the PAT part. So with real parts things may be worse. However one could also use parts with a relatively high drop, like the low leakage BAV199 diodes.

author: magic
author_rank: Super Contributor
timestamp: November 18, 2020, 08:33:57 pm
content: With transistors, broadband noise at high currents (mA) primarily depends on base spreading resistance. If it's, say, 100Ω then you get 1.3nV/rtHz equivalent voltage noise in series with each transistor base. In Dieter's configuration that's a total of 4nV/rtHz on the left side and 2nV/rtHz on the right and 4.5nV/rtHz differentially. This noise, and resistor noise and amplifier noise is all amplified 1700/200 = 8.5 times so we should expect more than 40nV/rtHz. Some transistors are better, some are worse, and I have no idea how diodes compare.Another thing is base current noise flowing through base resistance. For 10µA base current (2mA/200), shot noise is 1.8pA/rtHz and rather insignificant with the hypothetical 100Ω. But things get progressively worse below the 1/f corner.---A real world example of curvature correction in accordance with Brokaw's paper:https://zeptobars.com/en/read/AD584-reference-analog-devicesThe bottom right corner is the GND pad. Among a dozen laser trimmed film resistors (green) all over the die, there is a lone (pink) diffused resistor there. It goes in parallel with a film resistor and then in series with another two - this network is the "lower" resistor of the Brokaw cell (R31 per the datasheet). As temperature goes up, this resistor increases in value and the current through it increases linearly too, producing a quadratically increasing component in the reference voltage.

author: dietert1
author_rank: Super Contributor
timestamp: November 19, 2020, 09:19:23 pm
content: In Spice the diode noise formula is basically about the series resistance, with a spec of 45 Ohm for the CA3039. So we should have a source resistance of about 360 Ohms parallel with 1K and that noise times G = 7.5. They also mention flicker noise.For the time being i have a 24 hour log of the initial test setup, HP3456A with a 100 PLC sample every 5 seconds. With a reasonable oven and a clean board that circuit probably ends up at a stdev of 1 ppm or less.Regards, Dieter

author: Kleinstein
author_rank: Super Contributor
timestamp: November 20, 2020, 09:35:00 am
content: The relatively large series resistance with a typical large TC may be the reason for the unusual upwards curvature.So far the noise does not look that good. However there may already be some contribution from the meters reference. An important noise contribution can also be the OP, as it's noise is amplified about 10 times.Overall there is the problem that quite a bit of the voltage, maybe up to 50% but usually more than 25% comes from the amplified PAT part. Because of this the resistors to set that gain have a relative large influence. So the demand on these resistors is quite high. From the crude estimate tThe resistor drift would be attenuated only by something like a factor of 2 to 4. So I am afraid the bandgap reference is not going to give a good long time reference.

author: dietert1
author_rank: Super Contributor
timestamp: December 08, 2020, 03:18:04 pm
content: Some weeks later and after fixing the failed power supply of the incubator, the same CA3039 based prototype still runs at 35.5 °C and its drift rate was -0.20 +/- 0.10 ppm/day (neglecting drift of the HP 3456A). Meanwhile i got some 1K5 foil resistors with 2 ppm/K spec. Also i learned how well an OPA189 chopper amplifier performs with the right set of capacitors around it. So i hope to improve on the noise.Regards, Dieter

author: MiDi
author_rank: Frequent Contributor
timestamp: December 08, 2020, 05:16:58 pm
content: Quote from: dietert1 on December 08, 2020, 03:18:04 pmWould be interesting to know how and which caps you used.

author: dietert1
author_rank: Super Contributor
timestamp: December 08, 2020, 08:42:12 pm
content: In addition to the usual supply buffer caps, i found that a 1 or 2 nF across the inputs helps, a somewhat unusual circuit with an opamp. That cap absorbs charge injection spikes of the input MUX and should sit close to the input pins. Then for loop stability there needs to be a larger cap parallel to the feedback path, like 10 nF. That one should be a low leakage cap in order to save precision. I used this setup with an OPA2189 and both input offset voltages remained less than 10 uV. In that circuit i had a third capacitor from positive input to Gnd, in that case 4.7 uF. But that depends on the application.Today i also received diode arrays i ordered from ebay. They are 1983 Fairchild FSA2720P and appear to be monolithic, too. Don't know yet whether there is a substrate diode as in the CA3039. A first prototype with those diode arrays has seven diode chains with forward voltages of 5.436 to 5.440 V when fed from 10 V over 1 KOhm resistors. One of the chains went from 5.440 to 5.478 V when feeding 10.5 V instead. So the differential resistance of the main chain that runs at 4.5 mA will be about 76 Ohm. Noise should be lower this time.Regards, Dieter

author: MiDi
author_rank: Frequent Contributor
timestamp: December 09, 2020, 11:35:14 am
content: Quote from: dietert1 on December 08, 2020, 08:42:12 pmThanks Dieter!This is very interesting, I plan to evaluate that type of charge injection filter, but for different application.The idea came fromST AN4587 - Input bias current of a chopper amplifier.Keep an eye on output voltage noise figure, it may degrade with this type of filter.Have done some simulations and optimizations for my purpose, diving deeper into this would better fit in appropriate topic rather than here.

author: dietert1
author_rank: Super Contributor
timestamp: December 09, 2020, 12:34:23 pm
content: Concerning output noise: I hope you understood that the feedback network will have a capacitive divider parallel to the usual resistive one. The 1 to 2 nF across the inputs is something that those opamps can drive without instability, i mean that's the limiting spec. The 10 pF proposal in the ST paper sounds a bit strange to me. Probably results from trying to keep some bandwidth. I was talking about a DC amplifier. Please have a look here and at the attached document:https://www.eevblog.com/forum/metrology/low-frequency-noise-of-zero-drift-amplifiers/msg3094594/#msg3094594Regards, Dieter

author: kleiner Rainer
author_rank: Regular Contributor
timestamp: December 09, 2020, 01:33:51 pm
content: I found an old thread about discrete bandgap references here on eevblog:https://www.eevblog.com/forum/projects/discrete-band-gap/msg787286/#msg787286The stuff by "Analog Innovations" is very interesting, this is pro stuff - the guy designed mixed signal ICs at Motorola and later as a design consultant.More design information from IEEE solid-state circuits magazine:http://www.seas.ucla.edu/brweb/papers/Journals/BRSummer16Bandgap.pdfHope that helps.Greetings,Rainer

author: dietert1
author_rank: Super Contributor
timestamp: December 09, 2020, 03:12:24 pm
content: Did you notice that the stuff in that old thread is mostly outdated (1970s)? Of course it is interesting to see the history, but those pioneers were not interested in uV but in mV and they did not look at noise. They didn't have integrated chopper amplifiers. Progress of commercial band gap references towards metrology level appears to be slow, so it may be worth a closer look. If one of the contributors five years ago arrived at a conclusion or solution, i am interested. If the OP wants to show his results, too.Regards, Dieter

author: kleiner Rainer
author_rank: Regular Contributor
timestamp: December 09, 2020, 06:31:19 pm
content: Dieter,what exactly did change in the chemical and physical properties of silicon in the last 60 years? If you want to design analog integrated circuits, you still have the same limitations of silicon that Bob Widlar and Dave Talbert had to circumvent in 1964, when they designed the first integrated opamp for Fairchild, named uA702. BTW only a few years later, George Erdi designed the first precision monolithic opamp, the uA725, wth low input noise voltage and current down to 10Hz (and 1974 the OP07 together with zener zapping for wafer trim of the offset voltage, and then the LT1001 and LT1013, only about 40 years ago...).It helps to remember that we are standing on the shoulders of giants, who invented the basic building blocks we still use.If you want newer information about bandgap references, I have this:https://www.iee.et.tu-dresden.de/iee/eb/forsch/AK-Bipo/2007/AKB2007_10_Berkner_BG_Ref_Simulation_101102.pdfOh, sorry, still stone age (2007)Newer research on bandgap references seems to concentrate on CMOS solutions simply because they are needed for mixed signal systems. Most interesting research tends to hide behind paywalls, it seems.I expect that niche applications like ultra precision references will be harder to produce in the future, simply because they are expensive to manufacture, need special processing and fabs and the semiconductor companies can earn more money with less demanding mainstream products. Maybe we should relearn how to select and temperature compensate zener diodes - unless they also disappear like the low cost hermetic package? Or LT as a separate company for that matter?Greetings,Rainer

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: December 09, 2020, 06:39:09 pm
content: 1N829 and similar have gotten too expensive to use in many applications, typically $10 USD or more, and I bet the yearly production has gotten smaller and smaller. A DIY reference is a good thing to have in your back pocket and the exercise of developing and understanding it keeps one sharp.

author: dietert1
author_rank: Super Contributor
timestamp: December 09, 2020, 09:04:08 pm
content: This thread starts with a report that at the German national institute PTB they tried to demonstrate a low noise, ppm-accurate band gap reference which can compete with the usual zener references, but to no avail until now. I think it is an ongoing quest without a good answer.Regards, Dieter

author: Kleinstein
author_rank: Super Contributor
timestamp: December 09, 2020, 09:58:39 pm
content: There has been quite some research on this topic and some commercial band-gap references are indeed low noise (e.g. LTC6655), but not really superior to zener based refs.The big downside that I see is the dependence on a stable resistor ratio to scale the PAT part. With the usual circuit the resistor drift should only be attenuated by a factor of about 2-4. So rather poor compared to the compensated Zeners that get a ratio of 50-100.The upside is that 8 x the bandgap voltage is relatively close to 10 V, so the scaling resistors would be less sensitive. For a 10 V reference it may thus still be competitive - just the critical resistors change: the 7 V zener has critical 7 to 10 V scaling. The bandgap has critical PAT scaling but possibly less critical voltage scaling.Going unconventional I would prefer the JFET based (JRV) reference as in a parallel thread. It is shown to be low noise (especially for the higher frequency range) and 2N4391 are still available in TO18. Extreme parts (threshold near 10.6 V) may even directly give a near 10 V reference.

author: dietert1
author_rank: Super Contributor
timestamp: December 09, 2020, 11:23:52 pm
content: Meanwhile i finished a second prototype with the 7-fold diode arrays and put it into the incubator. Noise improved and there is no longer any significant excess noise over the noise of the HP 3456A. This time i see a maximum of the TC curve. Since the new diode arrays have much lower diode resistance the maximum is now closer to the ideal band gap voltage 8 * 1.22 V = 9.76 V, maybe at 9.88 V. The adjustment resistor went from 1K5 down to about 1K05.Need more time to locate the flat top and watch the drift, but looks promising. Probably will try to get those diode arrays in ceramic case. The plastic ones were US $ 1.95 each, CDIP will be bit more expensive.Regards, Dieter

author: EmmanuelFaure
author_rank: Regular Contributor
timestamp: December 10, 2020, 12:52:50 am
content: An old application note for the LM194/LM394 matched NPN transistor shows a schematic for a bandgap reference :https://www.ti.com/lit/an/snoa626b/snoa626b.pdfThose parts obsolete for >10 years, but Analog Devices are still manufacturing a matched NPN pair in an hermetic package, the MAT12. It's maybe worth a try?https://www.analog.com/en/products/mat12.html

author: dietert1
author_rank: Super Contributor
timestamp: December 10, 2020, 01:40:35 pm
content: Thanks for the LM194 app note, but please try to understand why that circuit outputs 20 uVrms noise on 1.21 V output. It means about 60 uVrms on a 10 V output, in case you have eight of those. As far as i remember a good zener reference like the LTZ1000 is specified at about 1 uVrms. That band gap circuit isn't even close.When you wire up transistors as proposed, you are essentially testing the band gap of their base-emitter diodes. The left LM194 transistor gets about 0.6 V / 40 KOhm = 15 uA. But this is the collector current. At hfe around 300 it means the base-emitter diode gets only 0.05 uA. The other transistor gets even 10 times less - only 5 nA.In my circuit the diodes get about 4.5 mA and at 0.64 mA. That makes a difference. The diode arrays i am using are good for 100 mA, so i may use them at 9 mA and 0.64 mA to reduce the PTAT term even further.Regards, Dieter

author: EmmanuelFaure
author_rank: Regular Contributor
timestamp: December 10, 2020, 01:51:08 pm
content: It was designed to be a low power reference, powered by batteries, as written in the text. The collector currents can be increased by changing the collector resistor values, lowering the noise...

author: dietert1
author_rank: Super Contributor
timestamp: December 10, 2020, 03:14:48 pm
content: Hope you will demonstrate your version soon.Regards, Dieter

author: David Hess
author_rank: Super Contributor
timestamp: December 10, 2020, 06:32:02 pm
content: Bandgap reference noise is high for the reasons given but it can be lower if operated at higher currents. Ultimately noise is limited by the base-spreading resistance so lower noise should be available in a discrete design by using ring emitter transistors and similar. The LM194 and MAT series of integrated transistors also have lower base-spreading resistance than common transtors.

author: dietert1
author_rank: Super Contributor
timestamp: December 12, 2020, 08:28:30 am
content: Here is a log of my second diode array based bandgap reference, a 5 sec time series with a HP 3456A in 100 PLC mode. The reference is in the incubator at 30 °C and we can see the thermostat cycle at a magnitude of about 11 ppm. We can also see the 1 ppm steps of the HP 3456A. The measurement is noise free at this level.Strange enough this reference shows zero TC at about 9.51 = 8x 1.19 V.Regards, Dieter

author: dietert1
author_rank: Super Contributor
timestamp: December 21, 2020, 11:17:43 am
content: Made a simple demodulator for the waves introduced by the incubator cycle (about 70 to 90 samples). First step is a running average of 20 points and then determine the average of nearby min and max values. In the end the modulation helps to overcome the 1 ppm = 10 uV resolution limit of the HP 3456A used as logger. In other tests an intrinsic noise level of 0.4 uV was found for similar measurements with a HP 3456A.The diagram shows per hour averages over one week. Vertical scale is 1 ppm per unit. The blue data points are statistically independent from each other. We observe a mixture of a process or multiple processes with very low noise. First there was some exponential decay, probably humidity. Between the 17th and 20th of December appear 24 hour cycles, probably ambient temperature.Noise as determined from hour to hour differences is about 1 uV, including the reference noise of the LM399 inside the HP 3456A. For the time being i think it's worth to put that band gap reference into a hermetic linear oven to make it into something useful.Regards, Dieter

author: tszaboo
author_rank: Super Contributor
timestamp: December 25, 2020, 12:53:54 pm
content: Quote from: dietert1 on December 21, 2020, 11:17:43 amYou can reduce the effect of that by putting a few bottled water int the chamber, and covering the PCB with styrofoam.

author: dietert1
author_rank: Super Contributor
timestamp: December 25, 2020, 06:20:27 pm
content: The reference board has already been wrapped and put it into a plastic bag to protect it from the incubator inner fan. The strong modulation of several ppm shown above is a fast reaction, maybe thermal EMF. Curvature at the flat top of the TC curve cannot explain that modulation.Soon there will be two precision ovens, similar to our LTFLU references. They will sit on a standard Fischer heatsink. And there will be a plastic cover with 1.5 to 2 cm styrofoam isolation inside. There is a 10K thermistor for fast temperature control. Fine control will depend on the main diode string voltage (TC about -16 mV/K). In order to check how low reference noise can be, i want two of those.Also i need to understand how to tune the new references a little closer to 10 V. And the TC curves may look different with different output voltages.Regards, Dieter