Isotech MicroK Temperature Bridge - DIY approach? - Page 1
Forum section: electronics / general

author: Echo88
author_rank: Frequent Contributor
timestamp: July 09, 2022, 12:11:34 am
content: Ive always been interested in very high accuracy and stability temperature measurement.Therefore the inner workings of contemporary devices like the Fluke 1595A/Isotech MicroK are of great interest to me and maybe also to people who are just interested in high accuracy resistance measurements based on the substitution principle.Fortunately the MicroK-designer Metrosol has published a couple of documents describing a few details on how they achieved the impressive specs of the MicroK (and the related automated resistance bridge calibrators).I tried to replicate/simulate those details in LTSpice for sensors like the PT100/25 (thermocouples not yet considered in the simulation) and am evaluating what kind of performance if achievable with a selfbuilt version, also since ill most likely will never be able to buy/have access to such devices.Of course temperature metrology in general is expensive when considering triple point cells/SPRTs and so on, no doubt about that.Attached are two simulation versions: a working one and a more detailed nonworking one, with a "keep warm"-current source matrix.Asking for suggestions on the circuit in general and these thoughts:As far as i understand it theres no need for a difference amplifier with its good CMRR like done in the HPM7177, since the measurement resistor is always connected to virtual ground and therefore doesnt see a shifting common mode voltage, is that correct?Maybe theres a better amplifier/adc-driver topology which might use ADCs like AD7177-2 / AD4630-24...i still have to work through the correct design of SAR/DS ADC drivers.Still unsure about the cascode and how to actually construct it in real life to get stable results.Switching could be done via solid state relays, which should work just fine with their <10pA lekage current and <10nV offset voltage. But maybe there are better switching circuits based on discrete implementations with JFETs/MOSFETs i dont know about yet.The DIY implementation lacks the bridge aspect when using a simple ADC, see publication3 point 4.Internal temp stabilized reference resistors like Vishay VHA/VHP are of course necessary, also visible in F1575A-teardown photos.Linearity calibration/sanity check can be done with a DIY resistance bridge calibrator, also developed by Metrosol as "RBCA". But that topic would be worth another post.The documents regarding the Fluke 1595A are also a great read, with their power supply design and dual ADC-technique.I include some links to the relevant documents.https://www.metrosol.co.uk/publications.htmMicroK design publicationshttps://isotech.co.uk/products/microk-precision-thermometry-bridge/https://isotech.co.uk/wp-content/uploads/2020/09/precisionthermometer-microKvol1.pdfMicroK-DShttps://eu.flukecal.com/de/products/temperature-calibration/digital-thermometer-readouts/super-thermometer-1594a1595ahttps://download.flukecal.com/pub/literature/Recent%20Advances%20in%20Resistance%20Thermometry%20Readouts.pdfF1595A design detailshttps://xdevs.com/doc/Fluke/1575_1590/Teardown/photos of older Fluke 1575Ahttps://ohwr.org/project/opt-adc-10k-32b-1cha/wikis/homeEdit:https://www.analog.com/en/analog-dialogue/articles/why-does-voltage-reference-noise-matter.htmlRefnoise raw/filtered considerations on AD7177 as examples. Paralleled LTC6655LNs might be sufficient compared to a ADR/LTZ1000 for a ratiometric bridge measurement where the resistor determines the stability, a Thermocouple-config would need a ADR/LTZ1000 of course.Edit2:https://www.eevblog.com/forum/metrology/theoretical-limits-1kohm/msg4289722/#msg4289722A 35page document analyzing the MicroK with commentary from Paul BramleyMany more details emerge from it, which makes it easier to understand the design choices and inner workings. It explains the cascode circuit in better detail, confirms the usage of OptoFETs, bootstrapped amplfiers, weighted measurement samples and so on.https://www.researchgate.net/publication/326682660_A_Fast-Multi-Channel_Sub-Millikelvin_Precision_Resistance_Thermometer_Readout_Based_on_the_Round-Robin_Structure/link/5b737203299bf14c6da24fbe/downloadA paper on a similar project which uses the AD7177-2.

author: maat
author_rank: Regular Contributor
timestamp: July 10, 2022, 11:24:32 am
content: I did build similar design for a temperature controller for my university project. I haven't published it yet, but it is in the making. Anyways, I don't mind sharing the details of that design. My design was onlysimilarin the regards, that it was built specifically for a 10k resistance, so there are no switchable ranges (but it could be added). The other major difference is, that the ADC is not a discrete design (or special ASIC), but rather an LTC2508-32. The design has two input, so two LTC2508-32. The input MUX is a MAX329. The input Buffers are two LTC2057 per ADC. I tried several other op amps (including composite techniques and bootstraping), but this one gave the best results. Btw. the OPA189 was horrible in this situation. The reference is an LTC6655-4.096, filtered using a composite amp with an LTC2057 and an ADA4898-1. The current source is built using an OPA827 and a good resistor (metal foil) and then cascoded using an SMP4338. The guard amplifier is another OPA827.I will try to answer some your questions or at least give you hint into the right direction.Quote from: Echo88 on July 09, 2022, 12:11:34 amWell, that depends on how far you want to go with the transfer. If you limit that to the 5% range of the µK, than it is not issue. If you would like to compare 100 -> 1k, then you will of cource need to cover that voltage range with a decent CMMR.Quote from: Echo88 on July 09, 2022, 12:11:34 amI can totally recomment the LTC2500-32 series, because this is more a problem of resolution than anything else. You are basically feeding the reference into the ADC and also using it to drive the current source. So most of the noise cancels out.Quote from: Echo88 on July 09, 2022, 12:11:34 amI used the MAX329 and the biggest 'issue' in the end is the PCB. You can see humidity creeping in. Covering the typical humidity range of 30-60 %rH this got me ~0.5 ppm uncertainty. Compraing to the second channel (or internal reference, I used a VHP101) this dropped to 50 ppb and was mostly due to cabling setup. So I simply used an autocal routine to get rid of the humidity drift. The tempco is about 10 ppb/K.I can get into further details if you have any questions regarding that design.

author: Anders Petersson
author_rank: Regular Contributor
timestamp: July 10, 2022, 12:24:22 pm
content: I don't grasp the details here, but if someone makes an open-source design I will build it.

author: Echo88
author_rank: Frequent Contributor
timestamp: July 10, 2022, 12:46:17 pm
content: Yes, please get as much into details as you can maatI would use a COTS-ADC like the AD7177-2, LTC2508-32 or AD4630-24 since the ADC-design of the MicroK is way to intricate for me and a lot of the details of the ADC itself are hidden in the FPGA.How much such the ADC limits me in terms of noise/INL etc. i need to calculate.Im still unsure wether a SAR or SD-ADC would be best in this case, as the SD-ADC is inherently integrating while the SAR would need to be filtered more to avoid picking up EMI? Need to read up on that topic. Also wether a FPGA is really necessary for data stream collection and if it has benefits comparing to sample averaging in the ADC itself.Apparently the AD7177-2 has lower noise compared to LTC2508-32 according to the following document, but im not fixed on using the AD7177-2, at the moment it just seems to be the best performing ADC for the task:https://www.researchgate.net/publication/325285614_Analog-to-digital_conversion_beyond_20_bits_Applications_architectures_state_of_the_art_limitations_and_future_prospectsNikolai Beev comfirmed the INL of ADCs like the AD7177-2 to be stable over time (fingers crossed), so i hoped the INL of the whole device could be improved by calibration with a DIY Resistance bridge calibrator (Metrosol RBCA) and subsequent INL-correction-fit (like branadic suggested for the HPM7177 from Marco Reps).Of course building/checking such a RBCA is another whole can of worms....I was hoping that the LTC6655 might be good enough regarding short term stability (0.1-10s?) to be able to be used in this project, paralleling them would get the noise down to LTZ-level.I need to see wether we have information on the forum regarding short term stability of LTC6655 vs LTZ/ADR1000.How exactly did humidity influence your measurements? By influencing the leakage currents/voltage reference drift? Yeah, regular autocals are necessary to ensure stable performance. The sample technique used in the MicroK is also interesting in this regard: weighted 3-sample-measurements, which were reversed and double weighted for the second sample to apparently cancel drifting offsets.For the MicroK-amplifier they apparently used differentially paralleled and series OP-stages to get low voltage noise and high linearity while bootstrapping helps with CMRR.I suppose your VHP101 is thermally stabilized by a heater or TEC?I will look into the ADC-topologies and the suggested LTC2508-32 and MAX329.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 10, 2022, 01:30:00 pm
content: I low resistance bridge or resistor measurement system works with a low votlage and can thus use ADCs with a 2.5 or 5 V full scale range.Much of the DC offset errors can be eliminated by reversing the polarity - it looks like the fluke system does this in some way. So they may not need AZ amplifier at many places.Due to the often low voltage (may be less of a problem at 10 K, but would be at 100 ohms) the noise of the ADC may be the lesser problem than the noise of the amplifier(s).From the describtions I have read so far, it is not clear to me if the current cource and ADC use the same reference. If they do, the common mode rejection for the amplifier / ADC input becomes a major concern.If they are fully isolared to allow easy switching on both sides, the references must be very low noise - at least for the shorter time like some 10 seconds.It depending on which of the 2 ways is used the reference are relatively uncritical or at least critical for the noise. The long term drift and TC would not matter.Using DC with a perodic polarity reversal is another way of saying that they use low frequency AC with a square waveform. This does not fully remove possible errors from dielectric absorption, though a low frequency helps - though the square waveform also contrains some higher frequency parts. So they may have to add waiting times after a reversal.

author: Echo88
author_rank: Frequent Contributor
timestamp: July 10, 2022, 02:51:55 pm
content: I was wondering wether they use AZ-OPs for the amplifier or some JFET-OPs like OPA140 or similar to ensure low Biascurrents that dont divert current from the current source. Maybe the biascurrents are also nicely balanced in the amplifier topology. The noise impedance is apparently "a few hundred ohms".The current is reversed during sampling to eliminate TEMF and drift, yes.The current source and ADC use the same reference, most likely a LTZ1000 "The microK employs “a very good buried zener reference”".Why would CMRR become a major problem when the current source and ADC share the same reference? One input of the differential amplifier is connected to the virtual ground of the guard amplifier and therefore theres no problematic common mode voltage.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 10, 2022, 04:40:59 pm
content: The CMRR would become an issue if they use the circuit they show in the fluke flyers with the resistors in series.The plan in LTSpice format is different and way less critical with CMRR. As far as I see it the CM signal is very low, just the lead resistange acting on the drive part and casode part.That plan looks much more sensible.When they use polarity inversion, there is no real need to use AZ amplifiers, as the offsets would also eliminate, though possibly with a relatively low frequency. As the system is a bit older I don't think they had the OPA140 available and for only some 100 Ohms BJT based amplifiers would be the more obvious choice.It depends on the noise level aiming for and the resistor range. For 10 K a single OP27 or similar may aready have too much bias and current noise and a JFET based solution can be better. For 25 ohms the BJT based solutions would be better unless using massive parallel ones.It is also not just the amplifier towards the ADC, but also the amplifier to provide the virtual ground towards the current source that matters. For the bias current the amplifier for the virtual ground is actually the relevant one. The amplfier towards the ADC is not critical with the input current or current noise as it is driven by the other amplifier and sees only the lead resistance.For the bias it is only the change in the bias - the medium value is just part of the test current.So one may end up with a combination of something like OPA140 or ADA4625 or discrete JFETs for the virtual ground and something like LT1028 for the ADC amplifier.I don't see a need for a super stable reference and a 1N825 or similar Zener would be good enough. For an ADC chip based version it would be OK to have a low noise bandgap refrence, as it is mainly about the noise maybe for a frequency range of 0.5 Hz to 100 Hz. So the TC and long term drift noise not really matters.Due to the relatively fixed voltage switching with JFETs would be possible to, at least for the critical side towards the current source. The side towards the amplifier is uncritcal.

author: Echo88
author_rank: Frequent Contributor
timestamp: July 11, 2022, 04:09:43 pm
content: Ah yes, the Fluke uses Standardresistance and Sensor in series, with a amplifier and ADC each. That reduces the current ref stability/noise requirements since they cancel out mostly when the samples of both ADCs are used, but leaves the necessity of amplifiers with super high CMRR to get the wanted ultra low error specs.Shame we dont know which ADC the Fluke uses, i suppose its a discrete Multislope-implementation?The MicroK uses a substitution topology to get around the CMRR-problem.Your statements regarding virtual ground/amplifier-OPs makes sense. I wonder why they didnt use the LT1028 or equivalent and instead chose a parallel/series-OP-implemetation that still only resulted in a few hundred ohms noise equivalent when the input bias current isnt relevant as you suggested.If the amplifier is supply-bootstrapped for additional CMRR (questionable if useful in this case with the limiting ADC), then im wondering if the voltage reference which is used for Vcm is able to be used with its fixed ground.Im wondering which switches should be used generally for switching the resistors: JFET-based (cumbersome, maybe level shifting needed?), OptoFET-based (AQW210S 35R, a few pA) or Analog Switches like the proposed MAX329 (2.5kR 1pA) or MUX36D04 (125R 1pA). At 10mA measurement current the MUX36D04 and AQW210S are usable, the MAX329 not. TEMF-problems due to the switch resistance * measurement current can be alleviated by usage of thermal jumpers like THJP0612 and are averaged mostly anyway.Still unsure about the ADC-choice. The AD7177-2 is easier to layout and its frontend is proven and measured by Nikolai Beev. The AD4630-24 on the other hand sports very good INL even when used with a single ended buffer and seems to be reasonably well to drive given its SH-precharging circuit, assuming EMI is taking care off.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 11, 2022, 04:25:28 pm
content: For the ADC choice a parallel configuration mainly makes sense when there is no single OP to deliver the performance.The LT1028 can be a bit tricky, as it is quite fast and only stable with some gain, but there is a similar version that is unity gain stable.The critical amplifier is still the upper one, and there the current noise does matter.For the switches the 2 sides are sparate: on the upper side leakage matter and JFET switching would not be a big deal, as the switches are at a virtual ground. So just a 0 and -8 V signal would be enough.For the low side leakage is not relevant and relatively simple CMOS like DG411 would be an OK choice, maybe with separate chips for the drive and voltage sense to avoid thermal effects.The switches that carry the current and those that measure the voltage are separate.For the Fluke series connection it is not just CMRR of the OP, but it would need some differential amplifier / INA with good INA: so also well matched resistors and maybe an adjustment / test for the CMRR.