6.5 digit multimeter challenge - Page 1
Forum section: electronics / projects

author: hve
author_rank: Regular Contributor
timestamp: April 11, 2025, 11:37:49 am
content: I have been looking for 2nd hand 6.5 digit multi-meters but they are still quite expensive minimum 150 USD, and still you would probably need to calibrate and fix stuff like displays, batteries elco's in order to really use it..So I was wondering is there a way to create 6.5 digit multi-meter for under 50 USD?Nowadays you can easily find a 24bits ADC CS1237 for 2 USD or lowerhttps://nl.aliexpress.com/item/1005007167589068.htmlwhich should in theory be more than adequate. Combined with a suitable voltage reference and a nice analog front-end... and one of those ultra cheap micro-contolllers like for example esp32 or ch32...Lets first select the most important component:Latest suggestion is to useADS125H02Of course it all depends the requirements, so in order to set some limits I would propose these baseline requirements:User interface controlled via an isolated computer interface: USB | RS232no WLAN" or similar RF radiators that might compromise the readings or the EMF in your lab.Voltage reference LM399 or better in terms of basically long term stability.Range requirements based on Agilent Keysight 34401A specs but only VDC and ADC ranges (No AC, No Resistance, just basic VDC and ADC)DC voltage 100.0000 mV 1.000000 V 10.00000 V 100.0000 V 1000.000 V DCDC current 10.00000 mA 100.0000 mA 1.000000 A 3.00000 AMinimum 5 readings per second on highest resolution.Overvoltage and over-current protection, and maintaining those specifications after significant voltage/current overload". (thanks tggzzz)

author: xvr
author_rank: Frequent Contributor
timestamp: April 11, 2025, 11:46:10 am
content: QuoteYes, and spend another $50000 for calibration and sertification

author: JohanH
author_rank: Frequent Contributor
timestamp: April 11, 2025, 12:02:37 pm
content: I built the one by Barbouri a couple of years ago (it's based on other designs on the Internet). It's max 40V, but nothing could prevent you from designing other input ranges.https://www.barbouri.com/2019/08/07/millivolt-meter-version-2/But good luck building one for 50 USD. And you still have to calibrate it.

author: squadchannel
author_rank: Frequent Contributor
timestamp: April 11, 2025, 12:08:58 pm
content: how do you calibrate it, unless you have an 8.5 digit meter?You should buy a hp3478A instead of building your own meter.https://www.ebay.nl/itm/306231916025

author: hve
author_rank: Regular Contributor
timestamp: April 11, 2025, 12:12:33 pm
content: Nice design! Yes of course, calibration is key, also for the vintage stuff that is on sale sometimes.The local uni will let me use their calibrator if I ask nicely..I will skip the 50000 dollar certification though

author: tggzzz
author_rank: Super Contributor
timestamp: April 11, 2025, 12:24:09 pm
content: Quote from: hve on April 11, 2025, 11:37:49 amAdd "maintaining those specifications after significant voltage/current overload".

author: JohanH
author_rank: Frequent Contributor
timestamp: April 11, 2025, 12:27:09 pm
content: I also recommend using something better than an Arduino. Even though I'm no experienced C programmer, I came to the conclusion that the Arduino floating point (32-bit) was limiting calculations and lost some accuracy due to that (and it's kind of slow). A processor that supports double type (64-bit float) would be much better, and faster if it supported floating point in hardware. Maybe it could be done in some other way. You can see the code here:https://github.com/hevanaa/6-digit-millivoltmeter/blob/main/MilliVoltMeter336/MilliVoltMeter336.inoIf I remember correctly, the previous designs by the other people used the absolute voltage as measured, then added a calibration offset. I never got this working correctly, there was always some offset at some point of the scale. I changed it to ignore the absolute value, instead measuring and calibrating three points, and linearly calculating the voltage in-between the calibrated points. That's why many digits matter in the calculation.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: April 11, 2025, 01:51:21 pm
content: If you think USD150 for a 6 1/2 digit DMM is expensive, you've probably got some loose screws somewhere. I am wanting a benchtop DMM too, but I can't really justify the cost of one just for my hobby. Instead I bought a Brymen BM869S for around EUR 180, and it would be difficult to find a DMM with better specifications for that price point.But that said, You can DIY a voltmeter with limited input range and protection for a smaller BOM cost, especially if you don't add the housing and power supply to the BOM. And once you factor in your time / "salary" for building one, it becomes a lot less attractive again. But also, Designing and building your own measurement equipment can be fun and educational.So, I'm not saying you should not DIY, but do stand still for a moment and think about what you want to achieve and what expectations are realistic. consider the different aspects of cost, resolution, accuracy, (also linearity), long term drift, calibration, robustness, etc.

author: hve
author_rank: Regular Contributor
timestamp: April 11, 2025, 02:39:06 pm
content: Not everything is just about cost benefit. In the end it could be, with the collective power of the forum, who knows...As I was looking around for a decent bench multi-meter, there was one on offer for 125 euro, it is an old Philips PN2534 powered by a good old 8031, and using a BZV14 voltage reference (5ppm per deg celcius) and some rusty terminals... Maybe at least replace the voltage reference probably and definitely do a re-calibration.Also I still think it can be done for less than 50 euros for only DC voltage and DC current and maybe even limiting range to 40 V instead of 1000V.Yes I have certainly some loose screws in there, be sure of that

author: hve
author_rank: Regular Contributor
timestamp: April 11, 2025, 02:47:29 pm
content: Quote from: Picuino on April 11, 2025, 12:39:29 pmI like this setup, the voltage reading is pretty stable!,Definitely al least 6 digits of resolution!

author: hve
author_rank: Regular Contributor
timestamp: April 11, 2025, 03:02:08 pm
content: Just a quick scan...LTC2400 (INL ±2ppm, 2.5μV offset, 0.3ppm noise) +- 8 usdCS1237 (INL less than ±1.5ppm, ±1.4μV offset, 180nV @ 10Hz noise) +- 2 usdStill I have no clue on analog front ends. It has been a while since I mada an opamp circuit...

author: Kleinstein
author_rank: Super Contributor
timestamp: April 11, 2025, 04:00:46 pm
content: For hobby use one may not need a true calibration. Some DMM types (e.g. HP34401) usually hold there accuracy even over very long time.For a DIY build the calibration problem is even larger, as most labs would not calibrate those. So building the circuit is only half the fun - testing and adjusting it can be the more challanging part, especially if one does not already has a better meter.The ADC is only a small part of a DMM. Alone parts like the case, transformer (may want extra shielding) and connectors add up quite a bit. One could save quite a bit if one accepts a lower maximum voltage (like 300 V) so that more normal relays and resistors for the divider could be used. A good 10 M divider, worthy of 6 digits alone can be on in the $50 range. Similar good shunts can get expensive.For the ADC chip, it is not so much the noise to look at, but getting a good INL. Here the problem is that the INL can depend on the drivers for the input and reference, especially with the lower noise types. So the INL specs from the DS are only a hint on what a good system can do. Ideally one would still need to check with the actual driver.The ADC chips are usually for a small input range like +-2.5 V or +-5 V (with differential inputs). It add some extra effort to get a +-10 V range with high impedance. In addition they usually want a 2.5 to 5 V reference and thus need an extra divider when using a 7 V reference. The divider for the ref. add to the costs / drift. It is possible (e.g. the SDM3065, Rigol DM3068 work this way), but not easy.I have build a DIY DMM around a µC controlled multi slope ADC. However the parts costs are more in the $150 range for roughly 7 digit performance and limited to some 300 V and DC only. There are a few parts to build cheaper with slighty reduced performance, but I don't think one would get down all the way to $50.

author: SiliconWizard
author_rank: Super Contributor
timestamp: April 11, 2025, 04:28:01 pm
content: The datasheet of the CS1237 is in chinese, if you're keen to read it.We can see a claimed ENOB of 20 and INL of 0.0015%, which is about 250 LSB. That's quite a bit for a 24-bit ADC. I think it'll be tough to have 6.5 actual digits with that alone.You can find better, but much more expensive 24-bit ADCs for this purpose.And, as Kleinstein said, the ADC is just one part of the deal. You'll need to care about the accuracy and stability of the references, attenuators, and make sure the protection circuits won't also degrade performances. All that explains the complexity and price of a 6.5 digit DMM.You'll be able to display 6.5 digits with a cheap solution for sure. If you want to see 6.5 digits, that'll work. If you want those digits to actually mean something, that's another ten years of work for a single person that just starts investigating the topic.

author: David Hess
author_rank: Super Contributor
timestamp: April 11, 2025, 07:14:47 pm
content: Quote from: hve on April 11, 2025, 11:37:49 amThe CS1237 is only linear to 16 bits or 4.8 digits, so it is not even close to sufficient.Besides the calibration problem, you would still need to design and implement a 6.5 digit input buffer and attenuators. If you are going to go to that much trouble, then use a better but inexpensive ADC like a MAX11202 or MAX11210, or for a still reasonable price, an LTC2376-20.

author: D.Burnette
author_rank: Contributor
timestamp: April 11, 2025, 08:11:15 pm
content: Depending on your needs, take a look at fleabay for an HP 34970a, and a 34901a/34902a card. Sometimes they show up real cheap...And the DMM card in those is virtually identical to the HP 34401a 6.5 digit DMM.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 11, 2025, 09:06:22 pm
content: With the INL specs for the ADCs one may have to check the fine print. There can be a big difference between the typical and max values. The max values are often set by the test limit: INL testing, especially with slower / more noisy one can take a long time and thus be expensive. This is somewhat similar to leakage specs, where there is also often quite some difference from max to typical.For the higher grade DMMs it is normal to have INL error limits that are several LSBs. At the high resolution this is kind of normal as testing is tricky and the INL is the much more difficult part than noise.

author: PascalNE
author_rank: Regular Contributor
timestamp: April 11, 2025, 09:10:32 pm
content: I've had a look at designing my own DMM (I'm still in the process) It does seem deceptively simple until you realise you need Input buffering high accuracy voltage dividers and amplifiers. PCB design also has to be very good to effectively implement the ADC and shield it from noise. And voltage references is an infinite rabbit hole you can go down. And if you want resistance measurement there's a lot of other considerations and precision parts necessary.Additionally implementing USB and WLAN can be tricky if you want high speed measurements.BUT if you aren't chasing sub 10ppm like me and don't mind 50-100ppm it gets a whole lot easier and would be possible for $50-100 in pure component cost if you don't mind something unrefined and are only looking to use it yourself.

author: PascalNE
author_rank: Regular Contributor
timestamp: April 11, 2025, 09:18:37 pm
content: There's also this project by Jaromir that might be of interest.https://hackaday.io/project/174022-diy-6-digit-multimeter(I've also attached the schematics he posted)

author: PCB.Wiz
author_rank: Super Contributor
timestamp: April 12, 2025, 12:51:56 am
content: Quote from: hve on April 11, 2025, 03:02:08 pmAnother part to consider could be Nuvoton NADC24, good noise specs but more modest on INL and not well defined on 50/60Hz rejection. (noise specs below, 31.25 sps ?)With cheaper parts, the data is more sparse, but I have seena on board calibration DACused to good effect on geological ADCs.Parts like 74AC74 and a good clock source, can create a PWM DAC that can calibrate check your ADC at multiple points, allowing you to get a handle on the INL effects.The template PWM is created from a MCU, and then an external clock and precision VDD is used on the separate 74AC74 to remove MCU noise and jitter.Addit: I thought I'd seen some work on here using a Pi PICO multislope (using the PIO?), but google finds thishttps://hackaday.io/project/190528-multislope-adc

author: David Hess
author_rank: Super Contributor
timestamp: April 12, 2025, 02:06:44 am
content: Quote from: PCB.Wiz on April 12, 2025, 12:51:56 amThe datasheet I checked for the CS1237 says 10 times that INL error *typical*, not even maximum.

author: NiHaoMike
author_rank: Super Contributor
timestamp: April 12, 2025, 04:16:07 am
content: It can be done, but not easy.

author: jeduffy
author_rank: Contributor
timestamp: April 13, 2025, 01:19:23 am
content: Someone has built their own:https://hackaday.io/project/174022-diy-6-digit-multimeterHowever it is *significantly* more complicated than just an ADC and a reference. Mad lad who made this one made his own multi slope adc.I'm working on a ~4 digit handheld one (https://hackaday.io/project/176607-hydrameter) and even that is far from simple.A big problem you'll run into is calibration, a 6.5 digit meter isn't much good unless it's precise and accurate, meaning you not only need to calibrate once to get precision, but maintaining that accuracy over time is also difficult.It's not impossible, I'd love to see this be a thing, and it's definitely already on my radar down the road, but unfortunately, not something that can just be thrown together easily.

author: David Hess
author_rank: Super Contributor
timestamp: April 13, 2025, 04:52:33 am
content: I did some quick calculations to check feasibility.6.5 digits will require an input buffer with common mode rejection of 126dB. An excellent part like the JFET input OPA140 is 120dB minimum, so does not quite make that requirement without grading. Bootstrapping the input stage would correct this, or OPA140s could be graded. The max input bias current of 10pA is too high, and even the typical specification of 0.5pA is a little bit too high, so external input bias current cancellation will be required.Automatic zero correction is commonly (always?) found in higher resolution meters, and maybe it should be used here, but I think a very careful design could avoid it at 6.5 digits without too much compromise. I am not sure how to implement automatic zero circuitry without impacting the input bias current.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 13, 2025, 07:11:03 am
content: The input bias requirement is not that strict. With the high resolution one no longer expects a good zero reading with an open input with 10 M input resistance. The typical specs are more like <50 pA ( a few claim < 20 pA) and an input impedance of > 10 Gohm for a 10 V and lower ranges.Another point is that a 6 digit meter does not need to have all errors below 1 ppm -- often the specs are more in the 3 ppm range.For the CMRR the really critical part is only the nonlinear part of the common mode sensitivity. The linear part would only change the gain so slightly, which would not be an issue. One still has the problem that a normal OP-amp amplifier may not be good enough, though the better types have a good chance to work.Automatic zero correction is not absolutely needed, but one expects a pretty stable zero (e.g. < 1 µV drift and LF noise over 4 hours). To reach this the auto zero switching at the input is one of the practical options. The difficulty with auto zero switching directly at the input is not just the extra leakage / bias. An even trickyer part is the switching current spike, if one wants to keep it small.An alternative is the use of a zero drift amplifier. Here the difficulty is, that these tend to have quite some input bias current - especially those for higher voltage. One could still use a selected unit of something like LTC2057 or OPA186. With bootstraping the supply there are more chips to choose from, though one may still have to check and if needed change. With the AZ amplifiers the worst case bias tends to happen more near the ends of the range and with resistance imbalance. If one avoids this, one can often get bias better than the typical specs. If the ADC itself needs some zero drift correction it is than relatively easy (CMOS switches like DG211, DG408) after an initial buffer or amplifier.One of the simpler ways is similar to the Keithley 2000, with an zero drift buffer at the input. A bootstrapped OP-amp as a buffer is relatively simple compared to the amplifier case. I would even go so far and avoid the signal routing before the buffer (excepts for a relay to switch in the divider) and if needed have a 2nd buffer for a 2nd signal path (e.g. sense H for 4 wire ohms). Signal switching for zero, current ranges etc. can than be after the buffer(s). The amplifier stage no longer needs to be super high impedance.

author: David Hess
author_rank: Super Contributor
timestamp: April 13, 2025, 09:02:31 am
content: Quote from: Kleinstein on April 13, 2025, 07:11:03 amThe input series protection network would allow an input bias current up to 2pA without any correction, but a 0.1pA input bias current is feasible for a 6.5 digit meter.QuoteThen this proposed 6.5 digit meter supports 7.5 digit resolution by current standards, or specifications could be considerably relaxed and designed for 5.5 digits making it much more easily feasible.QuoteLuckily, there are a couple of parts available now which can meet 6.5 digit accuracy to within a count, so no compromise is necessary, and bootstrapping solves the problem anyway.Back in the 1980s when dealing with precision instrumentation, we considered an error exceeding 1 count out of 100,000 to be unacceptable and achieved it, although this was with low impedance measurements and an OP-07 or graded and temperature controlled LM308 operational amplifier was sufficient.I remember once when there was an additional single count of noise in the initial design, the engineer said that should not be there, and tracked it down to the use of an RC oscillator suggested by the application note for clocking the ADC. An LC oscillator with higher Q had less jitter and solved the problem.QuoteI have not looked into how they achieve automatic zero yet without too much leakage and charge injection, but it should not be necessary at 6.5 digits.QuoteThe input bias current is bad, but the current noise is even worse. I remember when the Keithley 2000 was new and we were using all kinds of Fluke and HP high resolution meters and I asked my boss why we did not have some Keithley 2000s, and his comment was that they were way too noisy. I wonder if that was connected to how the Keithley 2000 input stage works.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 13, 2025, 10:11:04 am
content: The high resolution DMMs have very high input impedance, but there are still not made to measude very high impedance signals. The full accuracy is for a relatively low resistance (like < 1 K) source. The typical impedance the amplifier has to handel is some 100 K from either the 9.9M+100 K divider or some 100 K resistance as the main protection element.100 K is not ideal, but some AZ OP-amps can handle this OK, without too much extra noise from current noise.The Keithley 2000 uses and AZ amplifier (old version with LTC1050, new versions with some LME2xxx). They should be OK with the current noise. However the input bias can be on the high side. Not sure how much Keithley is selecting the parts there. The current noise is OK, though not great.Normal DMMs have usually input current specs of something like < 50 pA (a few like the 3458, fluke 8845 give < 20 pA or < 30 pA). 2 pA or even 0.1 pA would be more like an electrometer, no longer a normal DMM.50 pA with 100 K would give a 5 µV offset, but this would be essentially constant (the zero path tends to have a matched resistance) and can be subtracted internally.Meters with a non AZ amplifier usually have AZ switching and this causes some current spikes, that are larger than the small spikes from the AZ amplifier, but at least only at a low frequency (2 per conversion). It depends on the use case which version is preferred. the AZ switching version has the advantage to have no current peaks in the non AZ mode, though at the cost of higher drift and LF noise.

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: April 13, 2025, 12:23:33 pm
content: I have an idea: why not use standard passive components for the voltage divider and current shunt, along with a heated bed PCB and a thermal sensor like the HTC3 to maintain a stable 50°C chamber inside a closed enclosure—similar to modern 3D printers? It's cost-effective, the temperature remains stable enough, and we can use cheaper passive parts. For the digital components, we have many affordable FPGA options like GOWIN, or microcontrollers from WCH or Espressif. As for the ADC and DAC, there are also very inexpensive Chinese options available.

author: SiliconWizard
author_rank: Super Contributor
timestamp: April 13, 2025, 01:04:30 pm
content: Quote from: jeduffy on April 13, 2025, 01:19:23 amObviously. Once you get past the design issues (which can take several years of rabbit hole), you'll need something to actually test it and calibrate it against. Otherwise it's as good as dead beef. So, you'll need a better than 6.5 digit DMM to properly check a 6.5 DMM. For a hobbyist trying to cut off costs, that's not gonna work. Unless you have lots of time ahead of you and free access to a university lab, or something like that.

author: David Hess
author_rank: Super Contributor
timestamp: April 13, 2025, 03:48:37 pm
content: Quote from: Kleinstein on April 13, 2025, 10:11:04 amOne reason I keep those old Tektronix bench DMMs is that they have no input switching so no input charge injection.Quote from: ali_asadzadeh on April 13, 2025, 12:23:33 pmThat would only work if thermal coupling of sensitive parts was increased, which means potting or submerging the assembly. Voltage dividers already do that with hybrid or monolithic construction. I have some Tektronix multimeters where the voltage divider by itself is a potted module.In the past, little temperature controlled heaters were available which would mount to a TO-99 package to reduce operational amplifier drift. Parts like the OP-07 made that unnecessary.Quote from: SiliconWizard on April 13, 2025, 01:04:30 pm4.5 digit meter calibration can be done with a 0.01% voltage standard and 0.1% precision resistors, so 5.5 digits could be done with a readily available 0.001% voltage standard and much more expensive but available 0.01% precision resistors. 6.5 digit calibration without a calibration lab or better multimeter would be almost a heroic effort, but with time and expense I think someone could do it.I am doing 4.5 digit calibrations and graded inexpensive 49.9k 0.1% resistors for dividers as shown below, which is really not quite good enough because I need another digit of resolution.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 13, 2025, 04:28:02 pm
content: The Tek 501 still has auto zero switching at the front. It is inside the LD120 chip, right at the input. There is even not extra precharge phase shown to reduce the charge injection spike. What likely helps is that the switch is likely relatively low capacitance as it does not have to be low resistance.If done well, the charging spike is not that bad, but it is a tricky part to get right.For no switching spike (not even from an AZ amplifier or similar) one would need to look for old meters with a trimmed BJT based front end, like older Fluke 8502-8506 or Datron 106x. AFAIR there are different versions of the main amplifier. There is still the balance between dirft / low frequency noise and the charge injection spikes.I don't think one would really need a stabilized temperature. There are not that many critical resistors and matched arrays are available if needed. It is mainly the high voltage divider where the TC matching could be an issue. Here one however has heating from the resistors itself and the outside temperature is only part of the problem. The TC matching part can usually at least be tested - so one can design more around typical specs and not upper limits.The bigger issue is the long term drift of resistors and ADC gain. Here sensible specs are hard to get. So ideally the design would find a way to avoid relying on long time stable resistors or similar (e.g. use capacitive dividers / multipiers or use the ADC for ratio measurements / checks).

author: David Hess
author_rank: Super Contributor
timestamp: April 13, 2025, 05:44:34 pm
content: Quote from: Kleinstein on April 13, 2025, 04:28:02 pmThat is the 4.5 digit DM501A, and 3.5 digit DM502, which use Siliconix ADC chipsets. Most DM501As found on the market have blown input multiplexers and as designed had poor linearity.QuoteThe 4.5 digit DM501, which is the oldest Tektronix design, has a dual JFET with trimmer on the source side for offset adjust. The input offset voltage drift and input bias current increase during warm up are almost insignificant. This meter of course has other problems, but the input stage is not among them.Conveniently, the unobtainable dual precision JFET is readily available from Digi-Key at a good price, although they seem pretty rugged.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 13, 2025, 07:02:06 pm
content: With only 4.5 digit resolution and a smallest range of 2 V the demand on the input offset is rather modest for the DM501. The simple dual fet is no longer a real option at the 6 digit level.

author: David Hess
author_rank: Super Contributor
timestamp: April 13, 2025, 10:15:17 pm
content: Quote from: Kleinstein on April 13, 2025, 07:02:06 pmI did not mean to recommend a dual JFET for the input of a 6.5 digit design, although it could be done with selected parts and other steps. Tektronix went that route with the DM501 because at the time, there were no high precision JFET operational amplifiers available, so they made their own.For a 6.5 digit design, I would use something like an OPA140 for the input buffer and bootstrap it if necessary which is not much extra complication. The input offset drift would require compensation if a selected part is not used, or the temperature of the OPA140 could be controlled. I would prefer this to continuous automatic zero operation if it is not required.I think an OPA140 could work up to 7 digits, but not 7.5 digits because of flicker noise.

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: April 14, 2025, 08:45:59 am
content: What about Slkor(SLKORMICRO Elec.) SLA333https://www.lcsc.com/product-detail/Precision-Op-Amps_Slkor-SLKORMICRO-Elec-SLA333_C2936072.htmlpriced @ 0.36$

author: Kleinstein
author_rank: Super Contributor
timestamp: April 14, 2025, 09:26:59 am
content: The SLA is a kind of typical, 5 V supply range low power AZ amplifier. The input bias specs are a bit high. It is more a super power type. It could still be an option for an input buffer with bootstrapping. There may be other more suitable choices. E.g. the MCP6V66 as a more common ,western type.The OPA140 and related OP-amps can indeed have quite low noise. I just had a look at data from my ADC, that uses an OPA141 at the integrator. With 16 PLC integration I get some 3 µV_pp of noise over 1 hour in non AZ mode even with not that stable temperature. Most of this from the OPA141 low frequency noise with a noise gain of 2 and an OPA145 with noise gain of 1. So expect some 1 to 1.5 µV_pp of noise from an OPA140 as buffer or amplifier without AZ switching and extra noise gain. This could indeed be good enough for a 6 digit meter, for a 10 V and 1 V range, though no longer great for a 100 mV range. With separate build AZ switching one still has the option to turn the AZ switching off. So one has the choice between the current spikes and a bit more drift / LF noise.

author: Simmed
author_rank: Frequent Contributor
timestamp: April 14, 2025, 09:31:01 am
content: Quote from: ali_asadzadeh on April 14, 2025, 08:45:59 amlooks like the clone of AS333/NCS333the chinese version does not specify the nV/Hz noisethe AS333 is around 80, the NCS is 62i think COS182 is their attempt to clone Ti OPA182unlike the 333, the COS182 at least attempt to specify 10nV/Hz noisein their "expensive" price scalethere is a perculiar GM4500 (> us$1)then there is the tokmas (copy infineon logo haha) OPA2335 with offset of 1uvthis is interesting with 38nV/Hz but 0.3uv pp noise ?on paper it says zero drift, but i dont know what version is their zero drift

author: David Hess
author_rank: Super Contributor
timestamp: April 15, 2025, 01:35:00 am
content: Quote from: Kleinstein on April 14, 2025, 09:26:59 amThe 40 volt OPA140 series includes:OPA140 0.5pA typ 120dB CMRR min 0.23uVpp 120uV max 1uV/C max Single-Supply, High PrecisionOPA141 2.0pA typ 120dB CMRR min 0.23uVpp 3.5mV max 10uV/C max Single-SupplyOPA145 2.0pA typ 118dB CMRR min 0.33uVpp 150uV max 1.5uV/C max Single Supply, High Precision, Low PowerOPA1641 2.0pA typ 120dB CMRR min 3.5mV max 2uV/C max Single Supply, AudioWhat caught my eye is the combination of low input bias current, high CMRR and Aol, low flicker noise, and low drift, and costs are reasonable. There may be a better lower voltage part, but I was only interested in 36 volt replacement applications. The comparable lower voltage CMOS parts all have too much flicker noise.The temperature drift you measured would be unacceptable even though the OPA140 is excellent in that regard, so it needs to be handled somehow. Maybe parts could be selected for low input offset voltage drift like we used to do. The amplifier could be externally temperature controlled. Automatic zeroing before every conversion cycle seems like the best way. I need to take another look at solid state switching schemes with low leakage current.What are alternatives to the OPA140? Chopper stabilized with external input bias current cancellation?I am less familiar with what Analog Devices supplies these days because of their much higher costs. OPA140s were available at $34 per 10 last time I checked, and the OPA1641 audio version is almost as good for half that price.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 15, 2025, 05:36:19 am
content: I see mainly 2 options for the front end:1) an AZ buffer amplifier from a bootstrapped OP-amp. This could be something like an MCP6V66 or AD8628 (higher cost but a little better) or OPA388 (low noise but maybe higherbias and current noise) and a simple OP like TL061 for bootstrapping the supply. AZ switching, Signal routing and amplification would be in a 2nd stage. A SD ADC chip may need an extra divider as part of the signal routing and some may not need amplification (internal PGA).Some SD chips may bave the switching internally. Though not super cheap, the ADS125H1/2 may be an option for a simple circuit, as it can directly accept +-12 V.1) AZ switching and source selection at the front, followed by a good JFET amplifier (e.g. OPA1641). An ADC chip may again need attenuation and for this maybe an extra buffer. For the AZ switching I like the idea to have the precharge part directly at the input, like show in the this thread:https://www.eevblog.com/forum/metrology/analog-frontends-for-dmms-approaching-8-5-digits-discussions/Expecially with a simpler DVM only the main input may need the full pre-charge. Other signals would see a stronger switching spike, but are likely OK with this. If wanted the input could switch to a low bias/glitch free mode via the pre-charge buffer in front. So the user can have the choice between low drift with AZ switching or low bias / no glitch with an extra buffer (e.g. OPA145) and only minimal switch leakage.

author: peter-h
author_rank: Super Contributor
timestamp: April 15, 2025, 03:13:19 pm
content: Why not pick up a used HP3478A? I've got a few of them, 20+ years old, and they just get better and betterI then bought some references fromhttps://electron.plus/which are supposed to be calibrated to something super accurate. I see they no longer list them - REF50X on the PCB. But across my various multimeters, all of them are better than 0.01% despite not seeing a calibrator in decades.In reality getting better than 0.01% is very hard. I've been doing precision analog for many years. You can buy a 0.02% reference anytime and IIRC there are some 0.01% ones, and that will exceed anything you can make in production and have that accuracy and stability unless a lot of money is spent. I built a production calibrator for an analog product, money no object, 0.005% resistors, etc, TLV2333 op-amps, and it is of the order of 0.01% only.I have a MCP3550 chip as an option in a product I make and I can trivially knock up a "DVM" with that (it uses a top end ref chip) but then one needs to build multiple voltage ranges to be useful which is then way more work. Then one should calibrate it against something...

author: ivo
author_rank: Regular Contributor
timestamp: April 15, 2025, 04:26:38 pm
content: You need ADC with probably 22 bits of noise free resolution... I have looked at, in order of cost-MCP3562 (very cheap for a 'good' chopping 24bit), AD7190 (up to 23 noise free bits, also comes with a PGA to enable measuring small stuff without needing an amp), AD7172 (the entire 24 bit noise free at low sample rate, seems like they chopped it off a 7177). If you want something to play with you can pickup a TI ADS1256 module on Aliexpress.For reference, either LM399, ADR1399, or something with 1-2 typical ppm like a MAX6126 if you don't want the current draw.If you want higher voltage then I haven't yet tested how much worse SMD resistors would do vs a caddock divider for tempco and ratio stability.Then you also need to look at A) whether you want to be to measure down to true 0, B) whether it should be bipolar input; both of which can complicate the power supply setup you need.Does one care about ACV? Or having a current source for resistance measurements? Or being able to measure a shunt for ammetering? Are we being cowboy or studious with input protection?

author: David Hess
author_rank: Super Contributor
timestamp: April 15, 2025, 04:43:15 pm
content: Quote from: Kleinstein on April 13, 2025, 10:11:04 amForgive me for backtracking for a moment. I mean to ask this earlier but got distracted.So high resolution multimeters are avoiding the 9M+1M divider because its 900K output resistance would require too low of an input bias current, at the expense of requiring the input buffer to operate over a wider input voltage range?I have found this behavior in my 5.5 digit HP3478A particularly annoying because readings change between the 3 volt and 30 volt ranges with only moderate source resistance. Looking now, I see no specification for input bias current. I wonder how bad it really is.The 9M+1M divider on lower resolution multimeters is where I first noticed damaged input buffers with high input leakage. The zero was changing and decreasing from the 10M, 900K, and 90K parallel resistance of the input divider. The requirement for low input bias current is much easier to meet when the resolution only goes down to 100 microvolts.Quote from: Kleinstein on April 15, 2025, 05:36:19 amI have designed discrete bootstrapped low leakage switches before that operated in the 10 picoamp range at high temperature. I am going to have to try and design one which could do the switching for a multimeter front-end with even lower leakage.

author: peter-h
author_rank: Super Contributor
timestamp: April 15, 2025, 05:24:21 pm
content: QuoteTry achieving those "noise free bits" on a real PCBYou will probably need a shielded case over the ADC and any op-amps before it.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 15, 2025, 06:11:17 pm
content: Getting close to the SD ADC noise specs is usually not that difficult. There is still one slight hurdle from the ADC driver. The lower noise / high performance SD ADC often have quite hard requirements on the drivers for the input and reference. They may want speed that AZ type or precision BJT based amplifiers have a hard time to deliver.The tricky part with SD ADCs can be getting the specified INL. Here the drivers can make a difference. Another part is testing the INL. The tricky part often forgotten is that the INL of especially the lower noise SD ADCs depends on the driver part.The input bias can look like an issue for a 9 M - 1 M divider. However a 9.9 M - 100 K divider with more gain after the divider is just as sensitive to bias current. When it comes to offset drift and noise the 9 M-1M divider is acutually at least as good, if not slightly better. The point is more about avoiding switching a high voltage. The 9M-1M divider would need a switch that can withstand a high voltage when a higher voltage range is used. So one would need something like relay switching. Leakage at the switch would be very critical for the higher divider ratios. One coud use electronic switching with a 1:10 "divider" with an active divider (inverting amplifier), just like many autoranging DMMs do.For the reference the plastic case references like most bandgap reference have a sensitivity to humidity and long time drift that is quite high and for this reason not that suitable for a 6 digit meter. The THT metal case alone make the LM399 interesting.A divider made from SMD resistors can have an issue with a humidity effect from swelling of the PCB causing mechanical stress to the resistors. With one resistor type one can still get some canceling out from symmetry. For my DMM version I have a set of 1 M SMD resistor (as a 10 M - 500 K divider). So far the ratio is quite stable over time (like < 2 ppm over some 3 years, though not that many points I looked at, TC matching in the 1 ppm/K range). If needed one could use some internal test for the ratio to the ADC. This is the way I got the drift numbers.

author: David Hess
author_rank: Super Contributor
timestamp: April 15, 2025, 08:12:05 pm
content: Quote from: Kleinstein on April 15, 2025, 06:11:17 pmI figure range switching is a solved problem since relays or mechanical switching can be used after the divider. I am more interested in how to lower the leakage of the automatic zero switching, for which I have some ideas.

author: ivo
author_rank: Regular Contributor
timestamp: April 16, 2025, 05:27:35 pm
content: Quote from: David Hess on April 15, 2025, 04:43:15 pmAFAIK they're avoiding the divider simply so one has the option to not put a 10M resistor in parallel with your signal source. Many times 10M doesn't matter a bit (especially for 3.5, 4.5 digits) but at higher digits and if you're working with 10k, 100k resistances/impedances, as you said you can see values changing from its introduction.Many handheld DMMs only have battery voltage to work with for rails, so their primary ADC voltage input range might be 1-3V (and probably divide that in two so you can handle negative signals), hence for 99% immediately after the mV scale they are forced to use a 10M divider. A subtle point of differentiation for the handheld aficionado then is if the mV range has Gohm impedance (usually, not always), and then I think there might be a select few with that even on a ~1V range. And if they can keep the Gohm impedance when auto ranging and the input is small enough.Bench DMMs can flex their designers' muscles being able to work with supply rails taken from mains, so they can now extend that to +/-15V or similar things. Which means they can now implement an input stage which could handle +/-10V (or higher, etc) signals directly before they need the 10M divider again for the 100V+ scales. So in the past and still sometimes still in modern day you'll see dual-matched JFETs in a discrete input long tail design, so they can take a 10V signal directly with usually tiny bias currents. Nowadays there's also opamps which if you design with them correctly can achieve roughly the same effect.

author: hve
author_rank: Regular Contributor
timestamp: April 17, 2025, 08:07:43 pm
content: Wow what a lot of good comments!I might have underestimated the challenge a bit when first posting the question in a joyous spiritSo currently reading all comments and studying the schematic of the 34401A, interesting to see how it is working. Looking at the components that where used at the time, and how much has changed since then.Also when looking for suitable parts I am wondering, should I maybe change the specs?Is a range of [-10, 10] not to limiting for the choice of components, so should we lower the bar to [-5,5] as the primary range, so we have a wider choice of components...section maybe.. While I love AD and TI I also see a lot of Chinese vendors with pretty good components and they are good to come by and decently priced.For example when looking at these niceRS8554opamps for around one USD, they could be perfect for the reference section.There is a-lot to process in this thread, lots of very good comments, and I feel I need to follow up on this idea however insane it might be.So I want to capture your comments and suggestions in some sort of Wiki or on a github page. if you have some thoughts on this shoot..Also make it as modular as possible in order to cut the elephant in pieces, some initial thoughts on the parts:- Mains power section:Thinking to repurpose an old(er) 18-20V laptop power supply, they are mostly relatively decent and I am sure everyone has one in his used parts bin that can be used without any pain.Saves a little on e-waste also.- Power supply conditioning and distributionGenerating the +- rails and the MCU power.I am looking at the lectures ofon Common mode and differential mode filters.Highly recommended- Voltage Reference sectionGenerating the reference voltage and derivatives.- ADC section- Analog input sectionDC amplifier, what are the design criteria.- MCU control board + softwareAdditional requirements:- All calibration data should be in software, no trimmers if we can help it..- (optical or galvanic) Isolation between PC interface and the MCU.I have used thesemoduleswith great success in the past- Enable low(er) IO slew rates for the switching circuitry (if possible and needed?) in order to limit the interference of the digital circuitry

author: Kleinstein
author_rank: Super Contributor
timestamp: April 17, 2025, 09:12:25 pm
content: Since the 34401 times there are 3 major changes. One is that SD ADC chips got better and they are now a viable way to build a 6 digit range and even up to maybe 8 digits meter. An example are the Sigilent 3065, Rigol3068 and few more.The other point is that CMOS switches got better with less leakage. Ready made CMOS mux chips can simplify things quite a bit compared to JFET switching. For the AC part, fast ADC chips and µCs are now affordable, so that digital RMS instead of the old style analog AD636 RMS chip are now possible (e.g. like found on the Keithley DMM6500).The ADC chips usually come with a 2.5 or 5 V input range. It gets a bit easier if one limits the input range to such a lower voltage. However one would than no longer be capable to directly measure a 7 V zener reference in the prime range, e.g. to check for drift. So the extra effort for a 10 V good range may well be worth it. Having a relatively large range with high impedance can be an important feature.For the start I would not focus to much on saving a few cents on an OP-amp or ADC chip. The Chinese chips tend to come with a some complications in ordering them - the low price may be only for > 5 pieces. If in the US or Europe, ordering from Mouser may be simpler and more reliable.The point is to first have a reasonable simple circuit. Finding cheap parts is than a 2nd step. The active parts are only part of the costs. Also some of the resistor can add quite some costs.There are quite different ways to build a meter. The ADC effects the suitable front end with different voltage ranges and maybe internal gain steps.For the SD chips this can be quite different from the MS ADC in the 34401. Also different SD ADC chips can be different with the differential input range and whether the internal PGA part or buffer could be used. So the design can not be that modular. To compare ADCs one may have to incluce different front ends.For the power supply, a laptop power brick is a bad choice. They tend to be noisy and unless one has a large screen, one would not need that much power. I would more expect some 1.5 to 8 W needed. I would prefer a classic mains transformer, maybe as an old style heavy wall wart, though this come usually without a ground connection that can be a thing for grounding the case. It gets a bit less critical for a reduces maximum voltage and DC only and thus usually no mains measurement.For the 6 digit level the LM399 chip is pretty much the standard. Most of the 2.5 V or 5 V ref. chips are limited by long term drift.It could still be an option if one wants to cut corners here and accept more drift.Calibration in software is the natural choice anyway. The question can be if one wants the meter to also check the ranges against each other, either fully automated or with some user help. This could simplify the adjustment part - though one should at least initially also have a classical direct check per range.

author: hve
author_rank: Regular Contributor
timestamp: April 17, 2025, 10:49:00 pm
content: I did have a look on a 45 Watt Lenovo brick.Tested worst case scenario, without any added extra capacitance, the ripple voltage is horrible, 50HZ @ 4VPP and related harmonics...I expected to see some switching noise, but was undetectable in my setup so it must be below -80 dBm. (The inductance in the cable with the ferrite on the end probably got rid of most of it).Noise did not change when loading from 0 to 10, but its a topic for later..Lets focus on the most important part first!Cheapest ADC on mouser capable of minimum +-10v is this one:mouserData Acquisition ADCs/DACs - Specialised 24-Bit, 256kSPS, DAQ subsystemReference on board, select-able gain, highly linear, and only for 40 USD.... It looks like 90% of the hard work has already been solved by the AD engineers.Looks promising.. What do you think?

author: Kleinstein
author_rank: Super Contributor
timestamp: April 18, 2025, 07:49:22 am
content: The links somehow don't work, at least not from Germany.The ADAQ7768 is still only the ADC chip with PGA front end. A big downside and essentially no go for a DIY project is the BGA case, even though it is at least a rather large pitch version. So only for those who know how to solder BGA and have the equipment.There is a cheaper alternative: the ADS125H1 or ADS125H2. It has a little more noise (still well good enough), lower speed (still 40 kSPS), higher input bias (though unclear specs for large temperature range). On the upside a more DIY friendly case, better INL, +-10 V already single sided and a lower price (some $15 / $18).Because of the input current one may have to add a zero dirft input buffer, but that is a relatively simple and cheap part. It allows for simple (leakage no longer critical) signal switching after that, though it adds a little noise for the lower ranges (still OK). For a +-20 V range one could add a driven low side to generate the differential signal for the ADC. One would need this part with the ADAQ7768 already to get the +-12 V range.I would consider the ADS125H1 with extra input buffer a viable option. The reference would be either 1/2 the voltage from a LM399 or maybe a ceramic case 2.5 V ref. chip (e.g. LTC6655).

author: Andreas
author_rank: Super Contributor
timestamp: April 18, 2025, 08:51:02 am
content: Quote from: hve on April 17, 2025, 10:49:00 pmNope,you have to apply a 4 V reference externally to the (for hobbyist) unsolderable BGA package.The suggested ADR4540 has no good long term stability.I would want a long term stable buried zener reference in metal can package (LM399, LT1021CCH) or CERDIP package (AD586LQ).A real 6.5 digit instrument (HP34401A/K2000) has typical below 1 ppm linearity (max 3 ppm)So typical 6 ppm is not highly linear.There is no temperature sensor on board: -> the 6ppm/K drift spec is a problem.This cirquit is more like a 5.5 digit instrument.The targeted application is more for higher sample rates with reduced accuracy.Quote from: hve on April 11, 2025, 11:37:49 amWLAN: are you kidding. When handling with uV you do not want any RF around in your lab. I strictly ban mobiles and WLAN from my lab.5 readings/s: typical 6.5 Digit instruments are specced @ 100 NPLC (= 4 sec integration time with offset compensation) or 10 NPLC (0.6 sec) with higher noise so you will need averaging.The question is: what do you really need?- fast / accurate?- 6.5 digits with 5.5 digit specs? (linearity, noise, tempco?)- spec for long term stability?with best regardsAndreas

author: hve
author_rank: Regular Contributor
timestamp: April 18, 2025, 10:31:46 am
content: Quote from: Kleinstein on April 18, 2025, 07:49:22 amCorrectedQuote from: Kleinstein on April 18, 2025, 07:49:22 amThank you! I have put it on the radar!

author: hve
author_rank: Regular Contributor
timestamp: April 18, 2025, 11:25:26 am
content: Quote from: Andreas on April 18, 2025, 08:51:02 amYes I noticed the BGA part, I have soldered BGA parts in the past and it is always a pain. Might be able to do, with a special adapter board. But considering it is failing in the other aspects that where mentioned it is not worth the effort to look into this anymore. Thanks!Quote from: Andreas on April 18, 2025, 08:51:02 amNoticed in the past how WLAN screwed up my sensor readings, I should have thought of that- "no WLAN" is now a requirementAlso the 100 NPLC benchmark, I did not think of that, but it is important to get better requirements for thisNot sure how to write a requirement for that, or how it is implemented.I can imagine if you lock your measurements on the AC line using some sort of zerocrossing detector, you'll only make things worse. as the noise on the line will introduce significant jitter.If we take theADS125H02as a baseline, which can do 40kSPS, so you should have at least 666 samples in one single line period.So could you make the integration time less than 4 seconds due to the fact that you have more SPS to work with?I would probably add a decent low-pass FIR filter down-sampled to the desired output frequency, and skip on the averaging.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 18, 2025, 12:48:35 pm
content: The noise part is likely not that difficult. Modern ADCs can get 6 digit resolution (e.g. less than 1 ppm FS of RMS noise) already with relatively short integration time. Because of background mains hum one usually needs full mains cycles so in US some 60 SPS as the highest sensible speed for high resolution. The difficult part is the linearity and longer time stability, no more the ADC noise or resolution as quanzation limit. Noise can dtill be an issue for the reference.The ADS125H2 specs are for some 1.8 to 2.4 µV of RMS noise for 60 SPS, 10 V range depending in the digital filter. That would already be good enough for 6 digits. Noise gets a bit more difficult for a 100 mV or similar range because of the amplifier noise. For the lower ranges one may have to accept a bit longer integration or more noise.

author: ivo
author_rank: Regular Contributor
timestamp: April 18, 2025, 01:55:53 pm
content: Just to note, AD made a ceramic version of the ADR4540 (and 25 and 50), it's the D variant in LCC format. It's long term drift is basically an order of magnitude lower than the SOIC packages. One could even argue that it's better than an LM399 because it has roughly 1/3rd the 1/f noise, but probably not better than the newer ADR1399.

author: hve
author_rank: Regular Contributor
timestamp: April 18, 2025, 04:49:20 pm
content: Quote from: ivo on April 18, 2025, 01:55:53 pmThe ADR4540DEZ has some nice specs but I cant find it publicly available, LCSC list price is 51 euro with 0 on stock.It is hard to beat the LM399A(H) in terms of price, availability and performance.

author: Andreas
author_rank: Super Contributor
timestamp: April 18, 2025, 05:52:02 pm
content: Quote from: ivo on April 18, 2025, 01:55:53 pmHello,yes the ceramic SMD version is somewhat better than the plastic SMD parts.But usually you have to look which specs are left out from the data sheet.And since they are no buried zener references the long term stability is worse than a buried zener part.The problem with the LS8 package is that if you solder them to a epoxy board you are influenced by swelling of the epoxy by humidity.That is why I use dead bug mounting for the LS8-Packages.Noise is usually not that critical for integrating or sigma delta A/D converters.And be carefully with noise specs. They usually scale with output voltage. So a good value for a 2V part is factor 2 for a 4 volt part.The ADR4550D has a relative large temperature hysteresis ~3-4 ppm during a "extended room temperature cycle"The drift during initial temperature cycles is also larger than that what I get from a buried zener.Unfortunately I do not have long term drift data from LS8 package up to now.But I guess that it is so not much better than the MSOP8 package that it can reach a buried zener.with best regardsAndreasedit: example for AD586LQ buried zener

author: David Hess
author_rank: Super Contributor
timestamp: April 18, 2025, 06:16:16 pm
content: Quote from: hve on April 18, 2025, 11:25:26 amIn practice it seems to always work better to use a fixed integration time close to the line frequency. Tracking the line frequency, even without noise as described below, seems to lead to linearity errors or excessive noise for some reason, but I have seen instruments which had separate modes for fixed line frequency operation and tracking the line frequency. If the line frequency is tracked, then a local stable frequency reference should be phase locked to the AC line to remove noise.A base integration time of 1/10th of a second is common because it covers both 50 and 60 Hz environments.Quote from: hve on April 18, 2025, 11:25:26 amReplacing the averaging filter with a FIR filter that has a better response seems like a great idea, however simple averaging has the advantage of notching the power line fundamental and harmonics.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 18, 2025, 06:46:32 pm
content: The SD ADCs usually include higher order digital filters like SINC3. This gives very good suppression of mains hum, even if the frequency is a little off or the phase changes. The downside is that settling takes longer and consecutive readings are usually not independent. It is a bit like using some running average over the data. The slightly lower noise (at the same samplig rate) is from a reduced noise bandwidth and it comes with slower respondse time. It could still make sense to use an external clock to get a reasonable good match to the nominal mains frequency. Usually no real neat to measure the actual mains frequency (except fo maybe 50/60 Hz detection to also allow good 1 PLC).The LM399 is pretty much the standard ref. for 6 digit meters (especially as there are few new models that could include the ADR1399 as a better model). A divide by 2 to a 3.5 V reference can work with quite some ADC chips and dividers for /2 are readily available (e.g. as DIV23) and expected to be quite stable. For the long time drift one could measure the raw 7 V and this way also include long term drift of the ADC gain. This would also make a good test for the temperature effect from the ADC and ref. divider combined.

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: April 19, 2025, 09:19:58 am
content: what about the ADS131M02,ADS131M04,ADS131M08 family of ADC's? they are super cheap and have 2,4,8 channels and inputs are simultaneous too with PGA and we could use them to measure V, and I, so we can calculate power too

author: Kleinstein
author_rank: Super Contributor
timestamp: April 19, 2025, 10:02:16 am
content: The ADS131 has 3 times higher INL in the specs - this is not really 6 digit worthy. Already with the ADS125 the INL is a weak point.The PGA part in the ADS131 and most other SD ADCs is for low voltages only. So one would still need a front end with buffer, switchable divider and likely another buffer to get a 10 V high impedance range.The big point for the ADS125H is the included input stage for up to +-20 V differential. A similar external circuit would cost some $10.It can make the circuit really simple. If the bias is acceptable one would only need the protectio for the input.

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: April 20, 2025, 01:42:49 pm
content: QuoteADS131 can accept +–VREF differential input too, also it has a very attractive price (under 4$)