How do multimeters measure AC voltage? - Page 1
Forum section: electronics / beginners

author: LinuxHata
author_rank: Frequent Contributor
timestamp: July 18, 2023, 06:57:16 pm
content: Hello.I'm building my own multimeter with some specific to myself features.I'm not exactly sure, whenever I need AC measurements at all, but I'd like to ask about it, to determine, whenever I'm moving into correct direction.To measure AC, we have two ways - rectify input voltage/current and measure resulting DC.Another way is to sample input voltage with high resolution (say, 200 samples/sec), and, by measuring and comparing the lowest and highest peaks, calculate AC voltage based on that.The first method, rectification, will introduce significant errors, due to non-linearity of diode volt-ampere characteristics. But as I googled, there are some special ICs, like AD737, which do TRMS to DC conversion.2nd method, which is more modern, should work better, but I see a little issue here with existing ADCs - since most time, we well be measuring either 50hz or 60hz AC voltage, most ADCs have built-in filter, which does 50/60hz rejection, so this method will not work.So my question is, if I decide to do AC measurements in my DVM, which path should I follow? which will more price-performance effective?

author: radiolistener
author_rank: Super Contributor
timestamp: July 18, 2023, 07:19:54 pm
content: Quote from: LinuxHata on July 18, 2023, 06:57:16 pmBoth way you're talking about are incorrect, because they can be used when you know AC waveform, but you don't know it when using DMM.The correct way is to use RMS (Root Mean Square) measurement. In short, the formula is the following:So, you're needs to capture some sample interval to get values x1, x2, x3, ..., xNand then use this formula:Urms = sqrt ( x12+ x22+ x32+ ... + xN2/ N )The sample rate and sample size depends on measurement bandwidth which is required for your needs. Usual DMM has about 200 kHz bandwidth for AC measurements and using about 1 sec sample buffer.In order to get 200 kHz bandwidth, you're needs ADC with at least 400 kHz sample rate. But there is also another issue - frequency aliasing, which requires to use antialiasing filter before ADC to eliminate aliasing effects. It requires to use about 10-20 times higher ADC sample rate in order to keep your antialias filter slope within ADC working bandwidth. This is required to avoid aliasing images within captured bandwidth. So, you're needs to use ADC with sample rate which is 20-40 times higher than required measurement bandwidth.The sample buffer size depends on a lowest frequency which you're want to capture. For 1 Hz you're need for at least 1/1 = 1 second buffer.So, in order to measure AC voltage with 1 Hz ... 200 kHz bandwidth you're needs to use ADC sample rate Fs = 200 * 40 = 8 MHz and about 8 MHz * 1 second = 8 megabytes sample buffer.Also don't forgot to add low pass filter on ADC input. It's cut-off frequency should be between required measurement bandwidth and your ADC sampling bandwidth (which is Fs/2).Take into account that:1) low pass filter response is not flat near cut-off frequency, so you're needs to keep it above your required bandwidth with some margin.2) high frequency components should not get on ADC input, so it should be damped for at least 40-60 dB at ADC sampling bandwidth.For more precise 200 kHz bandwidth you can implement digital FIR filter which can cut-off frequencies between 200 kHz and Fs/2.

author: LinuxHata
author_rank: Frequent Contributor
timestamp: July 18, 2023, 07:26:00 pm
content: Thanks, that is really helpful, so built-in feature of most ADCs for 50-60hz rejection, will not cause any issues?And by the way, if set lower limit to 50hz and higher limit to 120hz, I guess, smaller buffer and slower ADC can be used, right?

author: radiolistener
author_rank: Super Contributor
timestamp: July 18, 2023, 07:32:49 pm
content: Quote from: LinuxHata on July 18, 2023, 07:26:00 pmif your ADC has built-in 50/60 Hz rejection filter, then such ADC cannot be used for AC measurement.At least for bandwidth more than 30-40 Hz. Usually such ADC is intended for DC measurements.But you can use some RMS to DC converter chip. It will convert AC voltage into DC voltage and then you can measure it with your low speed ADC.

author: radiolistener
author_rank: Super Contributor
timestamp: July 18, 2023, 07:37:56 pm
content: Quote from: LinuxHata on July 18, 2023, 07:26:00 pmYes, if you limit bandwidth, you can use slower ADC and less memory for sample buffer.But such bandwidth 50 Hz - 120 Hz is almost useless for practical usage.DMM which is intended to use for mains measurements needs for at least 10-20 kHz bandwidth.Modern SMPS working at 40-100 kHz and even higher, so your mains can contains a lot of harmonics and distortions with frequency up to 100 kHz and it needs to be taken into account for proper AC measurement.

author: LinuxHata
author_rank: Frequent Contributor
timestamp: July 18, 2023, 08:29:58 pm
content: Thanks!I tried to search ADCs on DigiKey, and most that come up, are with that built-in filter. Can you suggest any moderately priced, fast enough ADC, with SPI or I2C interface?

author: TimFox
author_rank: Super Contributor
timestamp: July 18, 2023, 08:34:08 pm
content: A nit-pick on the formula for the standard deviation (RMS) for a set of samples.The equation given, withNin the denominator, is correct for the standard deviation of the set of samples.As an estimate for the standard deviation of the parent distribution, with a finite numberNof measured samples, the denominator should be (N-1).In physics, we said that if that difference [(N-1)/N]1/2was important, then you did not have enough samples.In an AC measurement, we usually assume that the mean value of the parent distribution is zero (DC = 0) and compute the standard deviation of the samples.(In a "really-true RMS" measurement, the DC value is included in the computation, giving a value with the total power of the signal.)

author: robowaffe
author_rank: Regular Contributor
timestamp: July 19, 2023, 04:37:53 am
content: The multimetre just connects on as a voltage divider, with a high resistance line, and its the same as if it was a short, just disturbs the circuits less, but it would still work (whilst blowing your fuse) then the piddly amount of current going in, gets converted on a scale conversion to what the volts is.Dont expect any logical value unless u put it directly around a power source, I cant stand at looking a volt readings on oscilloscopes unless its directly on the power source.

author: dobsonr741
author_rank: Frequent Contributor
timestamp: July 19, 2023, 04:54:06 am
content: You can get a fairly high bandwidth simple to use RMS converter: LTC1968The ADC computational RMS is not for the faint hearted. If you want a simple start on it get an LP-MSPM0G3507 Eval board. Nicely supported latest incarnation dev environment included.

author: David Hess
author_rank: Super Contributor
timestamp: July 19, 2023, 08:23:07 am
content: Quote from: LinuxHata on July 18, 2023, 06:57:16 pmA half-wave precision rectifier is commonly used and will provide accurate measurements down to the noise limit of microvolts. The input offset voltage of the operational amplifier can be removed by using AC coupling at the input and output, which considerably relaxes the DC performance requirements of the operational amplifier.For true RMS measurements, typically an IC from Analog Devices is used, some of which can also do AC average measurements.QuoteA sampling ADC instead of an integrating ADC is used. Integrating ADCs include instrumentation delta-sigma ADCs like you describe. There are several suitable precision sampling ADCs available.The sampling method just as easily does average AC measurement, true RMS measurement, and DC measurement at the same time, but requires some digital processing.QuoteI think the largest difference in difficulty is that the sampling method requires DSP programming, including a digital filter to remove noise. The calculation for RMS is almost trivial.

author: EPAIII
author_rank: Super Contributor
timestamp: July 19, 2023, 09:00:04 am
content: Oh for Pete's sake! The poor guy is trying to build his first multimeter and everybody seems to think it must have true RMS? WHY?He didn't even say if this is going to be a digital or an analog meter. OK, it will probably be digital, but he really did not say so. So, he will probably have an ADC to convert the signals to digital. And basically those signals will essentially be DC. So far, so good.But he just wants to add the ability to measure AC and, reading between the lines, I think he means POWER LINE AC. He speaks of 50 or 60 Hz and a 200 Hz sample rate. He is not thinking about even audio signals. Perhaps he should, but he isn't.Those POWER LINE AC signals will, in all likelihood, SINE WAVES. Perhaps not perfect sine waves, but essentially sine waves.Multimeters have been made for under $25 (USD) for many decades now and they measure power line AC. They also measure audio AC. And other forms of AC. Perhaps not perfectly, but they do provide readings. And those readings use a multiplication factor based on the sine wave. They don't have true RMS ability but they do work quite well for power line measurements. And, if they are constructed properly, they can make accurate measurements of audio signals which are not pure sine waves.So, unless someone here wants to suggest an actual circuit, with chip IDs and all the component specs., then I would suggest that he, at least at the beginning, look at a simple rectifier circuit. Analog multimeters had separate scales for AC. Those scales took the characteristics of the rectifier into account by compressing the scale at low Voltage values. Digital meters either did not compensate for the loss in the rectifier or could EASILY just add a small bias Voltage to the AC scales. Again, the value of that bias Voltage would be based on the characteristics of the rectifier used.After looking into that and deciding if it would be accurate and would cover the types of signals he anticipates, he can, if he likes, look into true RMS.In a 45+ year career as a professional TV engineer I repaired, modified, calibrated (set up) and even designed and built audio, video, control, and RF equipment. I only once had a true RMS meter. And in that situation I allowed the building maintenance man to use it because it simply was not really needed for any of the things I would use it for. I did my job quite well without it. In my personal meter collection (about 15 or 20) I only have one meter with true RMS and I did not buy that one for that feature. You can get along just fine with 99.9% of the tasks in electronics without true RMS. Probably more, but then I may not be the sharpest tack in the BB.

author: EPAIII
author_rank: Super Contributor
timestamp: July 19, 2023, 09:07:57 am
content: One more point, in stating he is considering a 200 Hz sample rate for 50 or 60 Hz AC I think he is depending far too much on the Nyquist–Shannon sampling theorem.If true RMS is really needed, just how does four samples provide any accuracy if the waveform is not going to be a sine wave?

author: Kleinstein
author_rank: Super Contributor
timestamp: July 19, 2023, 11:43:21 am
content: The 50/60 Hz suppression is found with slow integrating ADCs, like SD ADCs. The measure AC from a sampled waveform one wants a relatively fast ADC, like 50-1000 kSPS. For the bandwidth one can use a anti aliasing filter to limit the BW to less than half the sampling frequency. Alternatively one can allow aliasing and this way also include higher frequencies, especially if uneven sampling times (kind random time sampling) are used. This could extend the BW. One would still want a rather high sampling rate to get a reasonable bandwidth (e.g. 10 kHz). One can do most of the RMS calculation in real time, without storing all the samples.This are usually sampling ADCs. A low end solution may be the 12 bit or even 10 bit ADC found as part of a µC. The dynamic range is a bit limited, but not that bad. With a 10 bit ADC one can be about at the 3.5 digit level, maybe a little better. There are somewhat different pros and cons to the digital RMS and more classic oder RMS chips or simpler active rectifiers.The analog RMS to DC converters are relatively slow reacting and have problems with low frequencies. Also the performance near zero is not ideal, as they have a hard time to compensate for their own noise and the classic ones like AD736 get slower the smaller the amplitude.The digital RMS solution ideally needs a way to measure the frequency to sample over whole periods. The power consumption may also be a bit higher.

author: Terry Bites
author_rank: Super Contributor
timestamp: July 19, 2023, 05:05:19 pm
content: ? why reinvent the wheel. As they say. "Dont build what you can buy".Life is short and the art is long.What don't available analog and digital mutimeters do for you?BTW if you want a simple RMS to DC thing, use an LTC1966.

author: dobsonr741
author_rank: Frequent Contributor
timestamp: July 19, 2023, 05:06:20 pm
content: I agree, given OP is building the first instrument, the simplest AC converter is most practical to get up and running. Either with a classic diode/opamp precision full wave rectifier or a single chip RMS converter, like AD737 or LTC1968, if affordable/available.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 19, 2023, 05:12:08 pm
content: If one wants an analog RMS converter also the AD8436 is worth a look: lower power and lower cost than AD737.

author: LinuxHata
author_rank: Frequent Contributor
timestamp: July 20, 2023, 06:53:39 am
content: Thanks!Actually, I'm not going to have AC measurements at this moment at all, because I don't need it. I just asked, in case I will do it, what efforts I need to put intoFor the multimeter which does not exist, please suggest me one like Keithley 179 - direct selection of modes via dedicated button for each function, no auto ranging, no rotary switch, no multi-function buttons, but big, bright LED display, pocket sized, and fast screen update time - at least 15 FPS. There are none. Maybe someone say that no one needs, but I don't care for other's needs, so I'm making one for myself

author: dobsonr741
author_rank: Frequent Contributor
timestamp: July 20, 2023, 08:19:11 am
content: Found one for you: “Display updates 150 times per second”:https://lygte-info.dk/review/DMMSiglent%20SDM3045X%20UK.html

author: Kleinstein
author_rank: Super Contributor
timestamp: July 20, 2023, 09:21:49 am
content: There are a few bench meters with fast update rate - though usually auto ranging.With the complications from safty, a DIY multimeters is not the most useful one, but can still be fun if one takes into acount the limitations. The AC parts is often relatively limited and could be handled as a later add on option.For the reading rate the brain is not very good handling a digitial display updating much faster than 5 times per second. So a faster reading rate is mainly a thing if data are send to storage or a PC. In my setup I have a choice of 24 / 12 / 6 / 3 / 1.5 updates per second and usually go for 3 or 6 for a fast manual reading.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 20, 2023, 11:52:09 am
content: Quote from: EPAIII on July 19, 2023, 09:00:04 amYes. This is some kind of test gear cult thing. For example, I have both as hobby and later professionally designed electronics for 30 years and never ever found any use for a "true RMS" feature in a multimeter, or heard anyone describe a real-world use case. There are some weird ones, like an electrician measuring highly distorted mains and still wanting accurate representation of how much real power would be used in a resistive load given that distorted voltage, but that's quite synthetic case!With AC (changing) phenomena, you almost always want to see the waveform, in which case you want an oscillosscope.For one's first multimeter, any really does.

author: LinuxHata
author_rank: Frequent Contributor
timestamp: July 20, 2023, 01:07:00 pm
content: Well I'm into practical electronics for last 30 years and I have to admit, only needed RMS measurement feature 2 times at allSo my DIY multimeter won't have AC measurement at allBut it will have 15 buttons to instantly select needed range and measurement type

author: David Hess
author_rank: Super Contributor
timestamp: July 20, 2023, 01:30:34 pm
content: Quote from: EPAIII on July 19, 2023, 09:07:57 amThe standard deviation calculation is not affected by undersampling. The standard deviation is calculated from the histogram of the amplitude which just takes longer to generate at lower sampling rates. What does matter however is that the period of time over which the histogram is generated is sufficient to capture the lowest frequencies of interest.RMS sampling RF voltmeters operate to GHz+ frequencies with sampling rates below 100 kSamples per second. They work by attaching an RF sampling front end to an RMS voltmeter; nothing more is required. If you attach the output of a Tektronix 7S11 sampler with S-4 sampling head to any low frequency RMS voltmeter, the combined instrument is a 14 GHz RMS voltmeter.

author: TimFox
author_rank: Super Contributor
timestamp: July 20, 2023, 01:40:56 pm
content: The RF sampling scheme works perfectly, so long as the sample rate is not coherent to the frequency of the signal.(This is not hard to avoid.)

author: Kleinstein
author_rank: Super Contributor
timestamp: July 20, 2023, 04:22:35 pm
content: If one has a µC as part of the system, it is not that hard to use the µC interal ADC to get AC by sampling and math. This way RMS is the natural choice and simple rectification would not be easier (more hardware, though less software).I can totally undestand the decision to skip on the AC part. My DIY high resolution DMM also is DC only, though limited AC via the µC internal ADC as an possible option for later.

author: David Hess
author_rank: Super Contributor
timestamp: July 20, 2023, 11:46:19 pm
content: Quote from: Kleinstein on July 20, 2023, 04:22:35 pmIn the past sampling ADCs were more expensive than integrating ADCs and digital processing was more expensive, so analog rectification or RMS conversion was preferred. Today sampling ADCs are still more expensive than integrating ADCs in the form of delta-sigma converters, but digital processing is incredibly inexpensive.A pretty credible multimeter could be made now using an inexpensive sampling ADC and microcontroller, but offhand I do not know of any like this.

author: Stray Electron
author_rank: Super Contributor
timestamp: July 21, 2023, 12:13:20 am
content: Quote from: LinuxHata on July 20, 2023, 06:53:39 amIt's not pocket sized but go check out the Keithley 194A. They're FAST and sometimes sell for peanuts on FleaBay. The autoranging can be turned off and I think it meets all of your other specifications. Download the manual and read it, you can learn a lot from it.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 21, 2023, 06:47:19 am
content: Quote from: David Hess on July 20, 2023, 11:46:19 pmIndeed, digital multimeters are still 1980's legacy where every component is penny-pinched. It's pretty interesting that the component BOM (not including case, fuse, etc.) for a $50 multimeter is still like $2-3 (5% of retail price), while the component BOM for a $300 oscilloscope is easily $100 (30% of retail price) or so. Talk about bang for buck!One could assume that in $((2011+11+1)), more advanced multimeters with <1MHz oscillosscope-like features such as waveform display, min/avg/max/rms statistics etc. would be commonplace even in sub-$50 class (given the triviality and low cost of doing that), but nope, we are still discussing about the freaking true-RMS functionality (which was a big thing in early 1990's.) as a decision factor between cheap and expensive multimeters.

author: Zero999
author_rank: Super Contributor
timestamp: July 21, 2023, 07:10:49 am
content: Quote from: EPAIII on July 19, 2023, 09:07:57 amSImplfying David Hess' point. It doesn't mattter if the sample rate is below the half the maximum frequency of interest. The waveform is repetative so all that's required is enough samples to give a decent level of accuracy. Obviouly it can't take too many samples, if the rate is low, because the display will refresh slowly..

author: dobsonr741
author_rank: Frequent Contributor
timestamp: July 21, 2023, 08:03:15 am
content: QuoteI want to believe in progress. But when I look back how do I use my multimeter:Is this battery dead?Is this supply rail OK?Is there a short?Is this resistor really 10K?Then I do not see the need for progress, neither do the manufactures.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 21, 2023, 09:14:14 am
content: The digital way RMS is not necessay with a sampling ADC. One can as well use a relatively fast SD ADC chip. There are plenty of such solutions for power metering, AFAIK usually with an SD ADC.There are some relatively cheap handheld DMMs that now include RMS from digital calculations as part of the chip, though usually with a rather limited bandwidth (e.g. ~ 1 kHz for the UT133A ) as they tend to use a low power SD ADC. For practical use this still has the nice point of fast respond, a big step up from a sluggish analog RMS chip.Of cause the cheapest solution is likely a µC internal ADC. Even a 10 bit ADC can give reasonable acceptable results. I have tried such a solution with an AVR and was surprized how well it works.In most aspects the DMMs are mature.A point where one may want improvements is with a DMM is the case of a mixed DC+ AC signal. It is a shame that some meters (inlcuding some otherwise good ones) may show total nonsense with rectified half waves. So a display with DC , min and max (with a reasonable BW) and maybe RMS would be great.

author: LinuxHata
author_rank: Frequent Contributor
timestamp: July 21, 2023, 11:38:03 am
content: There's a new trend of multimeters coming, I'm hoping to get sample by September, btwThese look like smartphones, they run android, have large touch screen and a lot of things, like display font size, color, etc. can be customized.And since they run stock android, you can have almost any app for any kind of statistical analysis or whatever....

author: David Hess
author_rank: Super Contributor
timestamp: July 21, 2023, 03:53:30 pm
content: Quote from: Siwastaja on July 21, 2023, 06:47:19 amThe only change I have seen is the replacement of integrator based integrating ADCs with moderate resolution integrating delta-sigma ADCs, but the later can be implemented incredibly inexpensively on a digital CMOS process.QuoteThe thing which amazes me is that they are still using analog RMS conversion or averaging, but I guess that is still less expensive than a sampling ADC and DSP.Quote from: Zero999 on July 21, 2023, 07:10:49 amJust to be clear, the RMS is calculated from the amplitude histogram, and the amplitude histogram is not affected by undersampling. A repetitive signal is not required, but enough sample points are required to limit statistical error.Quote from: Kleinstein on July 21, 2023, 09:14:14 amI knew some modern multimeters were using moderate resolution delta-sigma converters, but not that any were using digital RMS conversion.QuoteI have had bad results doing that because of poor linearity so I rarely even consider it. I would expect a microcontroller's 10 bit ADC to be 8 bits linear, so not even 2-1/2 digits.QuoteMy old Tektronix DMM916 measures AC average, AC RMS, and AC-DC RMS just fine. I would love to find a modern multimeter which is just as good. Its only flaw is an awful backlight. APPA might have one but they are unavailable in the US.Currently I am looking for a modern replacement for my Beckman RMS225. Ampro has some contenders but their accuracy specifications are suspect.

author: EPAIII
author_rank: Super Contributor
timestamp: July 22, 2023, 10:57:11 am
content: Well, if you are going to be that way about it, may I suggest that the easiest way may be to buy a meter with manual controls (rotary switch) and modify it with your features. That will save a lot of work in the actual metering circuits.Quote from: LinuxHata on July 20, 2023, 06:53:39 am

author: dobsonr741
author_rank: Frequent Contributor
timestamp: July 22, 2023, 03:23:35 pm
content: QuoteIf OP still wants to build a DMM, instead of waiting for the Android-ish one, two options, available on Analog.com:Sigma-delta, +/-20,000 count:https://www.analog.com/en/products/max1499.html$15.89Dual Slope, +/-40,000 count:https://www.analog.com/en/products/max134.html$22.16Might get a sample, too.

author: LinuxHata
author_rank: Frequent Contributor
timestamp: July 22, 2023, 09:56:05 pm
content: Yes sure, I'm still buildingmy prototype actually works, using built-in ADC of PIC18F46K20...

author: LinuxHata
author_rank: Frequent Contributor
timestamp: July 22, 2023, 10:19:54 pm
content: These maxim chips do look interesting but besides the price, they have slow read speed and there are even some trimpots, which suggests me that these are from the times when Reagan still was the presidentMy current setup consists of the PIC microcontroller, mentioned above, connected via TM1629A (LED display driver) to LED display, using built-in ADC with internal reference and TP181 as input amplifier. All this is on breadboard with a lot of jumper wires and definitely not worth to showI have ordered some goodies on LCSC, such as more precise voltage reference (ADR5040) and precision, 20 bit external ADC (CS1180), while "normal" PCB is being designed right now. Regarding the BOM, all these together will cost less than that single Maxim ICThere will be built-in lithium battery, Type C USB port for charging (isolated from multimeter circuitry)

author: dobsonr741
author_rank: Frequent Contributor
timestamp: July 23, 2023, 05:28:10 am
content: How many digits on the display? My measure for the home built DMM is to have a stable 0 LSB display with the input shorted.When I did that I had to go to a 32 bit sigma-delta (ADS1263) and several iterations on the auto zero on the frontend. So I ended up with a 5 1/2 DVM, with 150mV lowest range, and a stable LSB of 1uV.

author: LinuxHata
author_rank: Frequent Contributor
timestamp: July 23, 2023, 06:55:05 am
content: It has 4 digit 7 segment display for value and 2 digit 16 segment display for V, A, Ko, mA, Mo display.There are also 15 buttons for directly selecting the measurement type/range and 15 leds for displaying what is currently selected.

author: dobsonr741
author_rank: Frequent Contributor
timestamp: July 23, 2023, 07:06:24 am
content: What analog front-end and protections are you planning to use?

author: Kleinstein
author_rank: Super Contributor
timestamp: July 23, 2023, 07:12:55 am
content: The mentioned maxim ADCs are relatively expensive. If one wants a low cost dual slope there are still some ICL7135 around, but still a bit outdated as this chip is directly display oriented. So for old style analog adjustment and not for digital calibration constants.For 4 digits the µC internal ADC is not really enough for DC, but there are plenty of cheaper more µC oriented SD ADC chips, like the mentioned CS1180.

author: uliano
author_rank: Regular Contributor
timestamp: July 25, 2023, 05:09:08 am
content: Quote from: Kleinstein on July 23, 2023, 07:12:55 amCan you expand on this?I would also be interested in cheap DACs

author: Kleinstein
author_rank: Super Contributor
timestamp: July 25, 2023, 07:48:04 am
content: The µC internal ADCs are usual 10 or 12 bit (1024 or 4096 counts) and this is just not enough to get 4.5 digit (e.g. +-20000 counts), even with oversampling. Oversampling can help with noise but hardly with linearity. For AC use one gets usually averaging over multiple codes and linearity is less of an issue. The alternative analog RMS converters have a somewhat limited linearity.There are relatively low cost SD ADC chips like the mentioned CS1180, MCP3421, ADS1112 or some LTC24xx variants that are good enough for 4.5 digits in combination with a µC to do the scale factors in software. This is different from the old style display oriented chips that directly map ADC counts to display counts and are made for adjustment on the analog side (e.g. trimmers).The SD ADC chips may however need some extra input amplifier / buffer as a 10 M resistance divider can be quite high in impedance. The old DMM chips like ICL7135 usually also inlude the amplifier/buffer and not just the ADC.With 1 switch per range one would likely do the range switching old style with the mechanical switches, which quite a bit simpler than µC controlled switching for autoranging. This is at least if one does not need a higher CAT rating / high voltage rating.I have build a similar DMM circuit with MCP3421 ADC for DC and µC internal ADC for AC and 2 rotary switches for the ranges (1x voltage, 1 x current) - if interested I could share more details / pictures.

author: uliano
author_rank: Regular Contributor
timestamp: July 25, 2023, 10:04:46 am
content: Any "low cost" DAC?I'm trying to build a "measuring" PSU, so I need also a couple of 14-16bit DAC for setpoints, but they all seem to be in a different price class than the CS1180

author: MrAl
author_rank: Super Contributor
timestamp: July 25, 2023, 11:31:14 am
content: Quote from: LinuxHata on July 18, 2023, 06:57:16 pmHi,Yes, i remember the old days of purely analog meters that used diodes to rectify the AC into DC and then average it. They had a factor built in to convert to RMS, but either it was a sine wave, or you had to know the waveform and apply a "crest factor" for that particular waveform in order to get the RMS value.To make up for the diode nonlinearity they used a nonlinear scale on the face of the meter so it would work down to lower voltages. Nothing like we have today though, but then that is the world of pure analog movements.For what it is worth, RMS stands for the "ROOT of the MEAN of the SQUARE" which in the continuous math domain simply means first you take the square of the voltage, then take the mean of that, then take the square root of that, and that is the RMS value regardless of what the waveform is assuming you have the required measurement accuracy.Mathematically it looks like this:Vrms=sqrt(integrate(v^2,t,0,Tp)/Tp)The v^2 is taking the square of the voltage which is really v(t), the integration is to calculate the mean, and the square root is just taking the square root of the result. The integration is performed over the interval 0 to Tp with Tp being the period.That is the theoretical calculation.

author: David Hess
author_rank: Super Contributor
timestamp: July 25, 2023, 06:34:03 pm
content: Quote from: Kleinstein on July 25, 2023, 07:48:04 amThe difficulty of adding a 10 megohm buffer to a high resolution delta-sigma ADC should not be underestimated. Precision low input bias current operational amplifiers with sufficient common mode rejection to preserve the linearity of a high resolution delta-sigma converter are rare to non-existent. (1)The ICL7135 got away with it by correcting the CMOS buffer's common mode error as part of the automatic zero cycle. The same thing can be done with a delta-sigma converter by measuring the input offset voltage of the buffer *at the common mode voltage* which is the input voltage instead of ground or some other voltage, which is what the ICL7135 does.I think this problem could also be solved by bootstrapping the input buffer to follow the input voltage, thereby increasing the common mode rejection, but I have only seen this done in electrometer circuits where the common mode voltage is way beyond the input range of the high impedance buffer.(1) There are some really good modern parts now though like the OPA140.