DIY DMM and selection of main ADC - Page 1
Forum section: electronics / projects

author: technix
author_rank: Super Contributor
timestamp: May 10, 2015, 02:49:35 am
content: I am thinking about using the MAX11202 24-bit Delta-Sigma ADC in my VoltCard DMM. Its data sheet quotes a 20.5-bit noise-free precision and a differential input. So how good is this chip if used in a DMM?1) What resolution can I shoot for with LM399-based VoltCard Original reference? What about ADR01B-based VoltCard Mini?2) MAX11202 have a differential input. How to properly use that? If I want to make both input terminals floating, how to pull that off?3) Is it a good idea to use relays to switch the terminals internally and expose only floating negative and positive terminals?4) For the current measurement, is it enough to just use a relay to switch in the current shunt? Inputs are still floating and differential.5) For the resistor measurement, is a low side constant current sink design with a high side current limiting resistor a good idea? Remember that the sense terminals are floating differentials.6) also on the resistor measurement, if I need two-wire measurement instead of four, is it enough to just use relays to short the sense and current supply pins together?7) What is a proper I protection circuit? A bunch of MOVs, a poly fuse and a fusible resistor?

author: Smith
author_rank: Frequent Contributor
timestamp: May 10, 2015, 06:36:40 am
content: I don't know about the ADC. Most DMM's (and most good measurement gear I know) use a dual slope adc instead. This has speed accuracy and can be made quite cheaply (depends on the components used).1. It has been used in 6,5 and 7,5 digit DMM'. The 7,5 digit probably have hand selected ones. It depends on your PSU, shielding, components, layout........4. Remember most relays have trouble switching high currents. One resistor with an opamp and selectable gain (analog switch ala dg444) would seem like the way to go.5. DMM 's I know use different currents depending on the resistor range selected.7. Watch Daves video on the Fluke 87V. This is a quality and save device.Ever watched the keithley low level measurements handbook? Every one making decent measurements and gear should have read this.

author: technix
author_rank: Super Contributor
timestamp: May 10, 2015, 08:52:06 am
content: Quote from: Smith on May 10, 2015, 06:36:40 amI understand that the dual slope ADC can be done using an integrater and a Schmitt trigger and an optoisolater can be used to isolate the analog and digital sections. But that construction got me really confused and the handling of this frequency signal was still confusing to me.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 10, 2015, 11:23:12 am
content: The high end DMMs use multi slope converters, to get a really good linearity. The low power Sigma-Delta converters are less linear, but still quite good. So the MAX11202 or a similar ADC (e.g. LTC24xx) are limited by INL to something like 5.X digits of useful resolution. This is not bad and already needs quite some effort at the analog front-end. In times of LCD Display and USB Interface it's not necessary to really choose the number of digits up front. Also number of digits displayed is not the important parameter of a DMM, it's more about the noise, linearity and stability.For computer readout it's not problem to give more digits even if they are far from stable, just to avoid extra rounding errors.For a DIY DMM such a ready made ADC is a good choice - It's cheap, well available and reasonable high quality. Going beyond that needs quite an extra effort - usually more than just a dual slope converter.Switching can be done with relays - preferably bistable versions, and at low voltages also with CMOS switches and sometimes JFET circuits. Also photo-mos can be an alternative in some cases. All have there advantages in some applications.The type of protection depends on the measurement range, and the degree of protection required:Current ranges need some kind of fuse, preferable a HRC type. In addition the low ranges can use diodes and similar, to allow a single fuse of something like 3 A for all smaller ranges.The protection of the voltage ranges can use PTCs with sufficient voltage ratings and a divider with sufficiently rated resistors.The ohms ranges are a little more difficult, and might use a combination of PTC, MOV and possibly fusable resistors.

author: technix
author_rank: Super Contributor
timestamp: May 10, 2015, 12:36:06 pm
content: Quote from: Kleinstein on May 10, 2015, 11:23:12 amWith digital compensation (since a microcontroller is already called for in this design) can I calibrate the ADC nonlinearity away?

author: Andreas
author_rank: Super Contributor
timestamp: May 10, 2015, 02:52:03 pm
content: Quote from: technix on May 10, 2015, 12:36:06 pmHello,I am doing this with my ADCs. So it is possible.But INL is also dependant on temperature.So the compensation might work only around room temperature.Dependant on the ADC the INL adjustment is very time consuming.The LTC2400 which I use has the advantage that the correction curve is nearly parabolic. (See AN86)This makes adjustment of INL relatively easy.With best regardsAndreas

author: Smith
author_rank: Frequent Contributor
timestamp: May 10, 2015, 03:00:53 pm
content: I'm curious, what kind of measurements at what levels do you want to do with it?

author: technix
author_rank: Super Contributor
timestamp: May 10, 2015, 04:08:01 pm
content: Quote from: Smith on May 10, 2015, 03:00:53 pmVolts: 100mV full scale up to 200V full scale, AC and DCAmps: 10nA full scale up to 10A full scale, AC and DCOhms: 10 m? full scale up to 10M? full scale, 2-wire and 4-wire.Protection Cat II 600V equivalent.All AC ranges are switchable between true RMS and peak values.All ranges support autoranging (since a microcontroller is already used and range selection is already either relay operated or electronically switched, autoranging is just a bit of extra code in the MCU).Both sense inputs are floating.

author: Marco
author_rank: Super Contributor
timestamp: May 10, 2015, 04:08:53 pm
content: Quote from: technix on May 10, 2015, 02:49:35 amIsolated supply and a buffered rail splitter, with some series resistance on the output to keep it stable, to create a virtual ground. This is connected to the neutral input and to one of the ADC inputs.QuoteJust copy the 34401A input section.QuoteUse separate inputs for current measurement, accidents are too common even with separate inputs.QuotePolyfuse for low Z voltage measurement if you want it.Fuse for current ranges with some kind of voltage clamp across the shunt (careful of leakage for lower current ranges, the diode bridge clamp leaks quite a bit).From the high side of the shunt and from the voltage input you should have an input resistor string of around 100K which should be able to withstand short 10 kV range surges and continuous kV range voltages with the help of a TVS across the input buffer ... you can add fuse/ptc in series and MOV/GDT/spark-gap in parallel before the resistor string, but that's mostly just for peace of mind.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 10, 2015, 06:51:34 pm
content: Just a standard 1:1 copy of a standard multimeter is not very useful unless you need something special. There it's cheaper to buy a $400 DMM from one of the big players of maybe a used one. This would be still more reliable.There are a few point where a specialized HW might justify a DIY solution. This could be something like:- very good isolation by battery operation and optical interface, likely with limited ranges (e.g. DC only), to save power- high quality RMS with limited bandwidth (use digital sampling, similar to 3458, but with integrated ADC)- special AC handling, frequency selective- very low cost with limited ranges (e.g. DC only), possibly multi channels, possibly Data-logging- extra large display (though this might still be just an addition to a standard DMM with PC interface)If it's just for learning, exercise than I would start at a lower level and keep it modular. You may add ranges as you need them.

author: cncjerry
author_rank: Unknown
timestamp: May 10, 2015, 06:57:34 pm
content: I've been playing around with building my own as well. I have a pair of 3457a so in my case it is an exercise. initially I am working on references for both voltage and current and playing around with integration. When you get into it you realize how important stability, shielding, temperature etc is and how hard it is to control the variables.I was thinking about using a separate 24bit ADC but will most likely roll my own dual slope. I would like to hit 5.5 digits on my own.

author: technix
author_rank: Super Contributor
timestamp: May 10, 2015, 07:34:10 pm
content: Quote from: DiligentMinds.com on May 10, 2015, 05:23:57 pmSelf exercise and interest mostly, and I do hope to shoot for a 8.5 or maybe 9.5 digit DMM in the future. Noticed the DMM is built around my VoltCard reference module which is designed to be interchangeable and have its calibration stored digitally? My DMM project have its EVERYTHING modularized, interchangeable, and digitally compensatable.It will be a four-board construction if not five:* The base board, which is all digital but with precision analog considerations, is really just MCU, screen, keypad and USB interface. It talks to other boards using digital interfaces like SPI (e.g. the current ADC, MAX11200, speaks SPI) and I2C (e.g. EEPROMs storing calibration data, mostly 24C32, speaks I2C).* The voltage reference takes the VoltCard form factor (9-pin SIP in 20-pin size, with 0.1in pin pitch) and is used throughout the analog section. (not only ADC use it, the analog front end generates the constant current for the ohms ranges based on it too.)* The analog front end board have all input protection and preamplification, and outputs a voltage to the ADC board. This is the only place things like ultra low noise, high voltage or low leakage matters. Range and scale selection is controlled digitally using a few PCF8574s (speaks I2C to MCU) and relays.* The ADC is on its own board, taking the amplified input from the analog front end board and digitizes it. The MAX11200 won't really call for a big board, but rolling that Mark/Space circuit will certainly call for a bigger board.* Time base, if used, have its own board, and have a RTC built on it. I will use 32768Hz as the standard frequency for this DMM project. Possible solutions include DS3231 DTCXO (this will be my first option as I have a whole bunch of them, and they have generally 2 to 3 orders of magnitude better tempco and drift, as well as initial accuracy than a watch crystal) or rubidium, both with optional GPS disciplining.So I will start off as a 6.5-digit (doesn't really matter as it is all software job, the screen is 16x2 characters) using MAX11200 24-bit Delta-Sigma ADC, LM399-based reference ("VoltCard Original") or maybe ADR01-based reference ("VoltCard Mini") for now and I will upgrade its precision and resolution by obtaining better parts and build better modules gradually.FYI, LTZ1000 is within my scope as the reference solution ("VoltCard 1000")BTW, since I already have a few DS3231 but I don't have a frequency counter yet, does it make sense for me to build one of that using DS3231 as the time base first? It would require a way simpler front end (just a null detector), DS3231 time base, some kind of microcontroller, screen and USB interface. And since it is obviously small enough it can even be made battery operated (internal li-po with built-in charger)

author: technix
author_rank: Super Contributor
timestamp: May 10, 2015, 07:48:52 pm
content: Quote from: Kleinstein on May 10, 2015, 06:51:34 pmI shoot for modularity that allows me to gradually upgrade it to some impressive specs, and allow me to build modules without making tradeoffs (extremely slow, 10 samples per second multi slope ADC that is capable of 9.5 digit accuracy versus a fast 20ksps true RMS delta-sigma with 6.5 digit accuracy, or ultra low leakage/noise front end capable of 10uV/10uA that is only Cat I 250V rated and DC only verses Cat IV 1000V rated with direct mains measurement support but poor low voltage/current performance, or instant-on 3ppm tempco ADR01B reference versus 0.05ppm tempco but takes a while to heat up LTZ1000 versus cryogenic superconducting Josephson junction primary reference, or if a time base is required rubidium or OCXO or TCXO, that is all just a module swap away)

author: Kleinstein
author_rank: Super Contributor
timestamp: May 10, 2015, 08:38:59 pm
content: The integrated ADCs want a ref. voltage of something like 2-4 volts. So it does not make much sense to have the ref. module to amplify the voltage to 10 V if you would actually need something like a divider of 2 or 3. A divide by 2 (and little more complicated by 3) can be done quite stable with a switched capacitor divider (e.g. LTC1043). So the ref. board should also have an output for the raw ref. voltage.Amplification and division is more like part of the ADC board.To get really good line frequency suppression, the ADC should use an external clock. So the notch is at 50.0 Hz, not 51 Hz.Such a DMM would have two isolated sections, one connected to the inputs and one likely connected to USB and / or RS232. Things like keys and display should not be at in input section.Still the input section might use a simple µC instead of I2C port expansions, to allow for a little more intelligence here, so only few lines to the outside are needed. However a fast ADC for AC conversion could be somewhat tricky and need extra handling.

author: Marco
author_rank: Super Contributor
timestamp: May 10, 2015, 08:47:16 pm
content: Dielectric absorption correction is black magic ... getting a slope ADC up to or beyond the quality of LTC24xx or even a MAX11202B is not straightforward.

author: technix
author_rank: Super Contributor
timestamp: May 10, 2015, 09:09:00 pm
content: Quote from: Kleinstein on May 10, 2015, 08:38:59 pmNoticed my word "interchangeable"? If the reference is built into the ADC board I would divide the reference voltage down as well using a pair of matched resistors (0.5ppm relative tempco available in very cheap prices) and a dual precision op amp chip. However the problem here is the voltage reference have to be interchangeable so a single reference voltage have to be used and I chose 10V.

author: technix
author_rank: Super Contributor
timestamp: May 10, 2015, 09:18:41 pm
content: Quote from: Marco on May 10, 2015, 08:47:16 pmSo if not using slope ADC how to reach 32-, 36- or 40-bit accuracy? (for beyond 7.5 digits I prefer start talking in binary which is easier to handle in MCU. 8.5 digits is slightly worse than 32 bits)(p.s. I will stop dreaming about a 64-bit accuracy or that will have to use cryogenic superconducting Josephson junction primary reference backed by a Rubidium standard, lots of lily gilding in the analog front end, and wipe out HP3458A as secondary standards)About the data handling capability, AVR probably will hit a practical limit at 6.5 digits (24 bits) and I probably should move to PIC32 or ARM for 28-bit and beyond.

author: branadic
author_rank: Super Contributor
timestamp: May 12, 2015, 07:02:36 pm
content: For your info, ADI has introduced the AD7177-2, a 32bit Sigma-Delta ADC with 10kSps.32-bit data outputFast and flexible output rate: 5 SPS to 10 kSPSChannel scan data rate of 10 kSPS/channel (100 ?s settling)Performance specifications19.1 noise free bits at 10 kSPS20.2 noise free bits at 2.5 kSPS24.6 noise free bits at 5 SPSINL: ±1 ppm of FSR85 dB filter rejection of 50 Hz and 60 Hz with 50 ms settlinghttp://www.edn-europe.com/en/analog-devices-ad7177-2-32-bit-931-916-adc-in-distribution.html?cmp_id=7&news_id=10006376

author: Andreas
author_rank: Super Contributor
timestamp: May 12, 2015, 07:27:11 pm
content: Quote from: branadic on May 12, 2015, 07:02:36 pmMhm: when looking at the datasheet the 32 bit data turns out to be 28 bit conversion + 4 bit channel data.On the other side the INL is not bad for a sigma delta.For the mains supression I would wish me a higher figure.But it seems to be the price for the higher possible speed.With best regardsAndreas

author: technix
author_rank: Super Contributor
timestamp: May 12, 2015, 09:10:52 pm
content: Quote from: branadic on May 12, 2015, 07:02:36 pm24.6 noise-free bits at 5sps, we are talking 7 guaranteed noise-free digits equivalent here and with part selecting and matching probably squeeze 8 digits out of it. I am not sure if either its internal 2ppm reference or ADR02 can match up that performance... Maybe if I used that DAC I will have to use LTZ1000 reference?

author: paulie
author_rank: Frequent Contributor
timestamp: May 12, 2015, 11:38:39 pm
content: Quote from: technix on May 10, 2015, 09:09:00 pm10v is probably a mistake for what you described so far. That is what old fashioned meters and lab references used and volt-nerd dudes who live on a cloud. A 1980s cloud at that. Modern ADC use much lower voltage due to CMOS and bandgap technology, often with 3.3v or even 2.5v upper limit.As you get into this I think you will figure this out and discover designing a high precision DMM is harder than expected and there are much more important factors involved. I set out on similar project and after many blind allieys settled on ready to go module (5 digit amplified up to 6 digits). Very educational experience. At least for me.

author: technix
author_rank: Super Contributor
timestamp: May 13, 2015, 12:02:05 am
content: Quote from: paulie on May 12, 2015, 11:38:39 pmI am dialing things back a bit too here. I think I will start off with some ICL7129-based 4.5 digit DMM design with external ADR02 or MC1403 reference (turned out to be really cheap from Shenzhen.) This will be a more established basis point and those dinosaur chips are actually easier acquired, and the lower resolution also means lower parts requirement and my existing stock of 25ppm 1% thru-hole resistors can be used. Microcontroller, though, is not going away, so is the relay-operated frontend.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 13, 2015, 03:51:27 pm
content: The dual slope converter like ICL7129 are not that simple. There are 2 tricky point: one is the right choice of the capacitor. For 4.5 digits this should be a type with low dielectric absorbtion, so a simple mylar cap. is not a good choice. The 2nd point is adjustment ad the zero point. For 4.5 Digit resolution they often need some adjustment to a correction circuit to get the part around 0 right.For just 4.5 digits there a several low cost sigma delta converter that can also be used. Even a MCP3421 or similar might already do it and already includes a reasonable quality reference. However they usually need an extra input amplifier.The difference is that the old ICL... als generally display oriented, low DNL and limited resolution, so adjustment is analog. The Sigma delta converters are binary and more or less need a µC to drive the display. The extra resolution allows for numerical adjustment - so no need for the dividers and shunts to be in exact decimal ratios.

author: macboy
author_rank: Super Contributor
timestamp: May 13, 2015, 03:59:36 pm
content: Quote from: DiligentMinds.com on May 13, 2015, 05:25:07 amDo you even look at data sheets or just drool over marketing specifications?Sure, 1 ppm INL is quoted in the specs. But take a look at the data sheet, page 13, figure 19. This is INL vs input voltage for various configurations. Look at the INL when using the internal reference and compare to when using an external one. With the internal reference the INL ranges from -15 to +15 ppm! Yes youcanuse an external reference, and in fact youmustif you want good performance. The 1 ppm spec only applies with a 5 V external reference and input buffers disabled, and it is, of course, "typical" not guaranteed. Virtually every other chart in the data sheet gives data collected while using an external reference, usually 5 V (since that gives the best performance).

author: technix
author_rank: Super Contributor
timestamp: May 13, 2015, 04:46:48 pm
content: Quote from: macboy on May 13, 2015, 03:59:36 pmIf this is used in my DMM design I can live without the internal reference or the buffer - I don't need either actually. The voltage reference of this have to be based on LTZ1000 if not outright a Johanson primary reference, and the analog frontend will have its own low noise low distortion buffer.

author: macboy
author_rank: Super Contributor
timestamp: May 13, 2015, 05:45:23 pm
content: Quote from: technix on May 13, 2015, 04:46:48 pmIf you think that you are going to build a DMM with a built-in Josephson Junction as an internal reference, then this thread is pure fantasy.

author: Marco
author_rank: Super Contributor
timestamp: May 13, 2015, 06:33:21 pm
content: LTC2400 with INL correction is a lot cheaper than AD7177.

author: technix
author_rank: Super Contributor
timestamp: May 13, 2015, 06:48:38 pm
content: Quote from: macboy on May 13, 2015, 05:45:23 pmWell all my gear design is always modular so if either an external cryogenic physics package is used or somebody did manage to produce a small enough physics package to be used inside the machine, my DMM certainly can use one, since that is just a module swap away as always, as long as the module or adapter have the same pin out as the LTZ1000, LM399, ADR02 or MC1403-based reference modules.

author: Edwin G. Pettis
author_rank: Frequent Contributor
timestamp: May 13, 2015, 07:36:25 pm
content: Point of fact, the LTC2400 is just as good, maybe even a bit better in reality and is faster than this AD7177-2. The 7177-2 data sheet looks like something that Vishay puts out, 'typical' that isn't so typical, this is just sloppy!

author: tszaboo
author_rank: Super Contributor
timestamp: May 13, 2015, 09:13:59 pm
content: Quote from: technix on May 13, 2015, 06:48:38 pmWell, good luck making an LTZ1000 working nicely with any single chip ADC. The zener references create 7V, ADCs require 5V/2,5V/4,096V as their reference, and they pop above that.Not to mention your design ideas are filled with issues. The VoltCard (what is that anyway) design you are doing in an other thead is a prime example. LM317 constant current and single ended routing of the reference voltage... Resistor dividers in your measurement path. You might get 4.5 digits with these techniques, but nothing beyond that (for sure not 9,5). Do yourself a favor, buy a multimeter which has more than 6,5 digits, and start experimenting with components and circuits. This is not you just oversample and "she be arright".

author: technix
author_rank: Super Contributor
timestamp: May 14, 2015, 12:41:30 am
content: Quote from: NANDBlog on May 13, 2015, 09:13:59 pmI have long scrapped the LM317 constant current source in favor of a matched transistor pair current mirror based design. For LM399 the driving circuitry is one current mirror, one TL431 and one small resistor, and the noise and tempco caused by TL431 turned out to fell lower than LM399's own noise levels and hence can be disregarded. If I am using LTZ1000 the constant current source will be two current mirrors and an ADR02. I have long posted the updated schematics and board design.And about dropping the 6.7V from LM399 or LTZ to some voltage that modern ADC can take, one matched resistor pair divider and I get about 3.35V out of it, and those 5V ADC are happy. Those matched resistor pairs does not need even need good tempco or precision as long as their value ratio and tempco ratio are close enough to 1 and the ratios have a low tempco and drift. That would be a lot easier to obtain.Differential output is achievable and I just didn't bothered with it yet. I will consider it now.VoltCard is what I call my swappable voltage reference modules for DMMs and adjustable voltage standards.

author: Marco
author_rank: Super Contributor
timestamp: May 14, 2015, 01:08:18 am
content: Quote from: NANDBlog on May 13, 2015, 09:13:59 pmYou could use some other voltage reference for the ADC and always use a divider on the input (both should have good enough short term stability not to matter). Then run a calibration cycle against the LTZ1000 every so often (or every measurement if you like).

author: technix
author_rank: Super Contributor
timestamp: May 14, 2015, 01:52:25 am
content: Quote from: Marco on May 14, 2015, 01:08:18 amSince ADR02B and ADR03A are just so cheap from Shenzhen I think if I use this idea I will use either of those as a short term reference on the ADC board, and internal calibration can be done by internally measuring the LTZ1000 (or for the 4.5 digit job LM399) with or without a matched resistive divider using every voltage range and note down the measurements as a coefficient in the EEPROM and apply this coefficient every time a measurement is taken. This will cancel out the combined ADC and analog frontend errors.For the ohms range the same voltage reference used by the ADC is also used by the constant current generator so as long as the percision resistor for the constant current sink don't drift the reference error cancels out. The only source of error is the ADC and canbe compensated.For the amps range I can just set up calibration lines to measure all current shunts using the ohms range and note down their resistances in the EEPROM. Combined with the calibration of ohms and volts range it will be okay.Inlinerity can be compensated by taking several measurements of the reference at different resistive divider notches (for the volts range, compensates the combined effect of ADC and analog frontend) or the reference divider resistors themselves (for the ohms range, compensates the ADC only) and plot out the inlinearity using a odd-th order polynomial and apply the antifuction of the polynomial to the ADC reading when taking measurements. Only requirement for the ADC here is monotonicity.Noticed that all compensation is done using math? This is what the MCU will be doing in the most time, and why the requirement of the analog portion of this design is a lot lower.

author: charlespax
author_rank: Regular Contributor
timestamp: May 14, 2015, 01:53:56 am
content: Quote from: technix on May 10, 2015, 07:34:10 pmSounds like a cool project. Are you blogging this somewhere I can follow your progress?

author: technix
author_rank: Super Contributor
timestamp: May 14, 2015, 01:55:50 am
content: Quote from: charlespax on May 14, 2015, 01:53:56 amThis project, for now, is just in the planning stage. I am not blogging this yet but you can watch my blog if you want:https://en.maxchan.info/

author: Kleinstein
author_rank: Super Contributor
timestamp: May 14, 2015, 07:26:51 am
content: Having the real, long time reference connected to the input is a good idea, so at least the 10 V range (or whatever is used to measure the 7 V reference) has no extra divider or amplifier. So there is no need to provide an adjusted 10 V reference, measuring directly the 7.x V is better.For use with a integrated A/D a good quality bandgap reference is more suitable. They have reasonable low noise, possibly better than the LM399, and the voltage can be of the right size. However plastic cases are somewhat sensitive to humidity. This allows the DVM to run even without the power hungry LM399 active. This may be important if running an battery.It is still possible to use the LM399 with filtering to derive the AD's reference, even if the divider and filter is not that log time stable. The simple divider and bandgap reference can have comparable performance. For just 4.5 Digits one may not need a LM399 at all, or only once every few months to do checks.

author: tszaboo
author_rank: Super Contributor
timestamp: May 14, 2015, 07:35:32 am
content: Quote from: technix on May 14, 2015, 12:41:30 amThat is exactly the sort of things I've talked about. And if you have a divider, which you made by any resistors which you think they are good enough, they are not going to be. Say, you select two 100ppm/K resistors 10 cents each, a different engineer selects a 3ppm/K bulk metal foil 10 Euro single package less than 1ppm/K tracking. You bin it, make sure it is the tracking is the same value as the expensive one.After 1000 hours, the resistors drift. Yours will drift 0.5% the bulk metal will drift 0.005%. Let's say this will not totally ruin your instument, in the first place, because you calibrate it out. So the tempco will be based on the new value of the resistor. 0.5%*100ppm, say the two resistors drift in the different direction, you ended up with 2ppm/K tempco, double what you matched the resistors to. And this was only one secondary effect I calculated, with real world examples.ADC has low input impedance on the reference pin, you need to buffer the divider. You select the OP07 because that is that you can get cheap on the Chinese market, let's hope it is not a 358 with different laser marking. That has 1.3uV/K offset drift, which is 0.3ppm/K for 3V. Bias current will double this error for you.So even with the LTZ1000, your extra circuit adds 2.6ppm/K triple of what the reference alone is capable of. It also takes the reference to the level of the 5V IC Vrefs. It is probably good for 4.5 digits as I said.

author: technix
author_rank: Super Contributor
timestamp: May 14, 2015, 07:49:50 am
content: Quote from: NANDBlog on May 14, 2015, 07:35:32 amThose "matched resistive dividers" I was talking about is like the ones you find in Fluke meters, those laser trimmed precision resistor networks built from one piece of original metal film over ceramic material and laser trimmed to value. They are guaranteed to match in both value and tempco to a very fine degree, and since dividers is all about ratios, those deviations cancel out.The OP07C I got is indeed OP07C - I tried to run 1MHz through it and it had trouble with it. LM358 won't. And give the price of LTZ I obviously won't pair OP07C to LTZ - it is only paired to LM399 which itself is quite noisy.

author: Marco
author_rank: Super Contributor
timestamp: May 14, 2015, 01:44:36 pm
content: Quote from: NANDBlog on May 14, 2015, 07:35:32 amThis does not matter on a 10V range (ie. ~1:1 divider) if you auto-calibrate against the LTZ1000.

author: splin
author_rank: Frequent Contributor
timestamp: May 14, 2015, 04:37:57 pm
content: Quote from: Edwin G. Pettis on May 13, 2015, 07:36:25 pmI have to say I find your conclusions to be very surprising and unlikely. Do you have any evaluation results you can share, or link to, comparing the LTC2400's performance with the AD7177-2 which led you to that opinion? Or are they based on datasheet comparisons? Can you provide any specific examples of dubious 'typical' specifications in the AD7177-2 datasheet?I have found errors in many manufacturer's datasheets, not just AD who I generally believe to have a good reputation - with the exception of their website redesign perhaps.As to the LTC2400 being 'faster than this AD7177-2', are we looking at different datasheets? The one I have shows the latter to have a better noise performance at 10,000 SPS than the LTC2400 does at 22 SPS (ENOB = 22.4 v 21.6bits)!!!!Neither LT or AD provide minimum noise specifications, only typical figures.[Edit: Removed provocative and unwarranted 'smiley'.]

author: Marco
author_rank: Super Contributor
timestamp: May 15, 2015, 03:49:43 pm
content: There is more to live than INL ... no one is doing 8.5 digits +/- 1 LSD INL.

author: technix
author_rank: Super Contributor
timestamp: May 15, 2015, 05:21:49 pm
content: Quote from: DiligentMinds.com on May 15, 2015, 03:38:53 pmThis just occured to me - maybe the noise can be reduced by spectrum-based noise gating using fast fourier transform? Again, isolate & fix the error using maths. That does mean that I have to throw in a processor more powerful than an AVR though.

author: cncjerry
author_rank: Unknown
timestamp: May 15, 2015, 05:51:52 pm
content: Granted I don't know much about this but here are some findings:On the current source, I've been playing with a 399 as the reference and low noise op amps using a traditional CC op amp circuit without noise eliminating caps. The caps caused weird oscillation. Using that arrangement I get last two digit flicker on my 3457a in 6.5digit mode but long term drift that takes an hour to stabilize. If I adjust it to 9.99995 then I can get to last digit flicker when measuring the 1ma current source directly. I played with a Styrofoam oven with a heater and that helped a little but not perfect. I think the problem is related to the heating of the Bourne 10 turn pair I use for adjustment, duh. I also think buffering the output will be smoother but add noise as a tradeoff.I don't think you can get the performance using an IC DAC though the LT2440 with a separate vref (maybe an LT1460) as that explored by Paul in the N2PK VNA might get you 4.5 digits? Don't know.Discrete dual slope integration isn't hard to implement but then you need an OCX source or better for the clock and now you have more control but more variables. A valpey fisher low phase noise master clock will cost $120 new without OCX.So you need a vref for the CCS, the CCS, the clock, stable resistance sources,etc and I can see where it gets complicated pretty quickly. You can also program in run time stability curves to account for the startup drift.

author: splin
author_rank: Frequent Contributor
timestamp: May 15, 2015, 06:24:41 pm
content: Quote from: DiligentMinds.com on May 15, 2015, 03:38:53 pmQuite. Which begs the question why Keysight's '7.5 digit' 34470A has a linearity spec of .5ppm reading + 1ppm range. To be more precise the datasheet specifies the A/D Linearity - one might hope that it actually includes all parts of the signal chain from the input terminals as well as the A/D but marketeers rule these days so who knows (and is allowed to tell us)? E.G. what about the voltage coefficient of the input divider - is it likely to be linear on the 1000V range to < 1ppm? I don't know.However Keysight's view may be that its basic 24 hour accuracy is only 8ppm reading + 2ppm range anyway so 1.5ppm linearity error is well within that. Also the non-linearity is likely to be a reasonably smooth curve with much smaller deviations than maximum 1.5ppm over a limited part of the range - say 10% or less.Of course it could be that there are some nasty discontinuities in the linearity curve, but it still doesn't mean that the 7.5 digit resolution is useless. For example when measuring a voltage difference where the absolute values are not important. Providing the voltage difference is less than 50% full scale, it should be possible to average out some or most of the linearity error by adding various different offset voltages in series with the input measurements. Not perfect without knowing the shape of the linearity curve and a pain perhaps (especially as the offset voltages need to be stable over the time needed to make each pair of measurements) but if you can't afford an 3458A it may give you a way to use that resolution.Keithley's latest 7.5 digit DMM7510 is even worse with an ADC linearity spec of 1ppm reading + 1ppm range.Why is linearity specified that way for meters - what is the significance/source of the % of reading element given that meters typically will periodically autozero and calibrate full scale gain and thus INL should be zero at those points? ADC linearity is normally specified as a percentage of full scale either as maximum deviation from a line between the minimum and maximum inputs or a best fit line which gives the smallest INL. Corrected 8458A to 3458A

author: Kleinstein
author_rank: Super Contributor
timestamp: May 15, 2015, 06:30:38 pm
content: Noise wise these high end Sigma-delta converters (e.g. AD7177 or LT244x) are already quite good. Its already hard to get an amplifier that good in the 0.1-10 Hz rang, especially if you want something like GOhm input impedance. Multi chip slope ADCs usually use a higher voltage and are thus less noise critical when measuring 10 V signals. The sigma delta ADCs might have advantages measuring low voltages / currents , as there natural range is more like 2 V.The high end voltmeters get 7.5 and more stable digits only at stable temperature and 10-100 s integration time. So noise performance seem to be not that much better.The sigma delta converters are limited in INL. However for something like the LTC2400 the type of INL seems to be rather well behaved, so that an correction might be possible. If checked by measurements of the whole setup, there is no need for the converter to have guaranteed low INL - typical value might be reached. Also the input amplifier might need some corrections to reach a good INL, independent of the ADC.Something like 4.5 digit performance is already in reach of lower end SD converters, like a MCP3421 with internal reference - at least with 60 Hz line frequency.

author: Marco
author_rank: Super Contributor
timestamp: May 15, 2015, 06:35:43 pm
content: Quote from: cncjerry on May 15, 2015, 05:51:52 pmAre you sure this is necessary? The traditionalADC/jitter interactiondoesn't really do much at 1PLC sampling. Lets say we want 1 ppm for 50 Hz signals ... simply to get the 1ppm sampling position accuracy on a 50 Hz signal only requires jitter better than 1/(10e6*2*pi*50) = 3.3ns.

author: cncjerry
author_rank: Unknown
timestamp: May 16, 2015, 04:12:45 pm
content: Quote from: Marco on May 15, 2015, 06:35:43 pmYou bring up a good point but isn't a good clock necessary for accurate count at 7.5 digits? I've not looked at the master in my 3457a.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 16, 2015, 05:07:09 pm
content: For just measuring DC voltage at a rather slow rate, no accurate clock is required. The multi-slope ADC is somewhat comparable to PWM in demands, so it should be sufficient to get a time of something like 1 s resolved to something like 0.1 ppm. This may be difficult with a RC clock, but essentially every of the shelf crystal would be ok for 8-10 digits. Still the chip internal clock seems to be OK for 20 Bit resolution and more. Clock stability can be very demanding if you sample something like kHz signal at 16 Bit, but this a mainly to get the full resolution in signal phase too.Also getting the PLC timing about right, might help to get the best 50/60 Hz suppression. But no need to be more accurate than the grid stability. And even at 1 percent of suppression is already very good.