Community Bench Meter > ADC and associated - Page 1
Forum section: electronics / projects

author: Bloch
author_rank: Unknown
timestamp: July 09, 2011, 08:52:18 am
content: First stay on subject and be positive or please stay away from this topic!I would like to find out that kind of ADC that would work in a "Bench Meter"There are many different types of ADCSuccessive Approximation RegisterDelta – SigmaFlashDual SlopeAm I right in that when measuringDC voltage, Ohm, Diodethe ADC type has nothing to say?But that about AC.It have been a long time ago buti did learn that some ADC did not like the input to change under conversion. Is it just to select the right type or do there need to be sample hold circuit ?And that about RMS.AD637Calculate itMy ques is that a Calculate RMS there will need to have more than one measurement per sinus. But how many ? are there a standard for that ?I look forward to seeing your opinions.

author: jahonen
author_rank: Super Contributor
timestamp: July 09, 2011, 09:25:46 am
content: I think those types that reject 50/60 Hz line noise and harmonics naturally are the best choices here. That means either delta-sigma (with proper choice of modulator clock frequency) or dual-slope (with suitable integration time) might be a good choice.The choice of samples in RMS calculation would be so that samples per cycle is an integer much larger than 2, so there would be zero ripple on the result. It seems that many TRMS multimeters use a RMS-to-DC-converter chip, so there must be some reason to do it that way, instead of just sampling and calculating it by the definition.Regards,Janne

author: Bored@Work
author_rank: Super Contributor
timestamp: July 09, 2011, 09:49:25 am
content: Quote from: Bloch on July 09, 2011, 08:52:18 amWho the [unnecessarily offensive word removed] are you that you think you can dictate what we have to write here?

author: Fox
author_rank: Regular Contributor
timestamp: July 09, 2011, 09:53:26 am
content: Hi Bloch,your selection of the ADC depends on what you want to achieve.What accuracy and resolution do you want ,a 4.5Digit, 5.5Digit or even 6.5Digit Meter?What samplerate do you you want 3SPS or 30kSPS?I would recommend an Sigma-Delta ADC, many Bits for relative less money, look at the LT2410, LT2440 or any other LT24** series ADC or the ADS12** series from Texas Instruments, or look at Analog Devices.They all have Appnotes how to select and use ADCs, i highly recommend reading ADC Datasheets and Apnotes.For RMS calculation there aren´t that many chips, AD536, AD636, AD637, LTC1966, LTC1967, LTC1968 and one from Cirrus Logic (don´t now the number).As i said above Datasheets and Appnotes are very good resources.

author: Bloch
author_rank: Unknown
timestamp: July 09, 2011, 10:01:15 am
content: Can some one explain to me how the 50/60Hz filter will do to the measurement of a AC 50Hz ?If it filter 50Hz away then it will not be a true measurement .

author: jahonen
author_rank: Super Contributor
timestamp: July 09, 2011, 10:05:00 am
content: Quote from: Bloch on July 09, 2011, 10:01:15 amThe idea behind it is that you don't try to measure AC directly, but rectified and filtered version of it (false-RMS) or via a RMS-to-DC converter chip, so you are always actually measuring a DC.Regards,Janne

author: Bloch
author_rank: Unknown
timestamp: July 09, 2011, 10:21:11 am
content: Thanks JanneLOL I must read some more about that. Some times it is nice to know why it just works and other times it just fine to know it worksI have a keen eye for the LTC2460 / LTC2462. The internal voltage reference is a nice feature.I have not yet read the app notes but the LTC2462 with the deferential input must be the better choice here. But it is a lot more expensive 22$ vs 4$ on Digikey

author: Unknown
author_rank: Unknown
timestamp: July 09, 2011, 01:14:51 pm
content: Quote from: Bloch on July 09, 2011, 08:52:18 amADC type determines stability, linearity and noise, which are also important for DC measurements.Quote from: jahonen on July 09, 2011, 09:25:46 amThe Agilent 3458A can directly sample the AC voltage for low frequency AC. Trouble is the sampling rate. Even a fairly basic DMM will often have a few kHz of AC bandwidth, but only a sampling rate of a few samples per second.Quote from: Fox on July 09, 2011, 09:53:26 amI would be happy with a 4.5 digit design with matching accuracy. A good 4.5 digit DMM will have something like 0.025%/1yr basic DC accuracy. A good 5.5 digit may be 0.009%. A good 6.5 digit meter may be 0.0025%. Where are you going to get a resistive divider with this kind of stability? Voltage references also get pricey. How do you calibrate it?Quote from: Fox on July 09, 2011, 09:53:26 amGood luck with 30kS/s. A $1k+ Agilent 34410A will do 10kS/s at 5.5 digit. A $8k+ Agilent 3458A will do 100kS/s at 4.5 digits.Quote from: Fox on July 09, 2011, 09:53:26 amAll commercial DMM's I've studied appear to use some version of a dual slope ADC, the advantage is inherent linearity as long as your reference is stable and you've got a good capacitor. Almost all bench meters I'm aware of use a DMM designed from discrete parts, not an off-the-shelf one. The cheap bench meters with specs similar to hand helds are likely to be an exception.I don't really see a point in trying to do it different than all commercial guys. Reading the theory of operations in some service manuals for eg. HP 3455A/3456A may be useful. There are also some HP Journal articles (available online) that talk about the design of their DMM's. Eg. issues 1969-03, 1972-08, 1977-02, 1981-04, 1983-02, 1986-02, 1989-04. Fluke and Keithley seem to publish less technical details. The more recent the design, the more custom silicon, and the less documentation.

author: Bloch
author_rank: Unknown
timestamp: July 09, 2011, 02:15:28 pm
content: Thanks almI have not posted any specs yet. The reason for this i wanted to see that isdurable.Yes a 8.5 Digit Meter with a 1mS/s would be nice. No that is not that i am after. Just as a 2.5 Digit Meter with a 1S/s would be to easy and useless.If this need NASA tools and components it will be a fail.Will this be durable and yet usable tool ?4.5 Digit4S/s ok30S/s greate.As for Accuracy if some thing like this would be nice even if just closeAs for accuracy, if possible then grate. But again only if possible without be crazy hard to do (borrowed from Fluke)Voltage DCRange: 6.000V - 1000VBest Accuracy: +/- (0.3% + 1)Best Resolution: 0.1 mVVoltage ACRange: 600.0 mV-1000VBest Accuracy: +/- (2.0% + 2)Best Resolution: 1.0 mVAs for calibration. It should be plausible to make without calibration. Buy precise resistors and a Voltage reference there can match the selected accuracy.Dave did a greate vidio on counts / digits / Accuracyhttp://www.eevblog.com/2009/08/21/eevblog-26-multimeter-counts-accuracy-resolution-calibration/So now I think4 Digit / 10000 count will be ok

author: Fox
author_rank: Regular Contributor
timestamp: July 10, 2011, 11:02:49 am
content: If you it quick and easy look at the TC7129 from Microchip a 4.5Digit ADC with LCD Driver.If you want to do it discrete the MCP3550 is a easy to use and cheap 22Bit ADC.

author: FreeThinker
author_rank: Frequent Contributor
timestamp: July 10, 2011, 11:29:11 am
content: Quote from: BoredAtWork on July 09, 2011, 09:49:25 amHe did say PLEASE

author: EEVblog
author_rank: Unknown
timestamp: July 10, 2011, 11:36:08 am
content: I'd have a look at what is used in a high spec meter, like maybe the Fluke 289 or something.That would be a good reference to work from.EDIT:I found my Fluke 189 schematics. It uses the LT2415-1 Delta Sigma and LTC1968 RMS converter.I'm not allowed to post it because it's propriety, but if you want it (I only have some parts of it), email me.Dave.

author: Bloch
author_rank: Unknown
timestamp: July 16, 2011, 04:35:23 pm
content: Quote from: EEVblog on July 10, 2011, 11:36:08 amThanks that is great. Now i have the "big" parts. There may be others that are better but if they are ok for fluke. Then there are a chance that i can make a "usefull" unit with them.I did try to send you a email to you ( I did use the email icon under you name ) not sure if you did see it or it disapered out on the big internet

author: EEVblog
author_rank: Unknown
timestamp: July 16, 2011, 11:11:09 pm
content: Quote from: Bloch on July 16, 2011, 04:35:23 pmI got emails from two people and sent the files, can't remember who though...Dave.

author: Unknown
author_rank: Unknown
timestamp: July 17, 2011, 12:11:47 am
content: LTC2415http://www.linear.com/product/LTC2415http://cds.linear.com/docs/Datasheet/24151fs.pdfThat seems cool. 6 digits for 10$How about LTC2412 same thing but 2 channels?What kind of voltage reference is suitable for this thing? The best one from Digikey seems to be MAX6350. But what is "good enough"?

author: Unknown
author_rank: Unknown
timestamp: July 17, 2011, 01:56:39 am
content: LTZ1000 is probably the most stable reference you can find, but kind of spendy and overkill. LM399 is a popular choice for mains powered meters (heated references tend to draw too much power for battery powered meters), lots of 6.5 digit meters (eg. Agilent 34401A/34410A) use selected versions of this.No experience with the MAX6350, specs seem OK at first glance. Long term stability is somewhat high for 6 digits, but accuracy matching 6 digits is probably optimistic anyway, especially since few people will have the proper equipment for calibration.Keep in mind that the two channels are on the same power supply, so they're not floating relative to each other.

author: Bloch
author_rank: Unknown
timestamp: July 17, 2011, 06:28:09 am
content: Quote from: EEVblog on July 16, 2011, 11:11:09 pmI have now send you a email direct this time.Quote from: HLA-27b on July 17, 2011, 12:11:47 amYes it ischeapandbetterthan i did first expected. And it is so much easy to remove bits than add them in software.But it seems to me as a hard to get. Farnell, RS Online and mouser don´t have them ? only digikey and linear ? that cant be good.....

author: Unknown
author_rank: Unknown
timestamp: July 18, 2011, 07:23:44 am
content: Trying to imagine how the things would work.Greatly inspired by Keithley Low Level Measurements Handbookhttp://www.keithley.com/knowledgecenterFire away.

author: Fox
author_rank: Regular Contributor
timestamp: July 18, 2011, 02:43:10 pm
content: This is a pretty standard blockdiagramm for a multimeter except the Inputdivider.You draw a R2R ladder, as far as i can see this would be not a good choice, it won't give you a decade step per range switching and its input impedance would widely vary.Normally one would choose one of these voltagedividers (see picture Inputdivider).The first one would give you constant input impedance, but you have to use relays for range switching.With the second one the input impedance would vary a little, but you can use relays or analog switches, this variant is used in handheld DMMs.For the current switching a circuit like in the second picture is normally used.I hope you can see the picture, it is my first post with pictures

author: Unknown
author_rank: Unknown
timestamp: July 18, 2011, 03:29:02 pm
content: Quote from: Fox on July 18, 2011, 02:43:10 pmIt was not an informed decision to be honest. Just thought it would be easy on the Bill Of Materials. If it is going to affect the accuracy it is of course out of the window.Thank you for the correction.Any links to reading material?I am not sure I can make use of it but somebody else might.Edit:Here is a nice article. ABC's of ADC's from Nationalhttp://www.national.com/assets/en/other/ABCs_of_ADCs.pdf

author: Fox
author_rank: Regular Contributor
timestamp: July 18, 2011, 10:09:17 pm
content: Sorry no direct links, i just collected all i have from the manufacturers Websites e.g Linear Technology, Texas Instruments, Analog Devices ets. (Appnotes, Datasheets about 500MB all related to the DMM theme).For the reading material, i found the following books helpful:OP Amp Applications Handbook by Walt Jung ( Analog Devices)The Dataconversion Handbook by Walt Kester (Analog Devices)If you need specific Appnotes or Datasheet e.g ADC, Voltagereference; let me know

author: Unknown
author_rank: Unknown
timestamp: July 18, 2011, 10:21:11 pm
content: No worriesI just started another thread which I hope will be something like a catalog of relevant information. So hopefully datasheets app notes etc will slowly trickle there. Of course everybody is more than welcome to add on.https://www.eevblog.com/forum/index.php?topic=4194.0If those books are available online you can post links there too.

author: Fox
author_rank: Regular Contributor
timestamp: July 18, 2011, 10:53:29 pm
content: I got these books from Amazon, don't know if ebook editions are available

author: Unknown
author_rank: Unknown
timestamp: July 18, 2011, 11:46:33 pm
content: Both seem to be available. You only need to know where to look.

author: Zad
author_rank: Super Contributor
timestamp: July 19, 2011, 01:36:35 am
content: If you are finding ADCs difficult to obtain, Linear, Analog Devices and TI all have samples programmes, although Analog Devices keep adding more hoops to jump through, like engineers are some sort of circus animal. The irony being that it will filter out genuine engineers because only students and kid hobbyists have the time to go through the whole procedure for the more interesting parts. AD and TI also have some nice voltage references too, and I suspect Linear are the same.I would get a precision DC voltmeter running first, then you can add on whatever you need for current ranges, resistance, diode test etc. For AC I would probably use a precision rectifier circuit and a relatively fast sampling ADC, with RMS / Crest factor compensation in software.

author: Fox
author_rank: Regular Contributor
timestamp: July 19, 2011, 05:40:47 pm
content: Quote from: HLA-27b on July 18, 2011, 11:46:33 pmJust remebered you can get the OpAmp Applications Handbookherehttp://www.analog.com/library/analogdialogue/archives/39-05/op_amp_applications_handbook.htmldirectly from Analog Devices as Pdf for free

author: Fox
author_rank: Regular Contributor
timestamp: July 19, 2011, 05:47:23 pm
content: Quote from: Zad on July 19, 2011, 01:36:35 amYes Linear got some very good references like the LTC6655; 0,025% initial accuracy and extremely low noise.For the RMS calculation i would just use a of the shelf solution like an LTC1968 very easy to use TrueRMS converter.QuoteTry to download the Code Composer Studio IDE from TI for the MSP430 from outside the US

author: johnmx
author_rank: Frequent Contributor
timestamp: July 19, 2011, 09:00:43 pm
content: Check out the new Delta Sigma ADC from Microchip:MCP3903 - Six ChannelSynchronousSampling 16/24-bit Resolutionhttp://ww1.microchip.com/downloads/en/DeviceDoc/25048B.pdf

author: Fox
author_rank: Regular Contributor
timestamp: July 19, 2011, 11:26:47 pm
content: Yea very nice device especially for something like Daves multichannel Multimeter concept

author: Zad
author_rank: Super Contributor
timestamp: July 20, 2011, 12:44:34 am
content: Those channels wouldn't be isolated though, and the quoted 91 dB SINAD is only equivalent to a data depth of 15 bits.

author: Richard W.
author_rank: Regular Contributor
timestamp: January 21, 2012, 10:03:15 pm
content: Quote from: EEVblog on July 10, 2011, 11:36:08 amThe voltage reference is a Linear Technology LT1019 AIS8-2.5Can be seen on the teardown pictures. SMD code is 19AI25.just for sake of completeness