6 and half digital voltmeter; 1V-1.1V dc - Page 1
Forum section: electronics / beginners

author: Vindhyachal.takniki
author_rank: Frequent Contributor
timestamp: November 03, 2014, 08:00:12 am
content: 1. I have to make 6 and half digital multimeter with high accuracy & resolution with voltage range 1V to 1.1VdcReading should be updated every 1 sec.2. I have selected ADS1246 , will it be able to measure to that resolution i.e for measuring 6 and half3. If not, anyone else suggest other circuit

author: AndyC_772
author_rank: Super Contributor
timestamp: November 03, 2014, 08:13:41 am
content: I suggest something from Agilent or Keithley, and save yourself a great deal of wasted effort.

author: German_EE
author_rank: Super Contributor
timestamp: November 03, 2014, 09:42:04 am
content: As a) you only need to measure a small range and b) that range is under 5 volts why not use a conventional 32-bit ADC then convert using a small CPU?

author: mzzj
author_rank: Super Contributor
timestamp: November 03, 2014, 10:20:55 am
content: Quote from: German_EE on November 03, 2014, 09:42:04 amHow many "conventional" 32-bit ADC's you can list?

author: rob77
author_rank: Super Contributor
timestamp: November 03, 2014, 10:37:22 am
content: Quote from: mzzj on November 03, 2014, 10:20:55 amthere's a lot of 32bit ADCs, but the vast majority of them are for high resolution audio codecs.. and therefore not suitable for voltage measurement (some of them even got high pass filter to eliminate dc offset).

author: wraper
author_rank: Unknown
timestamp: November 03, 2014, 11:23:09 am
content: Don't seem to be adequate for the task. Rather look at the ADS1255/1256. However if you cannot determine which ADC you need, I really doubt you are able to design surrounding circuity to do the job.

author: Mark Hennessy
author_rank: Frequent Contributor
timestamp: November 03, 2014, 11:49:35 am
content: 32 bitaudioADCs?A quick "reality check" here. No need to get too precise - as we'll see...Approximately, 32 bits represents a dynamic range of 192dBOf course, dynamic range is the difference between the highest level possible and the noise floor.What noise floor can we get - assuming we don't super-cool the circuitry? Perhaps -120 to -130 dBu over the audio bandwidth (simplifying again, as measuring noise and deciding what units is a large topic in it's own right).So that means that 0dBFS must be +62dBu to +72dBu. Something in that ballpark.What is +62dBu?It's 976V RMS!Going to +72dBu would be approximately 3 times that number - about 3000V.So, in short: 32 bit audio ADC? It's ain't gonna happen. We're pushing our luck with 24 bit converters - when you actually measure them, their performance is often 22 bit or worse. There is only so far you can push the noise floor down at room temperature, and there's only so much level you can sensibly accept with solid-state technology. To get more than 24 bit performance, you'd need to attenuate the incoming signal - like a multimeter obviously does - but I'm not aware of that being done outside of test gear (e.g. Audio Precision).If any has a link to anaudioADC with32 bit performance, I'd love to read up about it.Of course, your audio box might use higher precision internally - 32 bits or more, often using floating-point maths - but we're talking about converters here. Also, to my knowledge, there isn't an industry interconnection standard for digital audio signals that carries more than 24 bits.

author: fcb
author_rank: Super Contributor
timestamp: November 03, 2014, 12:26:00 pm
content: 6.5 digit resolution implies reading from 0 to 1999999 (although you may count higher).This is something like 21 bits. This is pretty close to the ENOB on most 24 bit ADC's, so you'll have to be VERY careful with your design, references, component choices, mechanical construction.I would suggest that you tell us a little more about your motivation behind your project/problem. For instance, buying a 8846A or similar from Keysight or Keithley would be quite the logical thing to do, but perhaps you need to make many of these? - or they need to fit in a small place, or be battery powered? etc...

author: sdg
author_rank: Regular Contributor
timestamp: November 03, 2014, 01:21:28 pm
content: Quote from: Vindhyachal.takniki on November 03, 2014, 08:00:12 amWhy ?

author: rob77
author_rank: Super Contributor
timestamp: November 03, 2014, 03:40:30 pm
content: Quote from: Mark Hennessy on November 03, 2014, 11:49:35 amno-one is talking about 32bit performance, i'm saying there is shitload of audiocodec ICs with ADCs capable of 32bit (float) sampling, but those are not suitable for DC voltage measurement/sampling.for example these:http://uk.farnell.com/codecs/adc-dac-resolution/32bit/pg/110165544

author: free_electron
author_rank: Super Contributor
timestamp: November 03, 2014, 03:47:13 pm
content: get one of those analog devices ADuC microcontrollers. some come with 24 and more bit a/d converters.

author: mikerj
author_rank: Super Contributor
timestamp: November 03, 2014, 04:20:37 pm
content: Quote from: rob77 on November 03, 2014, 03:40:30 pm

author: Mark Hennessy
author_rank: Frequent Contributor
timestamp: November 03, 2014, 04:28:43 pm
content: Quote from: rob77 on November 03, 2014, 03:40:30 pmMost in that list appeared because they have 32 pinsCan you name an ADC or DAC that has 32 bit audio performance in this universe? Including a reference to the part of the datasheet that confirms that? You said there are "lots".Floating point does not count - that's just a way to describe numbers, like 2s compliment. FP is useful to maintain precision when processing captured samples, but in terms of conversion, it's not relevant.

author: fcb
author_rank: Super Contributor
timestamp: November 03, 2014, 04:39:01 pm
content: Quote from: Mark Hennessy on November 03, 2014, 04:28:43 pmThey are referring to AC coupled inputs, not floating point maths.

author: Mark Hennessy
author_rank: Frequent Contributor
timestamp: November 03, 2014, 04:48:28 pm
content: Quote from: fcb on November 03, 2014, 04:39:01 pmTo clarify, I only mentioned FP to pre-empt an attempted claim of 32 bit performance simply because a converter might spit out 32 bit FP. And I'm very aware of the difference between AC and DC. Obviously, this audio-related "fork" is somewhat OT for the OP, who wants to make a DC voltmeter with something like - as you pointed out earlier - 21 bit performance, but going OT is allowed here as far as I understand...

author: jaxbird
author_rank: Frequent Contributor
timestamp: November 03, 2014, 04:55:03 pm
content: Quote from: Vindhyachal.takniki on November 03, 2014, 08:00:12 amHow about a SiLabs 8051F350 + reference of choice?

author: fcb
author_rank: Super Contributor
timestamp: November 03, 2014, 05:44:15 pm
content: Quote from: Mark Hennessy on November 03, 2014, 04:48:28 pmWhat ADC puts out floating point??

author: Marco
author_rank: Super Contributor
timestamp: November 03, 2014, 05:50:47 pm
content: Quote from: Vindhyachal.takniki on November 03, 2014, 08:00:12 amIs the leading 1 included in that 6 and a half digits?Would it be easier to do a 1V reference/subtraction and a 5 and a half digit measurement in that case?

author: rob77
author_rank: Super Contributor
timestamp: November 03, 2014, 05:53:37 pm
content: Quote from: fcb on November 03, 2014, 05:44:15 pmaudio codec ADCs for example ? 32bit float.http://www.farnell.com/datasheets/1838839.pdf

author: fcb
author_rank: Super Contributor
timestamp: November 03, 2014, 06:03:20 pm
content: Quote from: rob77 on November 03, 2014, 05:53:37 pmInteresting...

author: Mark Hennessy
author_rank: Frequent Contributor
timestamp: November 03, 2014, 06:23:01 pm
content: Indeed. But only 17-18 bit performance at best. Perfectly respectable for home audio, obviously, but nowhere near 32-bit performance. Which is where I came in...Quote from: fcb on November 03, 2014, 04:39:01 pmI still don't see what you mean by this comment. Forgive me; I'm easily confused

author: XFDDesign
author_rank: Frequent Contributor
timestamp: November 03, 2014, 06:27:40 pm
content: If I'm trying to measure a DC voltage which lies within the range of 1.000000 to 1.100000 V, I'd not worry about the ADC initially. I'd start off picking the best reference I could (LTZ1000). Its drift is good enough. The resistive attenuator would have to keep with the <<1ppm/ºC drift too. Get it down to ~1.100000V, then use an AD8230 with a stupid low drift and an equally good resistor to gain it all up. Feed the input to the inverting terminal, and the 1.100000V reference to the non-inverting terminal, then Vout=G*(1.100000-Vin). This way, the range of interest can be maximized to fit the ADC's input. Suppose it were a single-ended input, referenced to ground with 5V max input, manage the InAmp to have a Gain of 50, and now the full-scale of the ADC is more useful for your input of interest. Supposing something like the AD7176 were used with a cutoff filter at 2.5Hz, the noise presented by the amplifier is around 2.5uVrms, which gives us about 22 ENOB for around 23.8nV/LSB with respect to the input (1.192uV/LSB / G=50 = 23.8nV). The primary value is that the bulk of the work is done on the signal of interest, which improves the workable dynamic range.This would still have a lot of fine tuning to deal with, though. Board layout will be important, and challenging. Components will be very expensive too. You will also want to download a copy of this:http://www.keithley.com/knowledgecenter/knowledgecenter_pdf/LowLevMsHandbk.pdf

author: Rick Law
author_rank: Super Contributor
timestamp: November 04, 2014, 01:35:04 am
content: There is something deliciously incongruent for a post inBeginnersto be on making something< 1ppm.Let us know how it worked out if you don't mind. I am curious.Rick

author: Vindhyachal.takniki
author_rank: Frequent Contributor
timestamp: November 04, 2014, 11:49:32 am
content: I have to make it on my own board. So cannot purchase directly 6 and half multimter.How it is done in multimeters like of FLUKE. Obviously companies won't open their schematic.But just how it is done with so much accuracy.meanwhile I am going ahead with ADS1246, 5 sps , external reference, separte digital and analog supplies , update reading every 2 sec, take average of 10samples & will see the results

author: Mark Hennessy
author_rank: Frequent Contributor
timestamp: November 04, 2014, 01:09:56 pm
content: Quote from: Vindhyachal.takniki on November 04, 2014, 11:49:32 amThere are plenty of schematics available if you search for them. Perhaps not exactly what you need, but plenty to give you some ideas. However, the schematic only tells you a limited amount. There is so much more to this besides selecting some silicon - for example, PCB layout and guarding and PTFE standoffs and thermals. Passives will be challenging. And how are you going to calibrate it?The classic trick to increase the resolution of ADC is to scale ahead of it. That's what all DVMs do. It's what audio test gear does - I'm told (haven't seen schematics) that the Audio Precision boxes have 6dB attenuation/gain steps before the ADC. I'm also told (by someone who works there) that the Prism Audio Dscope uses 2dB steps ahead of their 24 bit ADCs. The Lindos LA100 uses an 8 bit ADC to measure the DC output of the rectifiers, and the audio signal level is switched in 10dB steps. Using these techniques, you can cover a pretty much arbitrary range of signal levels using sensible and available ADCs (still waiting to hear about those 32 bit ADCs, BTW)But what you're doing - in effect attempting to measure 100mV riding on top of 1V - with 1uV resolution - is something that would challenge the most talented analogue design engineers. Could you tell us some more about the project? There might be a better or easier way to achieve the goal.Also, it might be worth checking out Dave's teardown of the Keithley 2015 THD, which will do what you want. It might be interesting to see what ADC they use, and to have a close look at the PCB layout, etc...

author: German_EE
author_rank: Super Contributor
timestamp: November 04, 2014, 02:40:15 pm
content: ADS1282IPW by Texas Instruments, 31 bit but close enoughES9102/9112 by ESS technology - 32 bitSTA013 by ST Micro - 32 bitThat makes three high resolution DAC devices found in thirty minutes.

author: Mark Hennessy
author_rank: Frequent Contributor
timestamp: November 04, 2014, 03:22:03 pm
content: Quote from: German_EE on November 04, 2014, 02:40:15 pmBest-case dynamic range of 130dB => 22 bit performance.QuoteIt appears the best-case dynamic range is 124dB => 21 bit performance.QuoteUmm - this is an MP3 decoder chip. No ADC in this one.QuoteNone of these have 32 bit performance.In my original post, I explained in very simple terms why32 bit performancewas impossible in an audio context with conventional techniques. I don't understand why this is confusing to understand. In light of that, if you come across a 32 bit ADC - that runs from 5V rails at room temperature - then surely you should be detecting "marking BS" of the highest order? After all, we're engineers, not audiophools

author: SharpEars
author_rank: Unknown
timestamp: November 04, 2014, 03:33:05 pm
content: Quote from: Mark Hennessy on November 04, 2014, 03:22:03 pmThank you for pointing out the limitations in these chips. It seems that many people on this forums are clueless when they read DAC/ADC specs and think that just because these chips are advertised as performing 32-bit internal processing that they can actually perform external conversions at a 32-bit level. If each bit represents 6 dB, find me a DAC that is advertising a range of 6*32=192 dB of SNR. An SNR of 132-135 dB (unweighted) is a stretch and limited to the DAC itself (not counting practically any other circuitry) and that represents about 22.5 bits.On the other hand with regard to communicating 32-bit audio data, it is being done already via USB (synchronous or assynchronous), for example see the OPPO HA-1. It's not a standard, per se, as far as I know, but it is being done. Of course, noone is expecting true 32-bit DAC conversion, since there is no such thing, just as there isn't for ADCs.It's all limited to the 21 to 22 bit range and even that is a fallacy in the real world, when you add circuitry besides the DAC/ADC, which inevitably has to be present.

author: Mark Hennessy
author_rank: Frequent Contributor
timestamp: November 04, 2014, 03:58:22 pm
content: Quote from: SharpEars on November 04, 2014, 03:33:05 pmExactly. Thank youQuoteThat's interesting to learn. I'm confident it's being done in other proprietary ways by other manufacturers as well. Whether we'll ever see anything like that in the pro world remains to be seen and is an interesting question. We're lucky that we're still able to use AES-3, that was defined as an (up to) 24 bit standard more than 30 years ago - very far-sighted of them given that 16 bits was state of the art at the time.

author: mzzj
author_rank: Super Contributor
timestamp: November 04, 2014, 04:02:32 pm
content: Quote from: German_EE on November 04, 2014, 02:40:15 pmFirst one is somewhat relevant but nowhere real 32 bits as already stated.Second is audio adc, hardly relevant for dc volt measurements?Third one is even further missI would start pondering around LTC2400/LTC2410, LTZ1000 reference, styrofoam cups and TC-matched Z-foil resistor(s) (networks) from vishay.If you need only resolution and not that much absolute accuracy just order one of the LTC24xx demo boards from Digikey, slap it together with your choice of arduino and call it a day

author: nfmax
author_rank: Super Contributor
timestamp: November 04, 2014, 04:06:16 pm
content: I know the OP asked about voltage measurement, but I'd just like to point out that 32 bit analogue to digital conversion is nothing spectacular - when the analogue quantity is a time interval! It's only 9 3/4 digits and the HP 53131A on my bench will do that comfortably.This is also a clue...

author: SharpEars
author_rank: Unknown
timestamp: November 04, 2014, 04:09:55 pm
content: Quote from: Mark Hennessy on November 04, 2014, 03:58:22 pmAs for audio gear, 16-bit is way more than enough (I'd go so far as to say 12 - 14 bit) and I say this being a certified "audiophool" owning an OPPO HA-1, ridiculous headphones and audio cables costing in excess of $100.

author: Andreas
author_rank: Super Contributor
timestamp: November 04, 2014, 05:42:34 pm
content: Quote from: Vindhyachal.takniki on November 04, 2014, 11:49:32 amHello,usually 6,5 digit multimeters have usually a LM399 voltage reference and a own ASIC for multislope A/D conversion with >=26 Bit.Accuracy is another theme.Stability is stated to be 24-35 ppm over 1 year (short term much better).Linearity is in the range of 1-2 ppm.The trick is that they age + select the voltage reference for several 1000 hours until they fit their needs.And also voltage dividers in this accuracy range are not trivial.With best regardsAndreas

author: rob77
author_rank: Super Contributor
timestamp: November 05, 2014, 07:45:25 pm
content: Quote from: mzzj on November 04, 2014, 04:02:32 pmbuying an used 6,5digit Keithley on ebay would be cheaper than sourcing those bloody expensive resistors from vishay and the ltz1000

author: XFDDesign
author_rank: Frequent Contributor
timestamp: November 05, 2014, 08:44:28 pm
content: Quote from: rob77 on November 05, 2014, 07:45:25 pmPretty much the truth there.When all is said and done, buying a used 6.5 digit Keithley, Agilent, or Fluke is going to be a cheaper way to go, with a known certainty of a working product.How many board turns will the OP budget? Figure $100 a shot for the PCB. Say 3 spins? Parts? The LTZ1000s are about $45 a piece. Better buy about 4 of them (1 to smoke, 3 to go through each turn). The ADCs? $20 a piece typically. Let's just say the analog BOM rolls in around $150/board. Then there are those damned precision resistors. Let's roll in another $100 there. All of the digital stuff, let's just say $50. With this hypothetical, it's looking to be about $400/board per run, with 3 runs as a newbie to "figure it out" we're now around $1200, and might only be at 4.5 digits because the minutia of high precision design isn't really grasped.Well now we can just go buy a Fluke 8845A new and pay for shipping.

author: Leiothrix
author_rank: Regular Contributor
timestamp: November 05, 2014, 09:26:53 pm
content: Why do you have to make your own board?What are you going to do with it?You really haven't given any details, frankly sounds like a homework problem to me.

author: Kostas
author_rank: Regular Contributor
timestamp: November 06, 2014, 07:36:33 am
content: Quote from: nfmax on November 04, 2014, 04:06:16 pmPerhaps a summing amplifier? A slope ADC?

author: David Hess
author_rank: Super Contributor
timestamp: November 06, 2014, 04:02:31 pm
content: Quote from: Kostas on November 06, 2014, 07:36:33 amA summing amplifier adds another set of resistors which need to track, another input offset, and more voltage coefficients of resistance.

author: Kostas
author_rank: Regular Contributor
timestamp: November 06, 2014, 05:51:49 pm
content: Quote from: David Hess on November 06, 2014, 04:02:31 pmOk, I don't pretend to be an expert, my knowledge is probably poor to modest, but let me explain myself:First of all, I tried to guess what nfmax alluded to and wanted to write anintegratingamplifier, but goofed and wrote summing. An integrating amplifier is part of a dual slope ADC and the resistor and capacitor tolerances aren't critical, are they? Anyway, another thing that AFAIK hasn't been mentioned so far is oversampling, which could improve resolution. Oversampling by 4ncould give n more bits.

author: David Hess
author_rank: Super Contributor
timestamp: November 06, 2014, 07:46:10 pm
content: Quote from: Kostas on November 06, 2014, 05:51:49 pmA summing amplifier and integrating amplifier are easy to confuse.Using a summing amplifier before the analog to digital converter would be a natural way to subtract 1.0 volt from a 1.0 to 1.1 volt range and expand the scale of the analog to digital converter. The original post mentions doing this by taking advantage of the differential input available on the analog to digital converter.The resistor and capacitor tolerances are not critical because integrating slope converters can calibrate their drift out but "not critical" takes on a whole new meaning with high resolution and linearity requirements.Oversampling will not help with linearity and extra resolution will not help with noise.I suspect the hardest part of doing what the original poster wants is verifying precision operation. Noise and drift including that produced by thermal effects will be significant.

author: mjkuwp
author_rank: Unknown
timestamp: November 08, 2014, 02:44:20 pm
content: just had a skim read of this and OP's other recent post. I am not sure absolute accuracy is required as the OP seems to be building a strain gage interface? maybe?https://www.eevblog.com/forum/beginners/calculating-force-from-strain/If this is a strain gage interface, the ADC needs to read a differential signal from a wheatstone bridge. The mechanical tolerances of the strain measurement might be a lot greater than that of voltage references and such....my off the cuff thought anyway.I thought that an instrument amplifier was a common way to do this. INA128 for example. am I wrong?

author: David Hess
author_rank: Super Contributor
timestamp: November 08, 2014, 08:39:01 pm
content: An instrumentation amplifier is the common way to do this but the other discussion thread is suggestive.If he is using a half bridge, then a precision resistive divider across the excitation would normally be used to produce the other output and if the excitation was 2 volts, then that explains the need for a 1 volt reference to subtract from the signal. A full bridge solves this problem and doubles the output signal but requires twice as many strain gauges.2 volts is pretty low excitation even for full bridge strain gauge which will have a typical sensitivity between 1 and 3 mV/V yielding an output signal of 2 to 6 milivolts full scale. Using a half bridge will cut that in half. That may explain his resolution requirements.Instead of absolute accuracy, it is better to either derive the excitation from the ADC reference or the ADC reference from the excitation. This produces a ratiometric measurement and cancel out some of the gain errors.An alternative to an instrumentation amplifier is to drive one side of the excitation with feedback from the difference in one of the differential signal outputs and ground. Then the remaining differential signal output becomes ground referred. This configuration avoids common mode errors in the instrumentation amplifier. This also supports ratiometric (mentioned above) measurements.Load cells suffer from a number of additional error sources. The modulus and zero change with temperature. There are linearity and hysteresis and creep effects. Typical accuracy when these are taken into account is more like 1 part in 10,000 so the ADC requirements are not high except in that the signal levels are relatively low.