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Old 31st Jan 2016, 4:42 pm   #1
Skywave
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Question A precision voltage calibrator

The initial part of this attached article is something that many of us have probably experienced: which voltage reading do I believe?

http://www.frankshospitalworkshop.co...reference.html

The chip as featured in that design is described here:
http://cds.linear.com/docs/en/datasheet/1021fc.pdf

Now before I take this any further, let me explain first that I am thinking in terms of what would be a useful voltage calibration reference for those uses in a typical workshop that many of us use, not something that is to NPL standards.

For the purposes of this thread, the attached article is separable into two parts: (1) the design of a calibrator; (2) actually using it. To avoid this thread drifting off-topic, it would be appreciated if we limit this thread to the design part only. (Usage of this calibrator - or any other one - can be dealt with in a separate thread).

Basically, the question I am asking is this: is this design for a voltage calibrator worthy of building for the purposes as described: to check the calibration of d.c. voltmeters, analogue and digital, that we typically use in our workshops? I'm inclined to say "yes", but a range of opinions is always worthwhile soliciting for with something like this.

As always, all responses will be appreciated.

Al.
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Old 31st Jan 2016, 7:00 pm   #2
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Default Re: A precision voltage calibrator

The LT1021C and LT1236 both seem to be very impressive - a headline spec of 0.05% basic accuracy isn't bad at all. Of perhaps more importance is stability over time - 15ppm (0.0015%) is very respectable (some manufacturers will "burn-in" their references before fitting them). Tempco is also good.

For context, the best-case DC accuracy of the top-line handheld Fluke meters is 0.025% - a more commonly found meter (e.g. Fluke 75) is 0.4%.

So such a thing seems like a worthwhile project to me, though I'd suggest that for the intended use, a cheaper reference would probably suffice. As he mentions the LM4040, I'll add that I have used it many times, and it's pretty foolproof - and the price is highly attractive too (64p plus VAT from Farnell). My experience suggests that the 0.1% specification is pretty conservative. If absolute accuracy really matters to you, I'm sure someone could measure it for you - indeed, most professional references come with a label stating the exact voltage as measured for reference.

He mentioned the multiple resistor trick - it works well in my experience, and is usually cheaper than buying precision resistors.

Given the cost of the LT ICs, I'd add as much protection as I can (inverse-parallel diodes, etc). I'd also study the datasheets extremely carefully to make sure I was using them correctly - e.g. is there a risk of the potential divider loading down the output voltage if you choose lower resistor values? Linear Technology have some of the best datasheets and appnotes out there.

BTW, I'd be tempted to split the 2k into 1k8 and 200 ohms to get 100mV. Most Fluke meters have a separate "mV" range, so it's nice to test that separately.

When using analogue meters on the 1V output, "loading effect" will cause errors as Zout of the calibrator will be 1.8k with the values suggested in the project. Into the 1V range of an analogue meter with a sensitivity of 20k/V, the reading will be 0.917V, an error of 8%. The error reduces to 3% on the 3V range of an AVO 8 - IIRC, the AVO is 1% at FSD?
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Old 31st Jan 2016, 7:09 pm   #3
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Default Re: A precision voltage calibrator

HTML Code:
When using analogue meters on the 1V output, "loading effect" will cause
Mark, Check please the apps on datasheet; select a 100mA version with an + transistor_Ok, not usable for discussed 1V output...
Karl

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Old 31st Jan 2016, 8:35 pm   #4
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Default Re: A precision voltage calibrator

As requested, I was referring specifically to the circuit in the original post; that circuit takes the 10V output from the IC and produces 1V with a potential divider (20k and 2k).

I've attached the circuit for reference

No bandgap reference can produce 1V natively, of course...

All the best,

Mark
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Old 31st Jan 2016, 8:39 pm   #5
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Default Re: A precision voltage calibrator

Simple ĎPrecision Voltage Referenceí projects have appeared twice in recent years in Everyday Practical Electronics Magazine, namely in May 2011 and June 2015, based on a custom IC - the AD588/AD587. I built the one that featured in May 2011. Itís a precision voltage reference, which - without any calibration or adjustment - will provide a 10.000V DC source within +/- 3mV, (an accuracy of 0.03%). I can't think of any applications for which we - as hobbyists - would require a higher degree of accuracy.

The unit requires 12V AC input, and supplies + and - 15V DC to the chip. Apart from the chip, there are four diodes, and a few resistors and caps, built on a small PCB, plus the requirement for the 12V AC input. As with all EPE projects, the PCB artwork can be downloaded from their website for a small fee or the PCB can be purchased.

Not having a precision Voltage Reference hadnít left a gap in my life, but as a compulsive builder of bits of simple test gear, I thought why not?

I had small 12V mains transformer, so made a little PCB on which to mount it and fitted it into an ABS project box along with a fuse in the secondary in case of inadvertent shorts on the output side. I etched a PCB for the device, but a PCB can be bought from EPE for £7.32 inc VAT, so for anyone without DIY PCB facilities it's not a big obstacle. As directed, I built the unit in a metal box - a cheapo aluminium one, as a diecast box of the correct dimensions was ludicrously expensive. I designed a front panel for it which I printed off and fixed with clear library film.

Not a lot more to say about it except that I've checked my trusty Toolzone £10.00 multimeter - probably the best buy I've ever made - and it reads exactly 10V. All my other multimeters are similarly accurate, including Maplin 'two for a fiver' ones - some reading just under 10.000V, some a little over. Iíve since built a homebrew FET AC/DC Millivoltmeter which I calibrated using the 10V Voltage Reference. (The last pic is of that meter, but it's just checking the voltage of a 9V battery - not connected to the Voltage Reference).

Frankly, for anyone not wanting to go to the trouble of making a precision voltage reference unit - and most won't - fresh mercury cells have a Voltage at 20C of exactly 1.3566V, which - if not used - falls slowly to about 1.3524V (99.7%) after a year or so. Silver Oxide cells have a voltage of 1.550V, so both types of cell provide a useful voltage reference source, more than adequate for hobbyist needs.

A few pics of the PSU and Voltage Reference are attached. The first pic is of my homebrew 12 V AC PSU, the second is the inside of the Voltage Reference, the third is unit boxed up, the fourth is the excellent 'Toolzone' £10.00 multimeter on test, the last is the homebrew millivoltmeter I built, which I calibrated using the Voltage Reference.

The May EPE 2015 Magazine, covering the 10V Precision Voltage Reference is here:

http://www.epemag.com/proj/0515.html

The PCB is £7.44 inc VAT from here:

http://www.epemag.wimborne.co.uk/acatalog/May_2015.html

The earlier June 2011 EPE Magazine project that I built is here:

http://www.epemag.com/proj/0611.html

The PCB is £7.32 inc VAT from here:

http://www.epemag.wimborne.co.uk/aca...June_2011.html

Hope that's of interest.
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Old 31st Jan 2016, 9:04 pm   #6
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Default Re: A precision voltage calibrator

In my opinion, you can end up on a slippery slope with stuff like this because it's easy to get caught up in all the specs and numbers. Before you know it you will be sweating over which reference works best in terms of ppm in various scenarios of time and temperature etc...

In my case, I've not adjusted any of my DMMs in 15 years or more. I do have some cheapo voltage references here and whenever I can be bothered to check my DMMS they all seem to be boringly consistent. The 3.5 digit ones obviously have limited resolution but they seem fine year after year.

I do have a 6.5 digit DMM here (Keithley 2015 THD) but I bought this for the onboard AF DSP to measure SINAD and also for the onboard AF generator. I really don't care how accurate the last couple of digits are.

However it does seem consistent with my old Fluke 45 in that it agrees within about 1mV at 5V and beyond that I just lose interest.

You asked for opinions ( ) and I just don't see the thrill or relevance of this stuff. DMM technology was boringly brilliant over 30 years ago and few people need this level of performance even today (myself included).
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Old 31st Jan 2016, 9:13 pm   #7
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Default Re: A precision voltage calibrator

"Volt Nut" is the term for this. There's a web site/mailing list: https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts

And yes, "Time Nut" is also a thing...

I agree; all of my Fluke meters are in-spec by quite some margin, and mostly they all agree plus or minus a couple of least-significant digits. My 2015 THD agrees with them, as does my HP 3468A.

But one can be confident with a large-ish sample size. What if you only have 2 or 3 trustworthy meters? That's when the doubt creeps in, and that's when a reference becomes a useful project. Also, it's useful for buying second-hand at meets, etc (although such a thing would need more functionality to test the AC ranges, plus current and resistance, and wouldn't need such precision). So, I can see both sides
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Old 31st Jan 2016, 9:51 pm   #8
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Arrow Re: A precision voltage calibrator

Quote:
Originally Posted by G0HZU_JMR View Post
I just don't see the thrill or relevance of this stuff. DMM technology was boringly brilliant over 30 years ago and few people need this level of performance even today (myself included).
"Boringly brilliant" and with a "level of performance" which may be superfluous as maybe, but all test kit needs to have its calibration checked from time to time. That is relevant, surely?

In the attached article, the max. and min. voltages shown on those DMMs are 10.21 and 9.85. That's a difference of 0.36 v., () and as a percentage of 10.00 v., 0.36 is 3.6%. To me, even with my humble needs, that is a significant error - and could easily be relevant in some measurements.

Alternatively, let me put that this way: suppose I need to adjust something to be 10.0 v. Generally, I'll readily settle for anything between 10.10 and 9.90, but 10.21 and 9.85? No: not if I'm using something expensive like a Fluke DMM (I own two) which have two decimal places (or more) of display.

Al.
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Old 31st Jan 2016, 10:06 pm   #9
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Default Re: A precision voltage calibrator

David: thank you for your contribution in post #5. The i.c. as featured in that post does indeed look promising - plus the availability of a suitable pcb. Definitely a strong contender here.

You also say "Not having a precision Voltage Reference hadnít left a gap in my life, but as a compulsive builder of bits of simple test gear, I thought "Why not?" "

That is something I can readily relate to - and indeed think the same way.

Unlike many other branches of engineering, when we make a measurement in electronics, we are almost invariably measuring the effect of, say, an electric current, and the resultant indication follows. Hence, there is the 'concept of trust' in that: can I trust that measurement? Knowing that you can gives a sense of assurance that you are on the 'right track' when repairing or designing something. In actual point of fact, (and in my experience), these are some occasions when small measurement errors are not ignorable.

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Old 1st Feb 2016, 12:30 am   #10
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Default Re: A precision voltage calibrator

Quote:
In the attached article, the max. and min. voltages shown on those DMMs are 10.21 and 9.85. That's a difference of 0.36 v., ()
But is that realistic? I've worked in RF labs all my working life. I see lots of DMMs and lots of test gear and it's rare to see a DMM that gives a duff reading like that. Even the cheapy (and poorly built) DMM built into my various old Thurlby PSUs stays an order of magnitude better than the DMMs shown in that link.

A meter that is giving a reading that far out probably can't be trusted if you were to simply 'correct' it against a voltage reference. I'd question the linearity of the metering and I'd even question how someone could have three out of five DMMs that were that far apart. But they don't look to be decent quality meters anyway.

However, I think the point I'm really trying to make is to advise people not to wander into the ivory tower world of the volt nut.
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Old 1st Feb 2016, 12:56 am   #11
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Default Re: A precision voltage calibrator

Having been through the loop "are my DMMs accurate" a few times, I think in your situation, and for (relative) peace of mind, I would just order one of these from China for <£10 and sit back. If you homebrew a reference you have to take a lot of things into account to get it a lot better than these. Such as thermal stress, variation with battery voltage etc.
These modules use the AD584K. It is accurate to +/-10mV at 10V so down at one last significant digit on a 3.5 digit meter (9.99 - 10.01V).
If you need something at a few ppm then there are companies in the states offering much more accurate reference boards on Ebay at £40 or so.
Great fun to homebrew but would you not rather be spending the time on mending things?
my two-penn'orth anway...
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Old 1st Feb 2016, 12:07 pm   #12
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Default Re: A precision voltage calibrator

Well that's a real showstopper Jeremy!

It's been an interesting thread, but really, Al's original query about the LT1021C and LT1236 designs was comprehensively answered by Mark Hennessy's response in post #2, though as you say, with ready made precision voltage references such as the one you cited being available so cheaply, it does beg the question, 'why homebrew?' and it makes no economic sense in this instance to do so. But then I don't see home-brewing as a diversion from restoration - I see them as twin aspects of my hobby.

Others have rightly questioned the relevance of the quest for precision to two decimal places, when for all practical purposes, it's largely an irrelevance to us as hobbyists.

The radios and other vintage electronics that we restore were made with components which - when new - often had a tolerance of +/- 20%, which does rather make it pointless to aspire to laboratory standards of accuracy. What the Precision Voltage Reference that I made was able to do, was to confirm to me that the cheap DMMs I own are quite accurate enough for hobbyists, differ little in their accuracy from one to another, and compare well to the 'big name' meters. Same goes for checking the calibration of my trusty Hameg 203-7 scope, on which - to me - the accuracy of the voltage calibration matters little as I mostly use it to observe waveforms - not to accurately measure voltages or frequencies.

Each to their own I guess.
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Old 1st Feb 2016, 12:45 pm   #13
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Default Re: A precision voltage calibrator

Sorry all, I didn't mean to stop an interesting thread!

I bought my voltage reference, I admit, a few years ago, as I was not interested in making something that I could buy so easily and cheaply. I researched voltage standards quite thoroughly via Voltnuts etc. And I learned a lot, but realised that I was not trying to replace a professional calibration lab.

The tiny standard cost me about £15 and gives me a reasonably accurate 5.000V. I was even able to test it against a calibrated voltmeter just in case. And it gets used at my radio club to check other members' meters of all sorts.

But I have to put my hand up that I love making and restoring testgear normally and probably spend as much time building and mending it as using it.

So each to his own, as you so rightly say. And on this forum, it's the diversity and depth of knowledge that is so extraordinary. Long may it continue.

Sorry again if I truncated an interesting thread.

All the best

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Old 1st Feb 2016, 2:21 pm   #14
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Arrow Re: A precision voltage calibrator

Quote:
Originally Posted by Skywave View Post
In the attached article, the max. and min. voltages shown on those DMMs are 10.21 and 9.85. That's a difference of 0.36 v.
Quote:
Originally Posted by G0HZU_JMR View Post
But is that realistic? I've worked in RF labs all my working life. I see lots of DMMs and lots of test gear and it's rare to see a DMM that gives a duff reading like that.
"Is it realistic?" you ask. I don't think that's the point; it's not relevant. The picture in the attachment is simply the author's illustration of why (in his opinion) one should have the means - and use that means, now and again - to check the calibration of an item of test gear.

I've worked in many RF labs. myself during my pre-retirement years - as have you. (From your location, it seem likely that you and I have worked in the same establishment). So you and I both know that it is usual to not only find the very best test gear in such places but that such test gear is always religiously re-calibrated on regular intervals and to very high standards. And that costs lots of money. In this thread, we are talking about a typical home-based D-I-Y enthusiast's workshop - and with a very limited budget: expensive external re-calibration services are out of the question for many of us.

Quote:
Originally Posted by G0HZU_JMR View Post
However, I think the point I'm really trying to make is to advise people not to wander into the ivory tower world of the 'volt nut'.
I am not an obsessive about accuracy, precision and resolution in measurements; I cannot speak for others. I take the simple attitude that those three parameters should always be in proportion to what the purposes of those measurements are for. But as I stated earlier, the ability to restore one's confidence in an item of test kit - especially if the cost of being able to do so is small in proportion to the benefit thus gained - seems worthwhile.

Al.
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Old 1st Feb 2016, 2:32 pm   #15
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Arrow Re: A precision voltage calibrator

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But I have to put my hand up that I love making and restoring test gear normally and probably spend as much time building and mending it as using it.
You, David, I and many others here have all of that in common.

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Sorry again, if I truncated an interesting thread.
You haven't; besides there's nothing to apologise for.

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Old 1st Feb 2016, 3:21 pm   #16
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Default Re: A precision voltage calibrator

I was looking for a good voltage and current reference instrument recently and I came across this site: http://www.voltagestandard.com/ which is connected to the articles. They have a small unit named DMMCheck Plus which caught my eye since it also has AC voltage and AC current capability which has always been the problem with my DMMs. I've never had any problems even with some of my 20 year old DMMs on the DC ranges, but the AC ranges have been off a lot (even for instruments that had proper RMS circuitry). I never bought this unit as I had other needs for my instrument but it looks really nice to me.
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Old 1st Feb 2016, 6:02 pm   #17
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Default Re: A precision voltage calibrator

Quote:
I've worked in many RF labs. myself during my pre-retirement years - as have you. (From your location, it seem likely that you and I have worked in the same establishment). So you and I both know that it is usual to not only find the very best test gear in such places but that such test gear is always religiously re-calibrated on regular intervals and to very high standards. And that costs lots of money. In this thread, we are talking about a typical home-based D-I-Y enthusiast's workshop - and with a very limited budget: expensive external re-calibration services are out of the question for many of us.
I think you are missing the point I was trying to make. At work we have a sub class of test equipment that is marked up as 'for indication only' and this includes various scopes, sig gens, spectrum analysers, counters and DMMs.

This test gear is 'too good to throw away' but also not considered worthy of a yearly calibration. In my experience this test gear maintains a reasonable accuracy for many years.

Here at home I have several old DMMs that were all bought very cheaply a very long time ago and they all still read 10.00V on the same DC test source. I don't think I've adjusted any of them in well over 15 years. In the case of my old Maplin Gold DMM it may be much, much longer than 15 years.

I'd also argue that if someone does a 'lot' of modern electronics work they will often be measuring 12V and 5V regulators and if the DVM was out by 0.2V at 10V then I think it would raise suspicion if the spread in readings over time across various 12V regs typically centred on 12.3V rather than 12V.

Quote:
Dekatron: I've never had any problems even with some of my 20 year old DMMs on the DC ranges
That's my experience too. Even a cheapy DMM I bought at a rally as a factory second for about £1 still reads 10.00V after over 15 years.

If I had a DMM that had drifted by 0.25V at 10V then I'd almost certainly bin it rather than attempt to fix it by merely readjusting the internal reference.

I tried measuring 10V with my two old DSO scopes here and they both managed to read it within 0.02V when I selected mean voltage on a suitably sensitive range. One scope is 20 years old and is an ex works scope that hasn't been used (let alone calibrated) for many years and the other is my Tek TDS2012. I've had this scope for over 10 years and it has never been away for any kind of adjustment or calibration. It only has a 3 digit display for voltage so I used a 9.8V test signal for this scope and it measured 9.81V. The old HP scope measured 9.98V for a 10V test source.

By varying the Y control I could add another +/- 0.03V on the scope reading but that still isn't bad for a scope.
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Old 1st Feb 2016, 10:51 pm   #18
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Default Re: A precision voltage calibrator

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I was looking for a good voltage and current reference instrument recently and I came across this site: http://www.voltagestandard.com/ which is connected to the articles. They have a small unit named DMMCheck Plus which caught my eye since it also has AC voltage and AC current capability which has always been the problem with my DMMs. I've never had any problems even with some of my 20 year old DMMs on the DC ranges, but the AC ranges have been off a lot (even for instruments that had proper RMS circuitry). I never bought this unit as I had other needs for my instrument but it looks really nice to me.
A friend has one of these and its very good. It uses a square wave for ac which produces good calibration results.

I agree about ac calibration. If you look at the specs for a modern DMM the ac is usually an order of magnitude less accurate than the dc.

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Old 2nd Feb 2016, 7:48 pm   #19
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Question Re: A precision voltage calibrator

Jeremy: you refer to many instruments which you own and imply that you are content with the validity of their readings. So what voltage source (or sources) did you use to give you the necessary re-assurance that those readings were accurate & reliable? Or did you rely on a simple probability / statistical basis?

Al.
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Old 2nd Feb 2016, 7:56 pm   #20
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Question Re: A precision voltage calibrator

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This test gear ('for indication only') is 'too good to throw away' but also not considered worthy of a yearly calibration. In my experience this test gear maintains a reasonable accuracy for many years.
That as may be, but surely you will also have access to test kit that falls into the 'other' category: "To be re-calibrated annually" (or some other regular interval). In government R & D and industry, that has certainly been my professional experience.

Al.
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