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Old 4th Mar 2016, 11:11 am   #61
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Default Re: A precision voltage calibrator

Yes,indeed. If my investigations indicate that both AD584 are defective, that is an option that seems worthwhile. Right now I have a number of other tasks I need to complete first (not all electronics / radio related). Since I retired and moved home, I've never been so busy!

Al.
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Old 4th Mar 2016, 11:59 am   #62
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Default Re: A precision voltage calibrator

I don't think it's been mentioned so far, but those Chinese kits use AD584s that have been rescued from scrapped equipment. That's how they get the price down; they're not fakes, as far as the community can tell. The date codes on the ICs is the clue .

Lots about these on the EEVBlog forum. For example: http://www.eevblog.com/forum/beginners/t17704/

Failures aren't unheard of, but they seem to be very rare. To have 2, of different designs from different suppliers, seems to be particularly unlucky. I went down the DIY route - which was probably for the best based on your experience
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Old 7th Mar 2016, 12:41 am   #63
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Default Re: A precision voltage calibrator

I've finally found the time to document my DIY DVM reference. Rather than clutter this thread, I've started a new one: https://www.vintage-radio.net/forum/...d.php?t=124491 - hope it's of interest
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Old 7th Mar 2016, 2:56 pm   #64
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Thumbs down Re: A precision voltage calibrator

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If my investigations indicate that both AD584 are defective . . . .
And those investigations are now complete. Removed each one from its pcb, used an external PSU and using the manufacturer's data sheet as a guide, configured each AD584 in its 'basic mode'. Result: output voltages low - in both examples. For me, that's conclusive. Salvaged a few bits; the rest went into the rubbish bin. It'll have to be the total D-I-Y route now; I've learnt my lesson when buying from certain sources.

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Old 7th Mar 2016, 3:11 pm   #65
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Default Re: A precision voltage calibrator

How low were they, Al?
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Old 8th Mar 2016, 12:44 am   #66
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Arrow Re: A precision voltage calibrator

The expected 10-v. output was 9.7v. on one and 9.4v. on the other. Those voltages were measured on a Fluke 73 DMM.

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Old 8th Mar 2016, 3:54 am   #67
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Default Re: A precision voltage calibrator

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One other thing that might be worth mentioning is that you can permanently affect the output voltage of a reference during normal soldering because the heat can age the reference. So if you do solder it try and use a heat shunt and be quick with the iron.
They could have been damaged during assembly, although I thought they were laser trimmed.
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Old 8th Mar 2016, 10:38 am   #68
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Default Re: A precision voltage calibrator

They are. I believe that G0HZU is referring to thermal hysteresis, which might change things during soldering. I don't immediately spot a value for this on the AD584 datasheet, but by way of example, the LM4040 reference specifies 0.08% after being cycled over the whole temperature range.
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Old 8th Mar 2016, 11:25 am   #69
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The expected 10-v. output was 9.7v. on one and 9.4v. on the other. Those voltages were measured on a Fluke 73 DMM.
How about the other output voltages, the reference voltage on pin 6, and the input voltage?
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Old 8th Mar 2016, 12:41 pm   #70
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Arrow Re: A precision voltage calibrator

The PSU / input voltage was +15 v.d.c. All the other output voltages from the AD584s were similarly and significantly low. Increasing the PSU voltage - but not exceeding the maximum rated voltage for the AD584 - did not produce any substantial change in those output voltages.

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Old 8th Mar 2016, 1:14 pm   #71
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Default Re: A precision voltage calibrator

Curious...

If pin 6 was low, that would be most unusual because that's directly from the bandgap generator, and these are usually very accurate because of fundamental laws of physics...

Perhaps leakage on the PCB would be a factor, but then your separate test would have eliminated that...

If both were out by the same amount, then your DMM would be the obvious suspect - the Fluke 73 does have an internal preset, so that is possible. However, your errors were 3% and 6% - as these were (presumably) measured with the same meter, it's unlikely to be the culprit. However, there is a chance that if your meter was out by 3%, then one IC was correct and the other was out by 3%. But that does seem unlikely, and your next step - whatever that might be - will help answer this one.

The fact the line regulation is OK suggests that at least some parts of the ICs were OK.

Were these the type that were tested before dispatch? If so, I presume the recorded figures were plausible?

Just about the only thing I'm left with is static discharge. But it would be extremely unlucky to damage two ICs - mounted on a PCB - in the same way. But what's left?

Oh - did you put a 'scope across the output to ensure it wasn't oscillating or picking up RF?

Would you like me to send you an LM4040? I have a fair few of them here. I'll measure the output first, and that will allow you to "reality check" your DVMs. FOC, naturally...
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Old 8th Mar 2016, 2:38 pm   #72
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Arrow Re: A precision voltage calibrator

I wouldn't go so far as to state that both AD584s were out of spec. to any reasonable degree of comparison. Since all the outputs on both devices were substantially below the stated figures, (and different), that was adequate justification for me to reject them as defective.

Although I only used one Fluke DMM to measure those voltages, a recent comparison has been made for the d.c. voltage indications on that particular DMM and two other Fluke DMMs (different models). That comparison was also extended to a pair of AVO Mk. 8 meters. Figures in the order of 10-v. 0.1 v. for the Flukes were obtained. The two AVOs indicated slightly more error. With regard to testing these AD584s, the magnitude of those recorded meter reading errors were, therefore, insignificant.

Yes, the outputs were 'scoped to check for oscillation: there was none.

As for 'testing prior to despatch', all I had was the assurance from the vendors that the items were "new, unused". Figures were only written on the case for one item - and those figures looked impressive.

I stated earlier that I have now been refunded in total for these items, so at least I have not made a financial loss; just had to spend a lot of time and effort achieving those refunds.

Al.
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Old 8th Mar 2016, 5:43 pm   #73
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Default Re: A precision voltage calibrator

As an alternative to the LM4040, the TL431 is found in many switched-mode PSUs. It's not as good as the LM4040, but most of us have them in our junk boxes. The TL431 is adjustable, but with "REF" connected to "CATHODE", you get 2.5V +/-2% with the worst grade, or 0.5% with the best. Good enough to quickly verify your DMM is OK in the context of the errors discussed earlier, but I wouldn't recommend the TL341 as the core of a DVM reference as the tempco is a bit on the high side.

Another "quick and easy" way to double-check your meters: irrespective of the state of the calibration of a Fluke 73, the reference will be comfortably within 1% of 1.24V. VR1 (an LM385) is to the left of the display.

It's a shame that we probably won't get to the bottom of the AD584 mystery... The small number of problems I've read about prior to this were related to the charging and switching circuitry; if they work at all, they are accurate. So this is a worrying development. As I said earlier, the ICs are recycled, so the completed modules have to be tested before shipping. It's a puzzle
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Old 8th Mar 2016, 8:04 pm   #74
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Question Re: A precision voltage calibrator

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As I said earlier, the ICs are recycled, so the completed modules have to be tested before shipping.
The two (defective) items came from . . . China. So there will be someone in China who will test those modules, prior to shipping, yes?

Al.
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Old 8th Mar 2016, 8:14 pm   #75
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Default Re: A precision voltage calibrator

Well, based on everything I've read about them, I believe so. Do I infer from your question that you perhaps doubt that

Did you see the link I posted earlier?

http://www.eevblog.com/forum/beginners/t17704/
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Old 8th Mar 2016, 8:47 pm   #76
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Default Re: A precision voltage calibrator

By 're-cycled', I understand that items that have been scrapped under the WEEE regulations and have eventually turned up elsewhere in the world - China, perhaps. There, someone has salvaged ('re-cycled') just the i.c.s and produced these units - in their hundreds, every day - and someone else has the task of testing the finished product and getting paid a pittance for doing that. Now in my life, I've had one employment where my job was doing just that - and not only is it totally soul destroying, it's also 'easy' for the occasional item to 'slip by'- untested. Needless to say, I wasn't there very long!

As for the link you posted earlier, I see that the top entry refers to a unit which is very similar to one of my defective ones - but the supplier is different to whom I bought mine.

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Old 8th Mar 2016, 11:37 pm   #77
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Default Re: A precision voltage calibrator

I certainly follow your reasoning, but twice, from different suppliers? I just can't shake the feeling that it's too much of a coincidence, even given the context...

However, curiosity has got the better of me, and I've just gone on a spending spree! Plenty of change out of 20 though. It'll be a couple of weeks before they turn up, but I'll report back as and when...
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Old 9th Mar 2016, 12:40 am   #78
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Arrow Re: A precision voltage calibrator

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I certainly follow your reasoning, but twice, from different suppliers? I just can't shake the feeling that it's too much of a coincidence, even given the context...
Two different suppliers; same type of chip: AD584. Both chips almost certainly 're-cycled / salvaged'. Both boards designed to do the same task. Source: same country - China.

In the final analysis, there are only two possibilities.
1. Both were faulty upon arrival. (Although they might have been O.K. when they left China).
2. I damaged both during initial testing.

Now I do not wish to sound unduly arrogant, but I am quite confident in my technical knowledge and skills to rule out possibility #2. Therefore . . .

------------------------

The two suppliers: just a bit of info. about each. I believe that what follows is in the Public Domain, so there should not be a breach of confidentiality.

First supplier. Board was a small pcb about 1 inch by 1 inch. Just the AD sitting on it and some jumpers. When I was in a dialogue with him (he was located in China), I distinctly got the impression that he was running a 'one man and his dog' outfit.

Second supplier. Board contained a few SMT components in addition to the AD; the pcb was housed in a perspex case. Now that supplier was located in the U.K., but it became apparent that he, in turn, got the items he was selling from his boss in China.

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Last edited by Skywave; 9th Mar 2016 at 12:50 am.
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Old 9th Mar 2016, 1:25 am   #79
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Default Re: A precision voltage calibrator

I agree that option 1 is possible. I also agree that option 2 seems highly unlikely.

But there is a 3rd option: perhaps your multimeters are ~3% off?

As the Fluke 73 has a preset adjustment, that is very plausible indeed - we have 10 or so Fluke 75s in the Funds lab at work, and when I calibrated them all a couple of years back, some were out by several percent. If you obtained it second-hand, who knows what the history is? This is what started this thread, after all.

So from everything that has been written so far, I haven't quite been able to rule out option 3. I can't say that it is the explanation - I'm just saying that I haven't yet seen enough evidence to discount it.

But I've offered a couple of hints and suggestions that could convince me. The checking of the LM385 (see post #73) in your Fluke 73 is the easiest/quickest option, but taking up my offer of a free LM4040 that has been measured before dispatch would also be good. You'd then have a "transfer standard" that you could use to calibrate your meters...

Re. your UK supplier - it sounds like he/she is a "drop shipper". Very common, and I've been caught out by that in the past. By "caught out", I mean that I paid a couple of pounds more to buy from a UK supplier in exchange for a faster delivery time, only to find that the thing came from China or Hong Kong. Live and learn
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Old 9th Mar 2016, 12:46 pm   #80
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Arrow Re: A precision voltage calibrator

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Taking up my offer of a free LM4040 that has been measured before dispatch would also be good. You'd then have a "transfer standard" that you could use to calibrate your meters.
It would clearly be very rude of me not to accept that generous offer - apart from its likely aid to throwing some light on this matter and its consequent utility value.
So, yes - I will gratefully accept your offer: thank you.
You have a P.M.

Al. / March 9, 2016 //
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