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Shunt Reference Problems

I?m working on a power supply design. It has 4 independent and isolated channels which use Microchip LM4040 2.5V references. It also has opto isolated readback for each channel which uses Analog ADR530B references and a tiny ST micro.

The ADR530B Temp Coefficient is specified as 15ppm typical and 40ppm max. There is a note saying ?Guaranteed by design, but not production tested ? .

When I ran it though a temperature cycle, I was a bit disappointed to get the results in psu_plot_1.jpg.

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For the temperature tests the power supply runs off the mains, its on board processor reports results to a PC outside the test chamber. The 4 PSU outputs are also monitored by a Keysight 34972A outside the chamber. I?m not bothered by the actual output voltages ? these are dependent on the cheap and cheerful LM4040s and within spec (although I don?t like the steps on the blue 15V channel).

The 15 and 5 V measurement errors plots come from comparing the measured (by 34972A) outputs with the self-reported output based on the ADR530B references and the temperature drift is much higher than 40ppm.

My first thought was that external parts, probably voltage divider resistors or maybe even the tiny processor ADC was causing the problem, so I replaced the resistors with expensive ones and added another 4 x

34972A channels to monitor the ADR530Bs directly.

This got me psu_plot_refs_ltcrs_1.jpg.

There?s still some bad stuff going on with the blue 15V channel ? but I don?t care about that yet ? concentrate on the last plot ? that?s the 4 ADR530B chips monitored directly, the temperature coefficients over the

5- 60C range work out at:

65, 56, 53, 53 ppm/C.

Well, it could still be my ADC or some other thing I?ve missed in my design, so I built a little test board with 2 x Microchip LM4040 and 2 x Analog ADR530B, there?s a picture of it set up in the temperature cycler.

Each ref chip is connected to a common 10V supply by a 2k2 resistor. Each ref chip is shunted with a 470nF cap. The results are shown in ref_chips_1.jpg

The temperature coefficients over the 5- 60C range work out at:

-27, 46, 50, 54 ppm/C.

The Microchip parts are within spec but the one with the blue trace has a seriously odd behaviour (this is NOT the same channel on the 34972A as the previous ?bumpy? LM4040).

The real problem is the ADR530B parts ? perfectly linear and smooth tracking of temperature, and out of spec.

I?ll be following this up with AD (of course) but I?d be interested in any comments.

MK

Reply to
Michael Kellett
Loading thread data ...

Only your first link is working. At least on my reader.

The 15V output (blue trace) in psu_plot_1 does seem to have some quirky behavior, but it may not be related to the problem ADR530B.

Have you tried this running a precision reference for the (Microchip) ADC?

Reply to
mpm

Does that mean that you are using the ADC in the micro with the ADR as its reference? Those kinds of ADCs aren't especially accurate, no matter how good the reference.

--

John Larkin         Highland Technology, Inc 

The cork popped merrily, and Lord Peter rose to his feet.  
"Bunter", he said, "I give you a toast. The triumph of Instinct over Reason"
Reply to
jlarkin

How fast are you sampling and is this a low-side (near ground) shunt current sense ?

If slow-ish sample rate, you could use that Microchip 16 bit differential input A/D that does a zero offset between every sample. Higher sample rates at lower resolution I think. Works really well for my current sense applications. It is also I2C though which may or may not be an issue for you.

Reply to
boB

What kind of "expensive resistors"? Do they have a low enough absolute tempco that even if the tempcos in the divider network conspire against you (there's no law they have to track together if they weren't matched for that purpose...) it's not enough to explain the observed error?

Reply to
bitrex

olated

If it really IS the ADR530(s), your best bet is to consider a thermal block (testing 0-80C, maybe heat to 90C?) for the sensitive parts. A little flu ff, a can, and a thermostat will make the thermal drifts irrelevant. And, you won't need the -B version. But, it's a nuisance at build time, and there's always the warmup period.

Reply to
whit3rd

At least you've got the point - the error does come from the AD530B parts. The problem with your suggestion is that the available space, power and cost won't support it. (There are 4 isolated channels and each needs its own reference and ADC.) What's bugging me is not that the voltage references are not perfect but that they do not meet their own spec. Thanks for the suggestion.

MK

Reply to
Michael Kellett

Actually the ADC is pretty good, the problem is the ADR530B, as is proved by measuring it directly both in the complete PSU and on a test board. Of 6 tested none meet the specified 40ppm/C mac tempco.

MK

Reply to
Michael Kellett

Thanks for the suggestion but it isn't the ADC that is the problem - it's the ADR530B voltage reference - see my reply to JL below.

I have had to fit the entire isolated voltage reference and ADC onto a tiny board that replaces an 8 pin SOIC part. If I respin the whole board other solutions are possible - but currently I'm more bothered by the implications of the ADR530B not meeting pec.

MK

Reply to
Michael Kellett

The dropbox link should get you all the diagrams - it does on Firefox.

The Microchip voltage references do have their own issues but they do meet spec. I'm not yet prepared to swear that the steppiness and other quirks are definately down them.

My big problem is that the top grade ADR530B doesn't meet its spec.

MK

Reply to
Michael Kellett

It doesn't matter what the resistors are or were - as I explained I 'added another 4 x 34972A channels to monitor the ADR530Bs directly' and when I did that they don't meet spec, and they don't meet spec on a test board either.

MK

Reply to
Michael Kellett

I don't think ADI is shipping bad parts. Sounds like some kind of setup problem.

--

John Larkin         Highland Technology, Inc 

The cork popped merrily, and Lord Peter rose to his feet.  
"Bunter", he said, "I give you a toast. The triumph of Instinct over Reason"
Reply to
jlarkin

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In your tests to attempt to isolate the reference- did you have nothing els e in the environmental chamber? If the power supply was there, as it appear ed to be in the photo, was it operating during the test?

Best regards, Spehro Pefhany

Reply to
speff

I wouldn't expect ADI to ship bad parts either - which is why I raise the subject at all.

The test board setup is so simple that it's rather hard to imagine what might be wrong with it. Yet the ADR530B parts had beattifully linear but out of spec temperaure coefficients. And the Microchip LM4040 parts had rather quirky (but in spec) behaviour.

I had hoped that someone on SED might have experience of good or bad behaviour of similar shunt references.

I've ordered some ADR250B parts and I'll add them to the same test and repeat.

MK

Reply to
Michael Kellett

Good point, thanks - it was there but not plugged in - if you look very carefully you can see the unnconected IEC mains connector at the bottom left (and it was unconnected during the test - I checked the testfile and all the PSU signals are at zero).

I've ordered some similar ADI parts (ADR525B) and I'll add those to the test board, and maybe a few others - the 34972A has lots of channels !

MK

Reply to
Michael Kellett

Be careful about EMI. We are in a high EMI location, and we see all sorts of strange rectifying effects.

I do wonder how they can guarantee tempco by design not testing, but sell different tempco grades. Maybe they assume that voltage accuracy implies tempco. Or maybe all the parts are the same but just the prices are different.

--

John Larkin         Highland Technology, Inc 

The cork popped merrily, and Lord Peter rose to his feet.  
"Bunter", he said, "I give you a toast. The triumph of Instinct over Reason"
Reply to
jlarkin

With reference back to the "two engineering fundamentals" before you could begin to assume that it's the parts and not a setup problem have to show that your measurement setup can in practice, not just in theory, measure a device with a 40ppm/C or better tempco operating to its manufacturer's spec in the first place.

Otherwise what justification is there to j'accuse the butler when the maid doesn't even have an airtight alibi yet.

Reply to
bitrex

IIRC datasheet temperature coefficient isn't dV/dT, it's Delta V (max P-P deviation over the specified temperature range)/Delta T.

dV/dT is allowed to be much larger in smaller ranges. (Offset drift of amplifiers is specified the same way.)

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
http://hobbs-eo.com
Reply to
Phil Hobbs

Well, there is some evidence that the test rig is OK because one of the Microchip parts does better than 40ppm. The nature of the test rig suggests that it is likely to work correctly ( the measuring device and power supply are outside the temperature cycler). I too would like to see a part actually achieve the spec under test so I've ordered some 2.5V parts. I would have liked to try some ADR530Bs from a different supplier but no one local had them in stock.

I've got a couple of LTC6652AHMS8-4.096#PBF (5ppm) kicking around so I'll try to graft them onto the test board as well.

MK

Reply to
Michael Kellett

That's an interesting point - you can read the data sheet that way but it's not crystal clear. From the perfectly linear tracking observed so far I don't think the parts will do any better over a wider range but the cycler can do it so I'll run the next test over the full -40 to 85 range.

MK

Reply to
Michael Kellett

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