Medium Precision DMM Verifier from Off-The-Shelf Voltage Reference Chips

[quantumvolt]

Since I have no way of getting real calibration of my Agilent 34401A (last calibration 2003), I am buying ref chips with a reasonable tradeoff between initial accuracy and tempco / drift.

The overall best best result until now is REF195E 5.0V at ROUGHLY 0.001% and 20uV noise. I will not try to quantify the drift or tempco, but the reading on the DMM has been between 4.99993 and 4.99996 (6ppm) day and night for weeks now.

I do trust not a single chip but have bought many cheap ref chips from different manufacturers (and also one board from voltagestandard.com and have ordered one from Geller's newest batch due this month).

By the law of averages you will get some readings too low and some readings too high and some quite spot on. Using 5 or more different chips from different manufacturers, all with initial accuracy spec of 0.05% or 0.01%, the probability of them all being totally whacked is close to zero. So this method implicitly gives you a verification traceable via datasheets to real instruments / production processes that are calibrated against standards.

This is not science - the only math being behind it being probability calculation and uncertainty reduction of mean standard error as square-root(n) for independent refs with equal expected value and standard deviation - but it sure beats defining 10V as being between the output of 6 and 7 AA cells (batteries).

Any tips for good cheap all-round ref chips that have had performance better than spec?

[PS Please do not turn this into a "initial inaccuracy can be trimmed away, what matters is stability" topic. This statement is true only if you have access to calibration. Thanks]


DMM > Serial > Arduino > USB > Processing

[Galaxyrise]

Something from http://www.voltagestandard.com/ perhaps?  The accuracy statistic isn't as good, but it comes calibrated.  And if you're in the US, sending it back for recalibration is pretty cheap, too.

What will you do with the reference to replace "real calibration"?

[quantumvolt]

As I mention in my post, I already have a Ref01 board from voltagestandard.com. A Geller board is in order and will probably be shipped ultimo this month.

The Ref01 is a neat product and mine is within spec 0.01%. The temporary problem is that the value has drifted in trend from around 5.00022 to 5.00045 Volt in the 2 months or so I have had it. The most stable of my other references have not had this trend relative to the DMM, so this leads me to think that the chip on the board needs more aging. As you say, the recalibration is cheap and even free for two years, so it will be sent back. But there is no point in calibrating a source that has trend drift.

Also, it might be lost in the post. Or it might fall down 5 feet while handled at an airport so the trim pot changes a little bit. So I will probably remove the trim from the board before I send it back for measurement. I would rather have a number that is off "5. 000 0...." but reliable, than a fragile pretty value that can be lost more easily.

Detecting the systematic drift in one source by comparing it to several sources and also the danger of losing a particular reference in post, by accident or for any other reason illustrates imo why it is useful to have at least 5-10 refs. Having even more and spreading them out on 1, 2.5, 5 and 10 Volt also gives you a better idea of your meter's linearity. You can also combine them to get several reference voltages without resistance dividers.

To replace real calibration I will do as you say: Send my "best" references one by one to "voltagestandard.com" and "gellerlabs.com" and others. But before I do that, I need to "become familiar" with the refs. And they have to be stable.  So I am in a process of may be a year where I buy a few different chips every month just to get a working idea of where 10 Volt is (I now have several refs that are better than 100 ppm with almost certainty). When I know by inter-comparison which of my 10-15 refs that are most stable, I will send them by post from Thailand to Europe or America for "postal order" calibration wherever I think a reasonable offer is given. I will part with one and one only for a given time period (change only one factor of a familiar system), and I will use several measurement services (it obviously increases the value of a measurement if two different persons / calibrated meters pins it at the same or close similar value).

I am also making a slow relay-based multiplexed LT2400 ADC to computer 20 Input Data Logger & Plotter that saves as much data you want to disk, and do all kinds of real-time and postponed statistics in Processing and C+ (using the same stuff as in the video in my post over). It does not need a more accurate ADC-voltage reference because it will only compare the noise and drift of my voltage sources. So I use whichever reference I have that is most stable as reference for the LT2400.

In time I will build better references. I have bought a single LM399 and LTZ1000 as toys to play with. Cheapbay fake or real - I don't care. For now they are toys. I need a fleet of commercial ready-to-use refs before I start to send high precision home brew voltage sources over the Atlantic or the Pacific for calibration to 5ppm or better.

So my next order is 4 x 2.5 Volt. According to spec I will get:

Low Noise: 0.25ppmP-P (0.1Hz to 10Hz) 625nVP-P for the LTC6655-2.5
Low Drift: 2ppm/°C Max
High Accuracy: ±0.025% Max
No Humidity Sensitivity (LS8 Package)
Thermal Hysteresis (LS8): 30ppm (–40°C to 85°C)
Long-Term Drift (LS8): 20ppm/?kHr
100% Tested at –40°C, 25°C and 125°C

http://www.linear.com/product/LTC6655

By putting 4 of them on top of each other on continuous individual independent battery power (9 Volt rechargeable, 2-3 in parallel, change one at the time so power is not interrupted) and using cheap 0.1% 5 Volt refs as preregulators, I will get a fairly good 10 V. If it turns out to be very stable, I can modify it for trimming  by using 5 chips (giving 12.5 Volt) and a divider.

The real nerds in this game baby-sits references for years. A slow tedious work with diminishing returns as you get closer to the practical boundaries for these kind of measurements ...

[branadic]

Wouldn't it be worth to find someone near your location, where ever this might be, with a calibrated DMM or a long-term stable reference that is continiously calibrated and to compare it with your references and/or DMM?
I do it similar, therefore I use a calibrated 7.5 and a 8.5 Digit Multimeter to read the values of a programmable Burster DC-Standard and set my privat 6.5 Digit DMM to the correct readings.

[quantumvolt]

I live in the jungle on the border between Thailand and Laos. The closest you get to a precision voltmeter here is a screwdriver with a test lamp for checking the mains 220 V ...

[PedroDaGr8]

Does it have to be a chip? You could theoretically always use a weston cell which has a very predictable aging and temp curve.

[quantumvolt]

Brilliant idea! But it will ruin the  fun of not being able to look forward to the next improvement (please don't say J. array). Like much else in life - hunting precision is an activity. No fun in buying 10.000 000 000 Volt at Seven Eleven...

And it is philosophically impossible to find the Volt as it defined by current (the number of charged particles passing through a cross-section over a given time interval). So may be I'll join the time-nuts instead and get me an atomic clock and a particle counter. What will I do with particles that are half way trough when time is out?

But for now, I collect and scrap chips. Next time I go anywhere big, I'll rig a small aluminum suitcase with the best refs running on small motorcycle batteries and get the stuff measured or formally calibrated. Who knows, may be I'll put in a temp reg and a microprocessor and SD card or other storage. When back home I can point at the time series on the laptop screen and say to myself "Oh, yes I remember that spike - it was during breakfast the day we went to the cal lab. No photos needed from that trip ... 

Anyone with a better idea?

[andyturk]

I live in the jungle on the border between Thailand and Laos.

Nong Khai? I used to live in Phnom Penh.

[Galaxyrise]

Sorry I missed that you had a REF-01! 

What will you do once you have a 10V standard?  Looking at the service manual, you need to be able to provide a lot of values to the meter (up to 1000V dc!) to use the built-in calibration procedure.  This is something I'm wrangling with myself; though I'm not in a jungle, I did want to be able to tweak calibration constants without paying for a "real" calibration. 

[quantumvolt]

...
What will you do once you have a 10V standard? ...

Very interesting question, thank you. I think I will do nothing. When I am thinking about it, the meter deserves to be left alone. In the menu it reads "REV 11-05-02", "CAL COUNT 54" and "MESSAGE 28 AUG 2003C". I guess it was produced in 2003 and has never been calibrated after being sold somewhere in Asia (I got it from an industrial recycling shop in Bangkok, but most factories here are Japanese or Chinese owned).

As for now I have a handful different brand 5.0 Volt refs that all are "guaranteed" to be within +-0.01%. Luckily some reads below 5 and some reads above 5, but they are all within the interval [4.9995-5.0005].  So I have no reason to believe that both the meter and the refs are totally out of spec. One of the refs has shifted a bit lately and seems to stabilize at 4.99998 Volt reading. This reading is low 4ppm. But the noise in the plot below is in the 5-20 ppm range.



So in my opinion I am very lucky to have got a 10 years old world class instrument (200 USD) and a 5 V reference chip (6 USD) that puts me in a situation where I really have no way of separating uncertainty/error between the measured value and the measurement.

Even if I was offered calibration, maybe I would say no. To me it is a bit like winning the lottery every time I get a confirmation that the meter's reading is "quite close, if not spot on". If - one day - I become very sure that the meter is out of spec , I will just adjust for that. Keeping a notebook (in your head or on paper) is a bit like firmware storage of unlinearity correction factors for artifact 10V/10kOhm calibrated instruments.

As I mention over, I am making a voltmeter from a LT2400 24 bit ADC. In front will be 20 relays multiplexed by an Arduino. Connected to a PC and a plot/statistics program, this logger can display and calculate statistics (also correlation) for up to 15 references + temperature DHT11 sensor, humidity, temperature measured from a diode/PN junction, variations in the power supply to the references, variations in mains AC, ...). If I get reason to doubt this instrument (which will use one of the references to measure all the other ones), I will store correction factors for 1, 2.5, 5 , 7.5 and 10 Volt in the software and let Arduino correct the reading by interpolation.

Imo it is all about fun, and it sure beats collecting stamps ...