A few months ago, I bought three cheap (AU$3ish) AD584-based voltage reference modules from a couple of Chinese eBay suppliers.
When the first couple arrived, I was a little disappointed to see that they came without the "Calibration Certificate" that was shown in the eBay photo. However, the third one did have the certificate, so I ordered 2 more from the same supplier. (And have a further 2 more on the way...)
(pictures below - I don't know how get them to go here)
Of course given the price, I was suspicious that they might be useless fakes, and I was almost certain that the "Calibration Certificates" would prove to be total bullshit. However, these units did appear to provide output voltages as per the specifications, but at the time I only had rubbish test instruments, and was unable to verify the measurements shown on the certificates with even the vaguest degree of certaintly.
But seeing that the 3 certificates I had were not all just reprints of the same figures, did instill a little confidence. Furthermore, they are hand-written with visible pen imprints.
Things changed recently when I acquired some decent voltmeters, a Fluke 8800A (5.5 digits) and a HP 3456A (6.5 digits). So I was now in a position to properly test these modules.
All the modules use AD584JH devices, with various date codes as shown below.
Also shown is the reference temperature from the calibration certificates (modules #1 & #2 had no certificate)
Date code Cal. Temp (C)
#1 0923 -
#2 1236 -
#3 1793 11
#4 d144 24
#5 9223 14
BTW, the modules that did come with certificates had other distinguishing features:-
- They were supplied in very thick ziplock bags (vs the usual thin bags)
- The AD584s on them have a duller matte finish (vs quite shiny) Especially #3 & #4
- They have smaller reverse polarity protection diodes.
Below are the claimed measurements from the certificates for each range (said to be taken from an Agilent 34401), and the
results I measured using the HP3456A and Fluke 8800A. These two instruments, although not calibrated, were in very close agreement, the Fluke sometimes reading 1 or 2 counts different in the last digit, usually on the high side.
BTW: I saw the HP reading 10.zeroes on a lab calibrator/reference when I bought it, and together with the close corroboration from the Fluke, I am very confident in the accuracy of its readings. The HP is assumed to be the more accurate of the two, and the figures shown below are from it. (in the default 5.5 digit mode, set to NPLC=10, with autozero and filter enabled)
The references were powered by a 16v bank of LiFePO4 batteries to eliminate any chance of power supply fluctuations or noise.
All equipment was powered on and acclimatized for several hours before the measurements were taken, at an ambient temperature was 19.5C.
Range 2.5v 5.0v 7.5v 10v
#1
Claimed --- --- --- ---
Measured 2.4983 5.0014 7.5003 10.0029
#2
Claimed --- --- --- ---
Measured 2.4967 5.0005 7.4973 10.0007
#3
Claimed 2.4990 5.0013 7.5002 10.0020
Measured 2.4990 5.0013 7.5000 10.0017
#4
Claimed 2.5001 5.0011 7.5026 10.0032
Measured 2.5001 5.0011 7.5026 10.0033
#5
Claimed 2.4992 5.0025 7.5018 10.0041
Measured 2.4993 5.0025 7.5019 10.0048
Averages 2.4987 5.0014 7.5004 10.0027
Conclusions
These modules do appear to be very stable and quite accurate, and when supplied with a measurement certificate, there is a good chance that the figures can be relied upon. (though I've also read that these can sometimes be faked).
It would seem they can provide a very cheap and reliable reference source for calibration of 4.5 digit (and possibly even 5.5 digit) multi-meters.
Also, the very close agreement between the certificates and my measurements gives cause for even greater confidence in the accuracy of my new test equipment.