Electronics > Metrology

Can 732As be tied together for an 'average"?

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acts238willy:
Dumb question... but since it's a matter of microvolts, can the
outputs be tied together for a 'average'? The 10mA would be
fighting among the several 732As. Will the internal resistance
of the outputs be enough, or will I need to put a resistor in series
with each output and tie the resistors together?
I plan on using my 'new' Keithley 181 voltmeter, which has a Gigohm
input... this should be enough to ignore the added resistance, right?
If resistance is necessary, could I use my pile of old L&N rams horn
4XXX resistors? I have eight 100 Ohm L&Ns and nine 1000 Ohms...
Need to do all the thermal and ground loop stuff, of course...

montemcguire:
This topic has been researched extensively, beyond just averaging references, to include the notion of calibration intervals, expected drift of each reference as obtained from calibration data, and also trying to predict reference drift. While it's not 'easy reading', and I'm not an expert at this, I do remember reading a paper from Fluke that directly addresses the topic of effectively using and calibrating multiple DC references to reduce uncertainty: https://us.flukecal.com/literature/articles-and-education/electrical-calibration/papers-articles/maintaining-10-vdc-03-ppm-o

I know that Fluke will gladly sell you a 'multi-pack' of references so that you can send some off at regular calibration intervals and have some at home as working references, so try to look for that as well. You're basically correct in the notion that multiple references can result in a better working reference volt, but I think it's worthwhile to study what's been done before rather than work out the details of some simple averaging system that doesn't seem to do everything that the experts have determined to be necessary or useful.

Again, I wish I could be much more specific, but I'm not an expert on this topic. Hopefully this 'tip of the iceberg' can direct your study to what's been done already, since it seems that a lot has been done that you can benefit from.

ap:
The 732A being a precision instrument should not be loaded. You do not know what effect this will have. The proper way is to use a low EMF switch, such as e.g. the Data Proof ones and measure the various outputs with a 3458A (transfer uncertainty 0.14ppm).
Optionally, you could build chopper amp based buffers and mix these together (observe chopper noise issues and so on, may have other issues). Since the voltages will all be very close, low TC/low drift resistors will do the job, no bulky LNs needed (they may have bad TCs btw.). Of course this setup adds uncertainties that you will have to figure into the uncertainty budget, but if you do not have a DP switch, it is kind of a low cost DIY solution if done right.

acts238willy:
Part of the reason for ganging 732s was to rotate one at a time
to buck the other group. The 6 together should be quiet enough
to see if the 7th one, which is doing the bucking, is noisy. The
7th digit on the 10V would be a microvolt and a 732B cal document
I've seen shows movement at the microvolt level. The only way
I can think of to show it's an individual 732 that's noisy is to buck
it against a group of them..........  or a JJA.
That's why I wanted to tie six 732s together and was wondering
if their outputs could do this.

TheSteve:
The 732A is short circuit protected and is supposed to fully recover without loss of stability so you wouldn't think its outputs are that delicate. The Datron 4910 uses 200 ohm resistors in series with each of its 4 outputs when in average mode. So if you did want to try it I would copy how Datron does it with the series resistors.