Two DVMs

I wonder if it was monolithic. More likely two dies.

<snipped>

It's supposed to be the best, so one variable a manufacturer can ignore, if the end price justifies it. They would still need to do a lot of work to get the performance, as other issues start to dominate at sub microvolt levels. Analog probably have a reference design for the pcb layout, and choice of resistors. Buying in thousands, they will get a price good enough to make them a viable choice.

Agreed, but if done right, say with batching to take care of spreads, that work only needs to be done once. Fine tuning with experience on actual performance just means a software update. Manufacturers can also specify tighter tolerancing for incoming parts, or get the vendor to do the work on curvature and aging.

Solartron and others got good results using such an approach. The Solartron 7081, 8.5 digit dvm. mid 1980's for example...

I will add that Volt-Nuts is a good source of circuit ideas and suggestions of components to investigate.

Really? A Keysight 34461A Digital Multimeter, 6.5 Digit, Truevolt DMM costs about $1,500 list, and the 34465A 6.5 Digit Multimeter, Truevolt DMM costs about $1,900 list. 75 ppm and 30 ppm respectively.

There is also a 7.5-digit option, 34470, for $4.400 list. Probably cherry-picked from the run of 34465A production. 16 ppm.

And then there is the 8.5 digit option, the 3458A Multimeter, for $14,400 list. 0.1 ppm

How good does this simple little voltage reference really need to be? Given what it takes Keysight to get to the above accuracies, is there any hope of getting anywhere close with just a hand full of components, even with calibration?

Joe Gwinn

I had my HP3478A calibrated & adjusted last year, MIT student lab surplus. I'm unsure if it was ever calibrated since it left the factory, there was no cert on it anywhere when I bought it.

It was within spec on everything except one test in the 3 Meg resistance range:

The work was done by Essco in Lowell, MA, about 30 miles north of Boston. total cost for the job was $170, about three times what I paid for the 3478A.. :-)

That would have been the 1N829.

You had to get the current to precisely 7.5mA to get the promised temperature stability. The one occasion where I saw one designed in, the engineer clearly hadn't bothered to get it precise enough - because the breakdown voltage at 7.5mA is in the range 5.89V to 6.51V and while he'd put in a trimpot to let him trim the output voltage to 10.000V exactly, he hadn't put a trimmer on the resistor that set the current to let him get that to exactly 7.5mA. It was pretty silly.

The integrated circuits that incorporate buried zeners presumably use laser trimming or on-chip zapping to sort this out on a chip-by-chip basis (which isn't cheap),

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