Simple DIY DVM Voltage Reference

[Mark Hennessy]

Really interesting project and a great write-up   

It just so happens I did a "statistical regulator" project on low-noise HV screen regulators for audio pentodes & tetrodes last year - the project write-up is here (not nearly as detailed/impressive as yours!): http://www.audio-talk.co.uk/phpBB3/viewt...=11&t=6204 (I'm called "Jack" on Audio-Talk ) - it's based on Morgan Jones' write-up in the latest (4th) edition of "Valve Amplifiers".

In my case, the primary objective was very low noise, not absolute precision - also, I was using long strings of low-voltage Zeners so temperature correction was applied using the ZPWL model - see http://www.audio-talk.co.uk/phpBB3/viewt...45#p128632

Hi Jack,

Thank you!

That's an interesting project - I'll read the thread properly in a bit, but I've already lost half an hour there

The funny thing is, I'd never heard of the term "Statistical Reference" before I decided to call mine that! But it's proof that there's nothing new - every time I have a good idea, someone else has got there first

BTW, I have one of those Tek DVMs. The 914, so not as nice as yours. And yes, I did haul it out yesterday, and it compared well to the Flukes

I know noise is not a design metric in your project, but the LM317 is a noisy old thing so I used a pair of depletion-mode FETs (the ubiquitous DN2450)  as the CCS, delivering about 10mA to the Zener string. Also, with the large number of Zeners I was using (up to 80), voltage accuracy for each was not important - it averaged out to a nominal 5.6V, though the temperature & current-related compensation was significant.

Good point - I haven't measured the output noise of mine yet. Easy enough to do, and will do so at the weekend.

I did think about a simple JFET current source, but dropout voltage was a concern, as was stability. You should see the pages of scribbles that I dreamt up! In the end, I picked the LM317 because it's totally "jellybean" - and I was a bit surprised at how well it performs as a current source (as a voltage regulator, they aren't bad, but they're not the best).

As you note, in a statistical regulator the voltage accuracy improves as a function of the square root of the number in the series string - obviously, for the first one in your string (2.5V), there is no improvement - the improvements will be for the higher voltages where more devices come into play.

Absolutely. Selecting the "best" sample for the first in the chain is critical - which isn't good for mass-production

It'll be interesting to see what happens as these devices are used. In fact, I'm setting up the remaining 25 samples on a breadboard so I can run them in over the next few months... I'm expecting these to drift. I'll also experiment with thermal hysteresis (probably using the cheaper D-grade parts).

In short, I'm really regretting buying the Keithley now!

As a DMM reference, into MΩ input impedances, this would be fine, but being the forum we are here, maybe worth emphasising "definitely not for AVOs"

Possibly consider a precision voltage-follower to lower the output impedance so the loading becomes insignificant?

I agree - the "x1" readings will be OK, but the "÷10" mode will exhibit significant loading effects - a couple of % at worst. But then, that's not far off what an AVO can do anyway.

Even into a 10MΩ DVM there are loading effects - the 135Ω in the output protection JFET is there, and that's before you've flicked the ÷10 switch. However, as soon as you move away from the Keithley and use "normal" meters, everything is suddenly very satisfyingly nice...

Trust me - I did agonise over this. In the end, I decided against adding a buffer - this was very much a KISS project, and the references themselves are not the last word in precision. Instead, I tried to minimise the systematic errors and point out the errors so people can decide for themselves.

By coincidence, I'm awaiting delivery of a second-hand Time Electronics voltage source - it'll be interesting to see how it compares. Sadly, they put the interesting stuff in a potted box, but even so, I'll get a few ideas from there. I'm thinking about building a current source next, in the same size box. Strictly, it'll be a current regulator - the current will need to be provided externally - but that gives maximum flexibility and utility while saving battery costs...

All the best,

Mark