among multimeters one instrument stands far and above the rest. An object desired for its accuracy, resolution and shear engineering beauty. I speak of course of the HP 3458A. That’s right, not Keysight, not even Agilent (though of course it goes by those brands too). The 3458A was released in 1989, when HP was still… well… HP. An elegant meter from a more civilized age. As the HP Journal documents, the 3458A was a significant engineering feat and has remained in production (and largely unchallenged) for the last 26 years.

But what, you might ask, makes the 3458A such a significant and desirable instrument? It’s all in the digits. The 3458A is one of the few 8.5 digit multimeters available. It is therefore sensitive to microvolt deflections on 10 volt measurements. It is this ability to distinguished tiny changes on large signals that sets high precision multimeters apart. think of weighing an elephant and being able to count the number of flies that land on its back by the change in weight. The 3458A accomplishes a similar feat.

While many engineers might desire the 3458A, it is an object of fascination to one particular group. I speak of course of the cousin of the time nut… the volt nut. Time nuts often find themselves needing to conduct highly accurate voltage measurements in order to calibrate their precision frequency standards and thus spawned a separate mailing list to discuss these topics. much of the discussion on this list centers around the 3458A, how to calibrate one, long term stability measurements, and how to care for their beloved meters. and of course reports of the occasional auction score (I have to admit to eyeing these jealously, after spending hours trawling eBay for a bargain and ringing numerous surplus stores).

At the heart of the 3458A lies a precision voltage reference. Voltages references are in principle quite simple devices, their goal is to output at all times an exact precise stable voltage of a known value. References have numerous applications but in the 3458A it’s used as part of its multislope ADC. The input voltage is effectively compared against this reference value in order to determine the input voltage. Its accuracy is therefore of paramount importance.

Roll Your own precision Reference

TiNs LTZ1000 based reference board
The LTZ1000 is an ovenized zener reference. You’re no doubt familiar with zener diodes and their clamping applications (if not Afrotechmods has a terrific tutorial). It’s their ability to “clamp” an input voltage to a specific lower value that makes them optimal for use as references. The zeners “clamp” (reverse breakdown) voltage varies with temperature. For better stability the LTZ1000 therefore uses an on-die resistive heater to keep the diode at a constant temperature (much like an ovenzied oscillator).

The LTZ1000 has found favor with voltnuts seeking to build precision references using this part. Some have used the reference board from the 3458A. but others, like TiN have designed their own. TiN has documented his work in exquisite detail over on the EEVBlog forums, describing not only the PCB layout but also extensive thermal modeling. That is what you see pictured above.

LTZ1000 based board from Izumo
Izumo Sangyouza has also been building LTZ1000 references. though unlike TiNs, few details are available, the boards look awesome however and though the forum posts are in Chinese, the videos included stand as testament to the accuracy of his references. But Izumo’s blog holds an additional surprise for the avid volt nut. The picture below perhaps says all that needs to be said.

The multimeter shot below uses the AD5791 in concert with a stable reference to provide incredible precision. While the post doesn’t specify, it’s a safe bet that one of Izumo’s LTZ1000 voltage reference boards is being used in this case. having recently played around with the AD5791, I can only stand in awe of Izumo’s work.

look at all those lovely digits

A liquid nitrogen amazing Josephson junction reference and HP 3458A

The LTZ1000 is the best generally available reference and the 3458A the best meter. but one question remains, what do you use to calibrate it?


The answer is to turn to basic physics, in this case the Josephson junction. The Josephson junction is formed from two superconductors separated by a thin insulating barrier. When cooled with liquid helium these junctions exhibit complex non-linear behavior, such that a frequency applied to the junction can be converted into a precise voltage. Luckily, producing highly accurate frequencies is something we’ve become very good at. Cesium standards (so called atomic clocks) are used together with the Josephson junction to create highly accurate voltage references.

The IEEE have create a video on the basic physics and the evolution of the Josephson junction. Through the efforts of  Quantum Electrical Metrology Division and others, cI fianchi che incorporano migliaia di tali giunzioni su un singolo dispositivo sono stati fabbricati e forniscono valori standard accurati all’interno dei nanovolt. Questa proprietà fisica di base è ora utilizzata per calibrare tutti i multimetri di precisione.

Il chilogrammo elettronico

C’è una svolta finale nella storia del dado Volt. Mentre la maggior parte dei nostri standard di misurazione si basano su una proprietà fisica fondamentale, il chilogrammo standard è un pezzo di metallo che si trova in un caveau a Parigi. Questo è ovviamente, piuttosto insoddisfacente. Nel corso delle centinaia di anni che questo grumo di metallo è stato seduto in giro, ha perso la massa come è stata pulita e gestita.

Le comunità degli standard ora cercano di sostituire questo con un “chilogrammo elettronico”. Il loro sistema equilibra la potenza meccanica di un peso sotto gravità e la forza esercitata da un elettromagnete. Questa energia elettrica è ovviamente misurata in termini di Josephson Standard Volt.

Brian Josephson ha vinto il premio Nobel per i suoi contributi allo sviluppo del riferimento Josephson Junction, forse rendendolo il dado di volt Ultimate. Ma per ora tali referenze giacciono fuori dal dominio dell’hacker medio. Forse un giorno, come gli orologi atomici prima di loro, le applicazioni industriali guideranno il prezzo verso il basso in modo che ogni dado di volt possa avere il loro standard molto personale. Fino ad allora la ricerca va sempre avanti.

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