[volt-nuts] Ye Olde HP3458A

[John Devereux]

"Poul-Henning Kamp" <phk at phk.freebsd.dk> writes:

> In message <87y5z97jwe.fsf at devereux.me.uk>, John Devereux writes:
>
>>What are peoples opinions on the likelihood of a newer model coming out
>>soon?
>
> p=0.
>
> Read the HP Journal article carefully, and you will realize that if
> you want to build a DVM an order of magnitude better than the 3458
> it will have connectors for both "power" and "helium", because such
> a instrument will crash directly into the thermal noise.

 :)

I have read that article before and was suitably impressed. But it was
not clear to me where their 100nV/rtHz "theoretical limit" came from. It
seems arbitrary rather than fundamental - it is the noise of a 600k
resistor. 

There are plenty of opamps (and transistor front ends) with <1nV/rtHz
noise. And plenty of resistors less than 600k:).

Also while digging around I found some patents that appear to improve on
the architecture. For example 4951053 describes a summing node switching
scheme that seems obviously superior to that illustrated in the journal
article. (In fact it predates it, maybe the article is not quite right?)

> The only place where you could drastically improve on the Hp3458,
> is the two internal references, and this is where much of the
> "value add" of Flukes 8508a comes from.
>
> I am somewhat dubious if we will ever see compact high-temperature
> superconductor based josephson voltage references and quantum resistance
> standards, but if they appear on the market, it would be pretty trivial
> to retrofit a HP3458 with them, provided they can fit in the box.
>
> The other argument for buying an HP3458A is that it is a much more
> interesting instrument to play with, up to and including downloading
> and running your own M68k programs on it.

Hey that sounds fun :) I wire wrapped a homebrew 68k board around then
too.

-- 

John Devereux