18 Dec
2008
18 Dec
'08
3:04 a.m.
On Dec 17, 2008, at 1:59 PM, Paul Koning wrote:
Today, the standard voltage references are subsurface zeners and
band-gap devices. The LTZ1000 is one of the former class, and one of
the best units available. It's used, for example, in the amazing HP
8458 DVM. Here are a couple:
http://www.neurotica.com/misc/tastyreference.jpg
> A DVM with
a built-in standard cell?? Neat! Got pics and/or
> schematics? I'd love to see that.
Tony> It was quite common at one time. The ones I've memtioned have
Tony> ther traditional H-shaped glass things in them...
Makes sense. A DVM needs an accurate reference. These days that's a
solid state device, the high end successor to the Zener diode. (I'm
not sure how they work -- quite possibly a completely different
scheme.) 
Before that came out you'd need something else,
and a Weston cell is
certainly a very good way to solve the problem.
Well, Weston cells usually exhibit big instabilities after being
moved, and don't settle down for weeks. Also, they are somewhat
temperature sensitive. I'd not consider it a good choice for
embedding in a device intended for anything other than cal/metrology
lab equipment.
In this case, I'd have expected a VR tube. They're far more
rugged. I have several related pieces of equipment from that era
which use VR tubes, though not a DVM per se.
-Dave
--
Dave McGuire
Port Charlotte, FL