[volt-nuts] 3458A reference boards on ebay

[Frank Stellmach]

Hello fellow-nuts,

do you already know about our monster discussion on DIY LTZ1000s on EEVBLOG?
http://www.eevblog.com/forum/projects/ultra-precision-reference-ltz1000/msg180731/#msg180731

There are several people who have successfully assembled such boards, 
and they are pretty stable, much more stable than the HP ones, and the 
precision resistors are also easy to obtain, if you use precision 
wire-wound, which are at least as good as the bulk metal foil, for this 
purpose.

The 002 reference is the same as the standard one, but is selected for 
lower drift.. assuming these run 24/365.

Otherwise, the may show big hysteresis and have to re-initialized for a 
period of time, to reach the same level of stability. standard boards 
are rumored to achieve the same level of stability after some years, also.

In DIY designs, but also in these HP references, simply reduce the oven 
temperature to 55°C for the LTZ1000A, and to 45°C for the LTZ1000. That 
will typically improve their stability by a factor of 2 for each 
reduction of 10°C, compared to the about 95°C, the original HP reference 
inside the 3458A is running (due to max. environmental temperature spec.)


To use these boards externally, simply put a metal shield around, give 
it a good ground, and a quiet, linear regulator (12V or so), and you're 
done. Once measured, you get an extremely stable (t & T) 7,1xV reference.


Amplifying to 10V by a ChopAmp will require precision resistors, which 
will account most for the annual drift, then.

If you really need that, use oil filled hermetically sealed Vishay 
types, e.g. VHP202Z. They really drift < 2ppm/ 6 years, and may have 
very low T.C. (dR / dT, physically correctly measured).
Or you may build a 2 stage PWM for that purpose.

Then you only need a precise ratio standard, to transfer the (known) 
7,1xV to 10V.. a 34401A will probably do the job for < 1ppm accuracy.

More details in the mentioned thread; my description in 5 or 6 chapters 
you may find on pages 12-16.

Frank