3458A reference boards on ebay
do you already know about our monster discussion on DIY LTZ1000s on EEVBLOG?
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
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.