>There are a number of reference boards supposedly removed from 3458As
on eBay.
>Assuming they are non counterfeit (not necessarily a valid assumption
for items on eBay), how easy would they be to integrate into a box to
make a stable reference?
>The LTZ1000A chip is about $50 new, but to make a stable reference you
need quite expensive resistors, so a used 3458A reference is attractive.
>But I don't know if problems making low thermal EMF joints might
totally ruin the potential performance.
David,
these 3458A reference boards are also quite expensive, they sell for
about 150-200$.
They may have problems with high drift and high noise, that's why they
show up on the 2nd hand market.
It's been demonstrated that it's possible to easily build such stable
references with an 40$ LTZ1000 (non A), and 5 precision wire wound
resistors ( <10$ each), and an LT1013, so BOM cost < 100$.
The emfs are not the biggest problem, can be mitigated by proper design
and thermal shielding.
The biggest problem with the 3458A and the DIY references is to amplify
precisely to around 10V.
These resistive dividers usually drift too much over time and
temperature to give reasonably stable 10V references.
I dfesigned an auto-calibration-divider, similar to the 720As 1st
decade, which amplifies the 7,15V exactly by 10/7, which is mostly
sufficient to calibrate the 10V range of a DMM.
Other solutions require these ultra- expensive Vishay hermetically
sealed, oil filled VHP resistors, which easily double the BOM cost.
Frank
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