[volt-nuts] Fluke 720A Kelvin Varley Divider Questions

Greg Burnett gbusg at comcast.net
Mon Sep 20 02:01:37 UTC 2010

Bruce wrote:

A more general question would be:

Is it possible to design a chopper stabilised null detector using modern
components with a performance equivalent to the 845A?

Whilst the noise of most chopper stabilised opamps is much larger than
that of the 845A, perhaps something like the approach used in:
could be used.

Since the low noise preamp is AC coupled a lower noise input using a
single ended JFET could be substituted for the one using the LSK389.


Good question/suggestion Bruce.

Maybe that approach could be used to re-design the first "half" of a HP 
419A, building a modern circuit into an existing HP 419A to save time/money 
and to come up with a nice little box that works well. (The HP 419A *was* a 
small form-factor, cute, attractive instrument, with built-in supply & 
batteries, so it might yet still have a use?)

I think the 419A can be purchased for practically nothing, if it can still 
be found. ...again, I'm afraid many of them have already gone to graveyards 
for scrap metal recovery.


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