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

[Randy Evans]

Does anyone know where and how to obtain the LK389 mentioned below.  Linear 
Systems has a high minimum order but i would like to pursue rebuilding my HP-419 
since the chopper doesn't work (as usual).

Randy



----- Original Message ----
From: Bruce Griffiths <bruce.griffiths at xtra.co.nz>
To: Discussion of precise voltage measurement <volt-nuts at febo.com>
Sent: Sun, September 19, 2010 2:17:51 PM
Subject: Re: [volt-nuts] Fluke 720A Kelvin Varley Divider Questions

Charles P. Steinmetz wrote:
> Bill wrote:
> 
>> You're kidding, right ?
> 
> I've never used one, and I'd be interested to know what you (and others) think 
>its failings are.
> 
> Best regards,
> 
> Charles
> 
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:
http://cds.linear.com/docs/LT%20Magazine/LTMag-V16N01-11-FETAmp-Williams.pdf
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.

Bruce


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