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

[Bruce Griffiths]

Since the dc gain of the amplifier using the LSK389 is unity a precise 
match of the JFET input offset isnt critical so one can use a selected 
pair of 2SK369BL.
These are readily obtainable on eBay.
I purchased about 20 and easily found several pairs with a Vgs match to 
within a few mV.

The amplifier that uses the low noise JFET's as its input stage is ac 
coupled with a dc gain of unity so that it could easily be redesigned to 
use a single FET input device with current feedback to its source.

Bruce

Randy Evans wrote:
> 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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