[volt-nuts] Accurary and performance : any experience with the HP 34420A ?

Bruce Griffiths bruce.griffiths at xtra.co.nz
Wed Aug 25 23:15:37 UTC 2010

> Hi guys,
> I read carefully all your debates about the voltage references and internal
> constructions of the famous 3458A or 34401A, but I wonder why nobody ever
> speaks about the 34420A. According to the datasheet, it seems to outperform
> the 3458A with small signal measurements. Does anybody have some experience
> with this meter or know what's inside ? Is it a tuned 34401A or something
> different ?
> Sam.
The service manual schematics indicate that its essentially an enhanced 
34401 with a higher gain lower noise dc amplifier, a few tweaks to the 
ADC and close attention to minimise thermal emfs in the input circuitry.
The dc amplifier input stage is a darlington cascoded differential JFET 
input stage (bootstrapped to the input device source common mode) with 
an active load whereas the 34401 merely used a pair of JFET source 
followers with bootstrapped drains.


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