[volt-nuts] 34401A Why 10M ohm default i/p resistance?

frank.stellmach at freenet.de frank.stellmach at freenet.de
Fri Apr 11 05:53:46 EDT 2014

In the manual (!), HP reasons the 10M standard input resistance:
"Normally, the multimeter’s input resistance is fixed at 10 MΩ for all dc voltage ranges to minimize noise pickup." 
 I explained that to myself like this: AC stray fields or noisy high impedance sources induce noise input currents in the DMM frontend.
The higher its input Z, the higher the noise voltage reading will be.
In that sense, 10MOhm 'shorts' those noise effects.

Anyhow, my 3458A is always programmed to have TOhm input Z as power up state, therefore I have to short the input jacks, whenever the instrument is not connected, or doing an ACAL. That avoids drifting of the input and unwanted relays actuations, when left open.

On the 34401A, it is very uncomfortable to set the input to high Z every time, as many keypresses are needed.

So agilent recognized that dilemma, and improved the handling on the new 34461A:
Changing input Z is only one additional keypress, as this feature is assigned to one of the soft keys.

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