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

Tony vnuts at toneh.demon.co.uk
Thu Apr 10 16:06:13 EDT 2014

That seems a more likely reason - matching users' expectations. It's the 
unexpected that trips people up - I doubt many casual users of DVMs ever 
see the manuals. I still think it was the wrong choice.

Tony H

On 10/04/2014 18:58, Joel Setton wrote:
> I think the 10 Meg default value became a de facto standard at the 
> time of VTVMs (vacuum-tube volt meters), as a convenient value which 
> reduced input circuit loading while remaining compatible with the grid 
> current of the input triode. Designers of early solid-state voltmeters 
> merely decided not to change a good thing.
> Just my $0.02 worth!
> Joel Setton
> On 10/04/2014 18:55, Steven J Banaska wrote:
>> As Tom said the 10M input impedance is used for the high voltage ranges
>> because it is a resistive divider (9.9M/100k) that can handle high 
>> voltages
>> without much drift. Caddock THV or HVD are fairly common in precision 
>> dmms.
>> Typically you will find a high impedance (10G) path that can be used for
>> the ranges 10V and lower, but the 10M divider can be left connected and
>> will work for any voltage range by changing which side you measure. 
>> As you
>> mentioned there can be an accuracy sacrifice when you have a high output
>> impedance from your source. I'm not sure why 10M is the default other 
>> than
>> it may extend the life of the relay that switches the 10M divider in 
>> or out.
>> Steve
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