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

WarrenS warrensjmail-one at yahoo.com
Fri Apr 11 14:52:21 EDT 2014

>"You can't have your cake and eat it!"
well said

Too true, that the 10 MΩ reduces the noise pickup, but **only** when the DVM 
input is **open circuited**, which is not so good for measuring anything if 
the signal is not connected.
Otherwise the noise pickup is a function of the impedance of the signal 
being measured, not the DVM's input impedance.
If you want the lowest noise pickup,  keep the DVM's input shorted. This is 
the best case for open circuit noise pickup & the worse case load when 
measuring anything.
If you want less than the 1ppm loading effect when using a 6 digit plus DVM, 
insure that it's input impedance  is > 1,000,000 times the signal's source 
impedance, which is worse case for open circuit noise pickup.

Anyone that thinks a 10 MΩ input on a precision DVM is a good thing, try 
using it to measuring a standard cell or the 1 volt, 1KΩ  reference voltage 
output from a Fluke 731 or 732, and see what that does.



>On 11/04/2014 10:53, frank.stellmach at freenet.de wrote:
>> Hello
>> 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."
>Oops! I'm ashamed to say I missed that!
>>   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.

You can't have your cake and eat it! You might be 'shorting' the noise
but you're equally shorting your signal; if you've got that much noise
then you've got some potentially difficult signal conditioning to do if
you want to make accurate measurements. 

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