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

[John Phillips]

It is more along the lines of building a voltage divider with stable
resistors.  A !G ohm voltage divider would be more expensive to build with
the same stability that you can get from 10M ohms. There are trade offs in
all designs. The cost benefit ratio just is not there. If you really need
high input impedance in a DC meter go  differential.  If you are looking
for AC circuits then stray capacitance will really mess you up with a high
impedance divider.



On Thu, Apr 10, 2014 at 12:27 PM, Andreas Jahn
<Andreas_-_Jahn at t-online.de>wrote:

> Hello,
>
> perhaps its just to save the lifetime of the input range selection relays
> to at least the warranty time.
> Just a guess.
>
> With best regards
>
> Andreas
>
> Am 10.04.2014 19:58, schrieb Joel Setton:
>
>  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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-- 
John Phillips