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

Brooke Clarke brooke at pacific.net
Thu Apr 10 16:58:28 EDT 2014

Hi John:

Because when measuring a source with a high resistance you get a different answer.
Some W.W.II electronics specified 1 kOhm/Volt meters and if you used a VTVM you got the wrong results.
If a test procedure specifies a 10MOhm input meter and you use a higher input Z then you may get wrong results.

Have Fun,

Brooke Clarke

John Phillips wrote:
> so why do you care what the input is as long as you know what it is and how
> to make it do what you want?
> On Thu, Apr 10, 2014 at 1:16 PM, Brent Gordon <volt-nuts at adobe-labs.com>wrote:
>> Pure conjecture:  So that the reading on the 34401A matches that on a $20
>> DVM.
>> Or stated differently:  So that the input impedance is the same as other
>> DVMs.
>> Brent
>> On 4/10/2014 8:23 AM, Tony wrote:
>>> There is no suggestion in the specifications for the 34401A that the
>>> accuracy suffers by selecting 10G ohm input resistance on the .1 to 10V
>>> range so why would they make 10M ohm the default? I can think of very few
>>> cases where having the 10M ohm i/p resistor switched  in is better for
>>> accuracy than not.
>>> On the other hand 10M is sufficiently low to produce significant errors
>>> on a 6 1/2 digit DVM for sources with resistances as low as 10 ohms.
>>> Measuring 1V divided by a 100k/100k ohm divider for example causes a .5%
>>> error - 502.488mV instead of 500.000mV. That might not be a problem but I
>>> wouldn't be surprised if this catches a lot of people out (including me)
>>> when not pausing to do the mental arithmetic to estimate the error. It's
>>> just too easy to be seduced by all those digits into thinking you've made
>>> an accurate measurement even though you discarded those last three digits.
>>> And if it's not a problem then you probably don't need an expensive 6 1/2
>>> digit meter in the first place.
>>> It's a small point I agree but it can get irritating to have to keep
>>> going into the measurement menus to change it when the meter is turned on
>>> when measuring high impedance sources (e.g. capacitor leakage testing).
>>> It can't be to improve i/p protection as 10M is too high to make any
>>> significant difference to ESD and in any case there is plenty of other
>>> over-voltage protection. OK. it provides a path for the DC amplifier's
>>> input bias current, specified to be < 30pA at 25 degrees C, but I imagine
>>> that varies significantly from one meter to the next, and with temperature,
>>> so not useful for nulling out that error.
>>> So why would they do this?
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