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

John Phillips john.phillips0 at gmail.com
Thu Apr 10 16:43:47 EDT 2014

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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John Phillips

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