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

[Brent Gordon]

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?