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

Tony vnuts at toneh.demon.co.uk
Thu Apr 10 10:23:14 EDT 2014

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? Could it be psychological? By limiting the 
drift caused by the i/p bias current to 300uV max when the meter is left 
unconnected? A voltmeter with a rapidly drifting reading (several mV/s) 
when not connected to anything is a bit disconcerting and would probably 
lead to complaints that the meter is obviously faulty to users who are 
used to DVMs which read 0V when open circuit - because they have i/p 
resistance << 10G ohms and don't have the resolution to show the offset 
voltage caused by the i/p bias current.

Personally I'd have though that the default should be the other way 
round - especially given that there is no indication on the front panel 
or display as to which i/p resistance is currently selected.

Any thoughts? What do other meters do?

Tony H

More information about the volt-nuts mailing list