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