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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