Hi Tony:
Fluke makes some DDMs that have what they call V-Check where they put a 1,000
Ohm resistor across the voltage input.
When testing lawn sprinkler valves if you measure the voltage across the valve with a Hi-Z voltmeter it looks normal,
but using the V-Check range on the DMM shows the voltage to be almost zero.
http://www.prc68.com/I/DMM.shtml
Have Fun,
Brooke Clarke
http://www.PRC68.com
http://www.end2partygovernment.com/2012Issues.html
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? 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
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