Gordon wrote:
Pure conjecture: So that the reading on the 34401A matches that on a
$20 DVM.
I assume you mean when the DVM is disconnected - otherwise you wouldn't
spend more than $20 on a meter! But I said that in my original post:
/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.////
/
Or stated differently: So that the input impedance is the same as
other DVMs.
Not really - that's a different reason. Other meters have a variety of
different input resistances but 10M is probably the most common however.
In any case, with the exception of matching the needs of a HV probe, the
higher the input resistance the better. Deliberately compromising the
performance to match cheaper models and making it harder than necessary
(a sequence of 9 button presses!) to de-select that error source, seems
to be a bizzare choice.
Tony H
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?
_______________________________________________
volt-nuts mailing list -- volt-nuts@febo.com
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts
and follow the instructions there.
_______________________________________________
volt-nuts mailing list -- volt-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts
and follow the instructions there.
