Possibly so, but I'd expect that the vast majority of HP 34401A 6-1/2
digit bench multimeters never see/saw > 1kV - even when CRT's were
common. Looking at the specs for several HV probes (1% accuracy was the
best I could find with a quick search), a $10, 3-1/2 digit DVM will be
just as accurate.
"Many of the accurate ones want to see a 10 meg input."
Well if you want *high* accuracy with such a probe, then you wouldn't
use a 34401A given that it's 10M ohm /P dividor has a tolerance of ± 1%
which would limit the overall accuracy to around .1% at best (for a
1Gohm, 1000:1 passive HV probe).
The 34401A only offers 10G ohm i/p resistance on the .1, 1 and 10V
ranges, switching to 10M ohm on 100 and 1kV ranges. So selecting the
100V range (much easier than using the menus to change the i/p
resistance) automatically selects the 10M ohm i/p resistance. Using a
1000:1 probe, voltages between 1kV and 10kV would lose a digit of
resolution compared to using the 10V range, but 5-1/2 digits is still
way more than needed given the .1% accuracy limited by the 34401A's i/p
resistance tolerance.
All in all, I think providing a minor convenience feature for HV probe
users (not having to manually select the 100V range) is a very unlikely
reason for selecting 10M as the default given that way more measurements
(source > 10 ohms) require the 10G ohm i/p resistance to justify using a
6-1/2 digit instrument.
Tony H
On 10/04/2014 16:07, Tom Miller wrote:
Think "HV Probe". Many of the accurate ones want to see a 10 meg input.
Also, some meters change input impedance depending on the selected range.
T
----- Original Message ----- From: "Tony" <vn...@toneh.demon.co.uk>
To: <volt-nuts@febo.com>
Sent: Thursday, April 10, 2014 10:23 AM
Subject: [volt-nuts] 34401A Why 10M ohm default i/p resistance?
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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