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

Tony vnuts at toneh.demon.co.uk
Thu Apr 10 15:58:19 EDT 2014

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" <vnuts at toneh.demon.co.uk>
> To: <volt-nuts at 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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