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

[Tom Miller]

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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