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

[Brooke Clarke]

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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