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

Tony vnuts at toneh.demon.co.uk
Thu Apr 10 17:18:02 EDT 2014

    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?
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