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

[Tom Miller]

Don't forget. There is accuracy and then precision. You should not confuse 
the two.

And many things use high voltages >1kv besides old crts.

T

----- Original Message ----- 
From: "Brent Gordon" <volt-nuts at adobe-labs.com>
To: "Discussion of precise voltage measurement" <volt-nuts at febo.com>
Sent: Thursday, April 10, 2014 4:16 PM
Subject: Re: [volt-nuts] 34401A Why 10M ohm default i/p resistance?


> Pure conjecture:  So that the reading on the 34401A matches that on a $20 
> DVM.
>
> Or stated differently:  So that the input impedance is the same as other 
> DVMs.
>
> 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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