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

[Tony]

Hi Paul,

On 11/04/2014 19:18, Poul-Henning Kamp wrote:
> In message <53482D9E.9060904 at toneh.demon.co.uk>, Tony writes:
>
>> I did find this explanation in the 3458A manual:
>>
>> When making DC voltage measurements, you can fix the multimeter's input
>> resistance using the FlXEDZ command. This is useful to prevent a change
>> in input resistance caused by changing ranges) from affecting the
>> measurements.
> Please don't forget that the 3458A is not just an 8.5 digit
> metrological wonder, it is also a precision 16 bit 100 ksample/s
> digitizer.
Yes I'm well aware of that but it doesn't make any difference if you're 
observing the display or the logged data - you would not want that 10M ± 
1% resistor across the input - unless you have no choice because the 
input exceeds 12V and you have to use a 100V+ range, in which case you 
have to accept the limitations of the instrument. I accept that it could 
help in some situations where you aren't bothered about absolute 
measurements but are interested in changes and the resistor helps to 
reduce the noise.

In such a situation I agree you would probably be logging the data 
rather than looking at the display but the sampling rate has no bearing 
at all on this issue.
> Many of the attributes of the input circuit are not there for
> voltage metrology.
>
I can't think of any - I'd be interested to know what you have in mind?

Regards,
   Tony H