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

[Steven J Banaska]

As Tom said the 10M input impedance is used for the high voltage ranges
because it is a resistive divider (9.9M/100k) that can handle high voltages
without much drift. Caddock THV or HVD are fairly common in precision dmms.

Typically you will find a high impedance (10G) path that can be used for
the ranges 10V and lower, but the 10M divider can be left connected and
will work for any voltage range by changing which side you measure. As you
mentioned there can be an accuracy sacrifice when you have a high output
impedance from your source. I'm not sure why 10M is the default other than
it may extend the life of the relay that switches the 10M divider in or out.

Steve



On Thu, Apr 10, 2014 at 8:07 AM, Tom Miller <tmiller11147 at verizon.net>wrote:

> 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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