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

[Tony]

Hi,

On 11/04/2014 10:53, frank.stellmach at freenet.de wrote:
> Hello
>   
> In the manual (!), HP reasons the 10M standard input resistance:
>   
> "Normally, the multimeter’s input resistance is fixed at 10 MΩ for all dc voltage ranges to minimize noise pickup."
Oops! I'm ashamed to say I missed that!
>   I explained that to myself like this: AC stray fields or noisy high impedance sources induce noise input currents in the DMM frontend.
> The higher its input Z, the higher the noise voltage reading will be.
> In that sense, 10MOhm 'shorts' those noise effects.
> 

Frank, I realise you know all the following perfectly well - so please 
don't take offense, but hopefully, for the benefits of others:

You can't have your cake and eat it! You might be 'shorting' the noise 
but you're equally shorting your signal; if you've got that much noise 
then you've got some potentially difficult signal conditioning to do if 
you want to make accurate measurements. Sticking a 1% tolerance 10M 
across the signal to cut the noise makes no sense at all - it's only 
going to make a significant difference to the noise if the source 
resistance is > 10M or so, in which case why are you using a 6-1/2 or 
even 8-1/2 digit meter? You might as well use a 1 digit meter - at least 
that wouldn't give any false illusions of accuracy. Wouldn't a capacitor 
(with a resistance >> 10M) across the input make more sense? If the test 
leads are long then 10M at the meter is going to reduce the noise more 
than 10M at the source, but the problem remains.

In the case of the 3458A using the 10M I/P resistance requires the 
source resistance to be < 100 milli-ohms for it not to show errors on 
the 8-1/2 digit scales - so OK. for measuring voltage regulators, 
batteries etc. but very little else including precision voltage 
references or the O/P of an op-amp etc. The 3458A does default to the 
high I/P resistance state though so you have to choose to change it to 
10M, unlike the 34401A which defaults it to on.

Even if you do know the precise resistance of the source allowing you to 
calculate the actual source voltage, the 1% tolerance of the 10M 
restricts the accuracy to 2 or 3 digits at best - so again it makes no 
sense to me to be relying on a non-precision resistor on precision 
instruments - even the 3458A's 10M has a 1% tolerance. You could measure 
the I/P resistance of your meter with another 6-1/2 or 8-1/2 digit meter 
but how many people do that? And how are you going to measure the source 
resistance accurately? What if its non-linear?

If you're relying on that 10M, chances are you're using the wrong 
instrument.

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//
/
A bit feeble though. If your measurement changes when changing ranges 
that is a *good* thing - it highlights that you've  got a problem and 
you need to think about it and decide which, if either, measurement is 
accurate rather than hide the problem by showing consistent, but 
consistently wrong readings. The a rather special case anyway, 
applicable to I/P voltages at the boundary between the 10 and 100V range 
- if its less than 10V (well 12V on the 34401A/3458A) then you've no 
reason to use a higher range.

I suppose it could be useful to observe a signal where you aren't 
concerned about the absolute accuracy but want high resolution relative 
measurements - eg. drift measurements or measuring the linearity of a 
slope. You need to know though that the 10M is not going to move the 
high impedance source into a non-linear region.

It looks like this is a HP feature; looking at the manuals for the 
Solartron 7081 and Datron 1071 meters they don't appear to be have the 
facility to lower the I/P resistance from >= 10G for ranges below 100V.

> Anyhow, my 3458A is always programmed to have TOhm input Z as power up state, therefore I have to short the input jacks, whenever the instrument is not connected, or doing an ACAL. That avoids drifting of the input and unwanted relays actuations, when left open.
I suppose that could be a slight concern although my 34401A  doesn't 
seem to drift much beyond 200mV - I guess it could sit around the 120mV 
switching threshold of the 100mV and 1V ranges and keep switching 
between them. I've never seen it though, and wouIdn't be concerned about 
the relay life even if it did (although the noise might be irritating).
> 
> On the 34401A, it is very uncomfortable to set the input to high Z every time, as many keypresses are needed.
> 
> So agilent recognized that dilemma, and improved the handling on the new 34461A:
> Changing input Z is only one additional keypress, as this feature is assigned to one of the soft keys.
>   
> Frank
> 
That's good to see. I think they just got it wrong on the 34401A

Tony H
> 
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