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Why do some multimeters have dedicated mV ranges?

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Robomeds:
I was looking at a Fluke 73iii I own and started thinking, why does it have a dedicated 300mV range?  I can understand why someone might want to measure a low voltage.  I'm not questioning the desire to measure sub 300mV ranges but really, why can't/isn't this incorporated into the autoranging DC voltage range?  Is there a significant change in input impedance in the mV range?
Incidentally I have seen this on several of my non-Fluke meters as well. 

Monkeh:

--- Quote from: Robomeds on May 18, 2013, 05:55:51 pm ---Is there a significant change in input impedance in the mV range?

--- End quote ---

Only about 2.99GOhm, typically.

Robomeds:
gigaohms.... hmmmm....  :-DD

Monkeh:
Your Fluke seems to be 10M on mV, though. Pays to check the specs..

Not sure why it has a separate range for the same impedance.

Robomeds:
Well the 87-iii and 73-iii show the same impedance in both the mV and V settings according to Fluke.  However, I do see that if I start the meter in high impedance mode the 400mV range does gain 3.99 Gohms of impedance.  By default the 87-iii has only 10Mohm resistance in either DC volts range.  That could explain why the 87 has such a range but not the 73-iii which doesn't have a high impedance setting. 


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