DMM input impedance -- FALSE specifications?!

[001]

Hi!

Many DMMs (Fluke 8020A, Mastech M3900 etc) marked as "10M impedance at ALL ranges" but is it true?
But I see simple resistive divider at schematic
9M resistor is on only at 1000v DC range, isn`t it?
900k for 200V range
90k for 20V range etc

What I misunderstud?

[schmitt trigger]

You are misunderstanding a lot.

What you are describing is the way analog meters worked, where actual current drove the needle movements.

[001]

Well
But what is the difference in simple dividers?
The example is

[Kleinstein]

The simple DMMs use mainly one of 2 type of input dividers:

1) A divider with 10 M total resistance and a very high impedance (> 1 GOhm)  amplifier from one of the taps. To input impedance is essentially constant, the extra amplifier impedance would be in parallel in the lowest range, but that would be only a < 1 % change.

2) A kind of inverting amplifier with a 1 M Resistor at the input.  The feedback Resistor is switched to adjust the gain of the initial stage, that can very well be smaller than 1.

[ogden]

Well
But what is the difference in simple dividers?
The example is

Sorry? What exactly you are asking? Please rephrase your question, be more specific.

[MasterT]

What I see, divider is the same for all ranges, R1-R6,  Input in "V" position connected to upper side of the R1 - 2x 4.5 M. Summ of the R1-R6 is pretty close to 10M.

[David Hess]

There are two common input divider arrangement for voltmeters which are briefly discussed in this application note from Intersil.

The one you are describing which is used in manual ranging meters like the Fluke 8020A and Mastech M3900 has a constant input resistance.  The decade resistor divider string usually adds up to 10 megohms.

The other style is used in automatic ranging meters because it is difficult to implement a floating high voltage semiconductor switch.  Instead of a series string of resistors, a single input resistor is used with a set of selectable shunt resistors with the switches at ground potential.  The disadvantage of this scheme is that the input resistance varies somewhere between 11 and 9 megohms.  Unfortunately these meters are usually advertised as having a 10 megohm input resistance which is a lie.

[001]

Great answer! Thanx a lot!