# Digital Multimeter Input Impedance

#### Danlar81

Joined Apr 19, 2019
87
Hello,

I was just looking at my Fluke 179 DMM manual and I came across the following nominal input impedance for Volts ac.
>10Mohm < 100 pF

I would like to check my understanding, but I'm assuming the < 100 pF is related to the capacitive reactance at some frequency, however it will not be less that 10M ohm?

Is this correct?

I have added a snip of the manual for context below.

#### ericgibbs

Joined Jan 29, 2010
18,212
Hi Dan,
You should consider the values as physical values.

Say for example Rin= 10megOhms and a 100pF across the input signal lines.

This is the physical 'load' the DVM will place across the circuit points you are measuring.

E

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#### Danlar81

Joined Apr 19, 2019
87
Hi Eric,

Ok, so for a 50Hz frequency Xc is approx. 31.8megOhms. Considering the parallel arrangement of Rin and C, this would equate to approx. 7.6megOhms.

If the frequency was increased to say 1kHz, Xc is approx. 1.6megOhms, and the 10megOhms could be ignored as it is so high in comparison.

So the DMM would be seen as an impedance Z of 1.6megOhms at a frequency of 1kHz.

Is this the correct reasoning?

Dan

#### Danlar81

Joined Apr 19, 2019
87
Hi Eric,
I did not see the picture before I replied, and assumed these would be in parallel.

Is that the correct distinction to make, considering the values are referenced to the DMM?

Dan

#### ericgibbs

Joined Jan 29, 2010
18,212
Hi Dan,
If we consider the 100pF to be across the measuring leads, the cut off-frequency would be determined by the source impedance being measured.
It is possible depending upon the internal design of the DVM the 100pF maybe distributed along the 10meg.?

This sim shows a Rsrc from 1k to 50k in 5k steps, using a 1V AC source test voltage.

E

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#### Danlar81

Joined Apr 19, 2019
87
Hi Eric,

Because the manual does not specifically identify a // parallel arrangement, the assumption that I had made is most likely incorrect.
I think I should consider the series arrangement as you have shown.

Dan

#### ericgibbs

Joined Jan 29, 2010
18,212
Hi Dan,
Do you have a Sine Wave signal generator?
E

#### ericgibbs

Joined Jan 29, 2010
18,212
Hi Dan,
Have you seen this clip from a 179 datasheet.?
E