Moral of the story made by Brian is:
Never assume your instrument’s input impedance!so it got me thinking about my DMM's. All of them got specified 10M impedance without any fine print notes, but let's check if it's true.
DIY DC voltage input impedance measurement
How to do this? It's quite easy. All you need is DC voltage power supply (the higher voltage, the better, 50V is fine) and current meter with uA range. Connect your power supply, current meter and voltmeter under test as shown on the schematic below. |
| DCV Input impedance measurement circuit |
From Ohm's Law, we know that for 10Mohm input impedance, we should get 1uA current flow from 10V voltage source. As 1uA can be quite low to measure accurately, we will use higher voltage.
In my case, I have about 44V at my disposal at this moment, so we should get about 4.4uA.
Brymen BM257, BM829 and BM867 test
Lets get started, first in line is my BM257 (on the left), current measured by BM867.
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| BM257 DCV input impedance test |
Second: BM829, current measured by BM867
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| BM829 DCV input impedance test |
And the last one: BM867, current measured by BM257
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| BM867 DCV input impedance test |
As you can see, all three of them limited the current to expected value, so they input impedance is about 10Mohm as in specs.
