Measuring voltages in a very high impedance circuit

[JC@]

I asked to Neumann and I had the answer to my question, but could you please explain me why my DMM loads down the circuit under measurement? :o
And why do they use in the circuit design such a high impedance?

>>... ... ...
If you look at the U67 schematic, you can see that you are trying to measure voltages in a circuit where the impedances are 400MOhm and 60MOhm. At those points you can only measure with specialized equipment, not with a standard voltmeter. ... ... ...>>

Thank you.
JC

[R ski]

The mics element generates very small voltages, the typical mic electronics
uses high impedance input to offset loading of the mics output.

Impedances in that range requires current measurements rather than
voltages.

Meaning microamps ma and insert the probes between the resistor
and its connection point.

[JC@]

R ski,

But why a very high impedance input circuit like this loads down a normal voltmeter so much it can't give a correct reading.
I don't keep the schem on hand, but on those points the expected voltages were something like +59 and +120 volts dc!
I mean I wasn't supposed to measure the element's output but to make sure that those voltages were there. This is a tube driven microphone with it's own power supply.

[Enzo]

If a circuit has 400M impedance, and your meter has 10M impedance, you cannot help but to load the circuit down to under 10M when yout meter is attached.

A scope that is well calibrated might have higher impedance. And of course expensive lab gear will work. A bridge can measure the voltage with little loading, but you probably don't have one.

What meter are you using?

[R ski]

A typical digital voltmeter has about 20 meg ohms source impedance. When
your probes are placed across a 400 meg source the meter will shunt
away the voltage present, therefor giving a false reading.

Getting back to the current reading is the most practical method of
determinig the voltage product. Remember voltage=current X resistance.

A typical high impedance tricky measuring situation of measuring a fraction
of a gram from a load cell. The same technique was used.

Remember the U 67 was design decades ago when instumentation wasn't
so easy to use.