"You can't have your cake and eat it!"
well said
Too true, that the 10 MΩ reduces the noise pickup, but **only** when the DVM
input is **open circuited**, which is not so good for measuring anything if
the signal is not connected.
Otherwise the noise pickup is a function of the impedance of the signal
being measured, not the DVM's input impedance.
If you want the lowest noise pickup, keep the DVM's input shorted. This is
the best case for open circuit noise pickup & the worse case load when
measuring anything.
If you want less than the 1ppm loading effect when using a 6 digit plus DVM,
insure that it's input impedance is > 1,000,000 times the signal's source
impedance, which is worse case for open circuit noise pickup.
Anyone that thinks a 10 MΩ input on a precision DVM is a good thing, try
using it to measuring a standard cell or the 1 volt, 1KΩ reference voltage
output from a Fluke 731 or 732, and see what that does.
ws
************
On 11/04/2014 10:53, frank.stellmach at freenet.de wrote:
Hello
In the manual (!), HP reasons the 10M standard input resistance:
"Normally, the multimeter’s input resistance is fixed at 10 MΩ for all dc
voltage ranges to minimize noise pickup."
Oops! I'm ashamed to say I missed that!
I explained that to myself like this: AC stray fields or noisy high
impedance sources induce noise input currents in the DMM frontend.
The higher its input Z, the higher the noise voltage reading will be.
In that sense, 10MOhm 'shorts' those noise effects.
...
You can't have your cake and eat it! You might be 'shorting' the noise
but you're equally shorting your signal; if you've got that much noise
then you've got some potentially difficult signal conditioning to do if
you want to make accurate measurements.
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