Problems in Measuring 200mV in audio range (AC)

r_s_dhar · 17th June 2002, 11:45 AM

I sat down the other day to do some Thiele-Small tests and discovered that my multimeter would not settle down to zero in the 200mV range. At first I put it down to an instrument fault.

I later discovered that I was using the multimeter very near the computer and that my DMM registers 4-24mV neary any power source!

Is it a general phenomenon or is there anything wrong with my DMM?

Please respond.

leroy · 17th June 2002, 12:07 PM

Prevent large (pickup) loops in your measuring cables, that means: run the two wires as close to each other as possible. If you do so, and short the wires your DMM should be displaying between zero and a few mV (never trust the last digit).

mrfeedback · 17th June 2002, 05:01 PM

Hello r_s_dhar,
Modern DMM's have very high impedence input, and are therefore prone to EM noise pickup.
Minimising the loop area of your meter leads will help to reduce this, and loading (50Kohm) the meter inputs will reduce noise pick up.
Cheap meters will hunt even with the inputs shorted, and good meters such as Fluke are much more immune.

Regards, Eric.

17th June 2002, 11:45 AM	#1
r_s_dhar diyAudio Member Join Date: Jun 2002 Location: Mangalore, Karnataka State, India	Problems in Measuring 200mV in audio range (AC) I sat down the other day to do some Thiele-Small tests and discovered that my multimeter would not settle down to zero in the 200mV range. At first I put it down to an instrument fault. I later discovered that I was using the multimeter very near the computer and that my DMM registers 4-24mV neary any power source! Is it a general phenomenon or is there anything wrong with my DMM? Please respond.

17th June 2002, 05:01 PM	#3
mrfeedback diyAudio Member Join Date: Feb 2002 Location: Perth, Australia.	Cheap Meter Hunting Hello r_s_dhar, Modern DMM's have very high impedence input, and are therefore prone to EM noise pickup. Minimising the loop area of your meter leads will help to reduce this, and loading (50Kohm) the meter inputs will reduce noise pick up. Cheap meters will hunt even with the inputs shorted, and good meters such as Fluke are much more immune. Regards, Eric.

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17th June 2002, 12:07 PM	#2
leroy diyAudio Member Join Date: Oct 2001	Prevent large (pickup) loops in your measuring cables, that means: run the two wires as close to each other as possible. If you do so, and short the wires your DMM should be displaying between zero and a few mV (never trust the last digit).