Test Instrument Confusion...

raveenvijendren · 13th December 2005, 06:32 AM

Good Day,

I would like to know the difference between a True-RMS digital multimeter and a non True-RMS multimeter. Thank you.

dhaen · 13th December 2005, 01:56 PM

A "True RMS" meter attempts to give a reading that equates to the RMS value of the waveform. I say attempts because there is a limit to the amount of distortion that it can tolerate before the reading becomes inaccurate.

A non ... meter requires a sine wave or wave of equivalent form-factor to read correctly. In other words it can't tolerate as much distortion.

So they are an improvement, but like all things, there are limits you must be aware of. It'll be in the "small print".

jackinnj · 13th December 2005, 02:04 PM

there are true RMS meters which are actually true -- these are thermopile devices like the HP3403C and its predecessors OR very fast ADC's (Linear Tech just announced some devices for this end.)

A good approximation of true RMS can be had with those HP meters employing the Analog Devices AD536 RMS converter -- basically a logarithmic converter which uses an integrating capacitor. The Tektronix DA4084 distortion analyzer also does a great job at measuring RMS.

I purchased an HP3403C for less than $20 on EBay -- and it compares exactly with my calibrated Boonton 1120 -- this instrument is a true bargain if you can find it --

13th December 2005, 06:32 AM	#1
raveenvijendren diyAudio Member Join Date: Nov 2005	Test Instrument Confusion... Good Day, I would like to know the difference between a True-RMS digital multimeter and a non True-RMS multimeter. Thank you.

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13th December 2005, 01:56 PM	#2
dhaen diyAudio Moderator Emeritus Join Date: Jun 2002 Location: U.K.	A "True RMS" meter attempts to give a reading that equates to the RMS value of the waveform. I say attempts because there is a limit to the amount of distortion that it can tolerate before the reading becomes inaccurate. A non ... meter requires a sine wave or wave of equivalent form-factor to read correctly. In other words it can't tolerate as much distortion. So they are an improvement, but like all things, there are limits you must be aware of. It'll be in the "small print".

13th December 2005, 02:04 PM	#3
jackinnj diyAudio Member Join Date: Apr 2002 Location: Llanddewi Brefi, NJ	there are true RMS meters which are actually true -- these are thermopile devices like the HP3403C and its predecessors OR very fast ADC's (Linear Tech just announced some devices for this end.) A good approximation of true RMS can be had with those HP meters employing the Analog Devices AD536 RMS converter -- basically a logarithmic converter which uses an integrating capacitor. The Tektronix DA4084 distortion analyzer also does a great job at measuring RMS. I purchased an HP3403C for less than $20 on EBay -- and it compares exactly with my calibrated Boonton 1120 -- this instrument is a true bargain if you can find it --

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