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From: jeff@nsr.hp.com (Jeff Gruszynski)
Newsgroups: sci.electronics
Subject: Re: digital voltmeter - how does it work?
Date: 21 Apr 1993 01:58:39 GMT
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MAMISHEV, ALEXANDER VALENTINO (avm1993@sigma.tamu.edu) wrote:
:      Hello, 
: 
:    Let me introduce a problem:
: 
:    When I measure a sinusoidal wave (voltage) with a digital voltmeter, using 
: AC mode, my output is an rms value (a peak value over 2 squared). / Right? / 
:    When I measure a square wave in the same mode (AC), my output is equal 
: to a peak value, actually, to the upper flat boundary of the wave.
:    I assumed, that a digital voltmeter makes some kind of integration of the 
: input value, and divides it over the wave period. / Right?/
:    Now, I used it to measure the same  square wave as above, but distorted 
: by high-frequency harmonics. Ideally, output should be the same, but...
: The output value was only about 10% of the previous one! 
:    Why? What is the nature of this output value? What does the voltmeter 
: actually measure? And what does it show?  


Re: RMS readings

Unless the DVM *says* it's measuring RMS, it's probably Average voltage.
The input is diode rectified and measured as DC.  If it says it's RMS
and but measures square/triangle/etc.  incorrectly, it's measuring
average and multipling by a correction that's *only* true for sine waves
(i.e.  Vave*(0.707/0.63) = Vrms).  If you want correct RMS for (most)
any waveform, you need a "True-RMS" DVM/DMM which literally does the
Root-Mean-Square calculation in either analog or digital circuitry.

: 
:    Related question (less important to me):
:    What are advantages and disadvantages of digital voltmeters to compare with 
: analog ones? 

The last significant advantage of analog (IMO) ** was ** being able to
"see" the signal if it was changing over time (e.g.  checking
electrolytic capacitors).

Anymore, most DMM have bargraphs, etc. that duplicate this.

: 
:    Thank you for your attention, you could mail me your opinion at
: avm1993@zeus.tamu.edu or open a discussion here. I would appreciate either 
: way.
: 
: 
: Alexander V. Mamishev
: 
: ____________________________________________________________________________
: Power System Automation Laboratory    <>   phone office (409) 845-4623    
: Department of Electrical Engineering  <>   phone home   (409) 846-5850
: Texas A&M University                  <>   fax   (409) 862-2282
: College Station, TX 77843, USA        <>   Internet: avm1993@zeus.tamu.edu
: ----------------------------------------------------------------------------
: 

--
================================================================================
Jeff Gruszynski
Semiconductor Test Equipment
Systems Engineer
Hewlett-Packard
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(415) or T 694-3381
jeff@hpmvd061.nsr.hp.com
jeff@hpuplca.nsr.hp.com
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