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AC RMS vs. True RMS vs. AC-DC True RMS... How to calculate?

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AC RMS vs. True RMS vs. AC-DC True RMS... How to calculate?

Postby Marcn » Wed Sep 02, 2009 7:53 am

I am new to Picoscope (just bought the 4224). Is there any information on exactly how the software's built-in functions computes AC RMS and True RMS? Is Picoscope's True RMS really AC+DC True RMS? My application is to determine puwer out of an audio amplifier when driven by a signal of pink noise. AC-DC True RMS is needed.

I want to add a formula and display trace for AC+DC True RMS Volts and then AC True RMS Volts without the DC offset and finally the DC offset by itself.

Then I need to do the same for the current measurements.

Then I can calculate power to the speakers with and without the DC component.

The application is to characterize non sinusoidal, irregular audio band signals going through a Class-D switching audio amplifier. The amps output is differential so I will tie up two probes that will have to be added together before the math functions of AC +DC True RMS are performed.

In Picoscope 6 there does not seem to be way to have the Math channels (or the measurement perform an integration of the data to allow me to set up the formula for calculating AC+DC True RMS. I would need to perfrom this for all data points in the sample and then calculate an average. Then I do this for the current and then I can mutiply to get the power result.

Also, when taking current reading by measuring voltage across a low value in-series resistor, how should I approach setting up the scope? Should I set the resistor/current measuring network as a custom probe and have the scaling function do the divide by the esistor value to get current then integrate each point to get AC+DC volts RMS?

Tnanks in advance for you help folks.

Regards, Marc
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Re: AC RMS vs. True RMS vs. AC-DC True RMS... How to calculate?

Postby Michael Hayes » Tue Oct 27, 2009 11:40 pm

Marc,
Any DC produced by an amplifier does not produce any useful output, in fact part of an amplifier set up is to set the DC to zero as it simply produces a constant physical offset in the voice coil, pushing it closer to end of travel in one direction it also produces parasitic heating of the voicecoil. In a Class B amplifier it produces waste in the output stage as well. In a class D, you should simply adjust the switching waveform to give 0VDC with no signal applied.
The problem of measureing power is then reduced to only measuring AC power. I would usually do this with single tone sinewave at a range of frequencies to also produce a frequency response (albeit a very flat one these days) I think the use of noise to measure power may be complicating things a little more than they need to be.
Regards,
Michael
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Re: AC RMS vs. True RMS vs. AC-DC True RMS... How to calculate?

Postby Jeff » Wed Nov 25, 2009 2:56 pm

Marcn: "AC RMS" is the RMS of the signal after the DC offset is removed, and "True RMS" is the RMS of the entire signal including AC and DC.

With PicoScope 6, you can measure power without using the RMS functions. Just set up a channel that equals voltage x current and then take the average (what PicoScope calls "DC Average"). There is an application note that shows exactly this procedure at http://www.picotech.com/applications/ma ... lysis.html. (Ignore the use of the RMS functions in the article unless you are interested in calculating the power factor.)

The User's Guide has not caught up with the new RMS functions in the software, so I'll fix this in a future version.
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