How to measure the Amplitude of a Sine Wave using a Digital Multimeter?

[RedDogAlpha]

Hello EEVBloggers,

This is my first post on the forum after watching hundreds of Dave's excellent videos. They are what got past wanting to tinker with electronics, to actually getting past the massive learning to curve to do something useful. Thanks Dave!

So after years of learning and experimenting I have purchased some "real" test equipment. I am currently trying to test the calibration of the equipment (mostly for experience), and one part of the test requires testing the Amplitude of the generated sine wave from a Siglent SDG-5082 Function Generator.

Here is my brand new test equipment:

Siglent SDG5082 Function Generator ($250 @ Amazon)

Siglent SDS1202X-E 200Mhz Oscilloscope ($379 @ Amazon)

Extech EX355 Multimeter

In the Siglent SDG5000 Service Manual on PDF Page 42 (Document Page 28)
they specify using a DMM to determine the amplitude variances.

However, my EX355 DMM does not have that function. And when I searched for products capable of doing Amplitude measurements, I found some bench top DMM's with price tags over $1000.

Now I know that my oscilloscope can measure amplitude, but not to the accuracy required to make a determination of pass/fail according to the Service Manual. Is there another way to measure Amplitude using the tools I have, or a cheaper option?

Thanks!

 

[Cliff Matthews]

Welcome! Use the REL function while taking a sample, it will zero. The next sample will be the change in amplitude. *edit Pg. 20 in the PDF manual states, the function applies only to Voltage, Current, Temperature and Capacitance modes.

[RedDogAlpha]

Thanks for the reply. Please correct me if I am wrong, but my DMM only measures True RMS, and pressing the <REL> button will only give me variances from the RMS value, correct? I believe that RMS is not the same as Amplitude, furthermore, Amplitude is not the same as Peak to Peak. Am I correct in those beliefs?

     A sinusoidal curve

"Sine voltage" by Matthias Krüger - SVG von de:Bild:Sinus-spannung.gif. Licensed under Public Domain via Commons - https://wiki2.org/en/File:Sine_voltage.svg#/media/File:Sine_voltage.svg

• Peak amplitude
• Peak-to-peak amplitude
• Root mean square amplitude
• Wave period (not an amplitude)

Furthermore, I should have pointed to the page I was referencing in the pdf, it is PDF Page 42 (Document Page 28). When I looked at the Page 20 you mentioned, I saw no mention of "Voltage, Current, Temperature and Capacitance modes". Are you referring to my DMM manual?

Thanks again![/list]

[Cliff Matthews]

Another document I guess..  http://www.extech.com/resources/EX35x_UM-en.pdf 
I'm no expert, but I've always calculated peak voltage as 1.414 times RMS.

[Brumby]

First thing:  RMS = root mean square.  It is a mathematical process that will produce a value which is more meaningful in many situations.  The classic explanation is that, for a given AC waveform, the RMS voltage is the equivalent DC voltage that will create the same heating in a given resistor as the AC waveform.

In reference to that diagram, you can replace the word "Amplitude" with "Voltage".  Amplitude is just a word to describe a particular attribute.  For the attribute being described here, you could use the word "magnitude" or even "Size".  In this specific example, the value of this attribute is being measured in volts.  What makes this situation a little more complex is that there is more than one "amplitude" measurement you can make.  Each has their own use and are all valid.

A "True RMS" meter will give you the correct result for any shape of waveform (within limits) - but for a sinusoidal waveform, this is very easy.

Even easier (for a true sinusoidal waveform), is the calculation of the peak, peak-to-peak or RMS from any one of those values.  The relationship of RMS to peak values is 1V RMS = 1.4142V Peak = 2.8284V Peak-to-peak.  The square root of 2 features in this.

[RedDogAlpha]

Thanks so much for your explanations. So I will use the <REL> function of my DMM, and then multiply the displayed value by 2.8284V to derive the variations of my signal.

Thanks again.

[Brumby]

That will give you peak-to-peak variations.

I suggest you get into the habit of writing down the measurement type next to the value.  eg: 2.7V RMS   3.82Vp   7.64Vp-p.  That way, you will know what the actual measurement is of.  If you don't, then the general assumption will be that it is an RMS value, but assumptions are not always reliable and unlabeled values are a bad idea, especially if you get into formal documentation.

[mainman]

Here's a easier to remember formula
voltage peak = VRMS/.707

[Brumby]

Understand the relationships and you can write your own formulae.

[The Soulman]

Here's a easier to remember formula
voltage peak = VRMS/.707

Yes, for sine waves, different shapes, different formula's.

From Extech dmm manual:

ACV frequency response is 45~400Hz

Accuracy above that is undefined.

Just a heads-up.

[vk6zgo]

Thanks so much for your explanations. So I will use the <REL> function of my DMM, and then multiply the displayed value by 2.8284V to derive the variations of my signal.

Thanks again.

As the positive & negative half cycles are not present at the same time, peak to peak does not represent a real voltage value.
It is convenient for measuring signal levels with an Oscilloscope, & does have a real lmportance when you
are looking at the dynamic range of a device, such as an amplifier or an ADC.

[RedDogAlpha]

As the positive & negative half cycles are not present at the same time, peak to peak does not represent a real voltage value.
It is convenient for measuring signal levels with an Oscilloscope, & does have a real lmportance when you
are looking at the dynamic range of a device, such as an amplifier or an ADC.

Wouldn't testing the amplitude variations of my function generator be a correct usage of peak to peak?

[RedDogAlpha]

Understand the relationships and you can write your own formulae.

That is true! I will work on that understanding. If there was ever something I have learned from electronics, it is that for every answer, there are multiple more questions derived from that answer. Like most other scientific things I guess.

Thanks everybody for your input!

[w2aew]

A few more points:

Make sure that you're measuring at a frequency that is within the specifications for your DMM - some will only measure to a few kHz maximum.

Also, make sure you understand the voltage setting on your signal generator and the effect of the Load setting - such as in this video:


And here's a video I made regarding the AC RMS (and true RMS) voltage readings on a DMM:

[RedDogAlpha]

Make sure that you're measuring at a frequency that is within the specifications for your DMM - some will only measure to a few kHz maximum.

From The Soulman for my DMM.

ACV frequency response is 45~400Hz

So it looks like I am limited in that regard. Maybe I should look into Siglent's Bench DMM models.

As for the video's, thanks for sharing them and taking the time to share your knowledge. I have already come across the first one covering input and output impedance concerns, as that was the very first thing I noticed when I set up my equipment and started testing. The ringing was really bad until I matched the impedance in the cables, and set the output of my function generator to Hi-Z.

The second one on RMS is insightful and opened my eyes to how easy it is to fool yourself into believing bad measurements. I am a Land Surveyor by trade and one thing I know to be a fact is that there is never a "perfect" measurement. The part that mentions that most DMM's are AC coupled is something I never considered before, but it makes sense.

Thanks everyone for all of your information and thoughtful approach to answering my question('s). I have definitely received my answer, and more. I think we can consider this topic closed.

[xani]

A lot (all ?) of digital oscilloscopes can measure common wave parameters automatically (my rigol gives either few chosen stats or a big table with all of them).

It is low resolution (most scopes are 8-10 bit) but still useful.

Also, your signal generator ( I have SDG1025 so lower series than yours) will automatically calculate output based on what load resistance you select

So it looks like I am limited in that regard. Maybe I should look into Siglent's Bench DMM models.

Even top of the range meters (like agilent 34465A)  go to only like 300kHz for truerms. Wrong tool for the job i think.

It would probably be vast overkill for most/all things you're doing. I've got:

* $50 6000 count one (that is at least 7 years old; you can get a whole lot more for $50 bucks now)
* cheapie 2000 count one (which i picked because it has separate power button) for when I need to measure current and voltage at same time
* UT210E because clamp
* 6.5 digit bench one (picotest M3510A)

The vast majority of time I just use my $50 one. It is just more convenient and unless you're doing some analog sensors the extra resolution/accuracy is just not needed.

I only really use my benchtop when:

* I want to measure something over time and dont want to constantly turn meter on and of to take measurement (or leave it on and drain battery)
* When I want to compare something to temperature; that particular one can display measurement of 2 things at once so I can show for example current + voltage or current + thermocouple's temperature

but that's not even something I *need* to do with benchtop, it is mostly convenience (altho I do plan to connect it to some linux box and add data logging at some point) as I could use thermocouple on my handheld DMM

[Mechatrommer]

Another document I guess..  http://www.extech.com/resources/EX35x_UM-en.pdf 
I'm no expert, but I've always calculated peak voltage as 1.414 times RMS.

Here's a easier to remember formula
voltage peak = VRMS/.707

+1.. 1.414 came from sqrt(2), 0.707 is 1 / 1.414... Vrms = Vpeak / sqrt(2)... logically its easy to remember just this... if we want Vpp, just multiply by 2... but this wont hold accurately for non-sinusoidal voltage.

[Brumby]

As I said before:

The square root of 2 features in this.

Understand the basic geometry of this graph and apply the square root of 2 to anything relating to measurement #3.  Multiply, divide, reciprocal ... it's all hanging off the square root of 2 - for a sine wave.