What's this Waveform's RMS? - Page 1
Forum section: electronics / projects

author: Luminax
author_rank: Regular Contributor
timestamp: December 16, 2016, 01:31:48 am
content: Hi folks!I have this waveform as attached below.It came from an RC filter which is connected between a microcontroller pin and ground.Unfortunately, the ?C is so old that only a few non-comprehensive datasheet is available on it on the net.The IC and Pin19 RC circuit also attached below and the ?C seems to be Panasonic's old 'discontinued' MN15G0804, thanks for forum member amyk for finding that one out.Now the problem is, our client describes the waveform as normal operation mode with Positive going 0 to 1.2 V peak-to-peak and a discharge (at least that's what it looks to me) time constant of 0.5The specs is to be measuring the voltage wave and making sure it's "below 0.5"... which I think is indicative of them wanting the RMS voltage?How do you calculate the RMS of a cyclic decaying capacitor waveform?

author: trophosphere
author_rank: Frequent Contributor
timestamp: December 16, 2016, 02:56:01 am
content: I am not positive about the spec wanting the RMS voltage butthis websiteshould help you in estimating the RMS voltage using either graphical or analytical methods.

author: Luminax
author_rank: Regular Contributor
timestamp: December 16, 2016, 03:59:40 am
content: Nice website! thanks.Using the graphical method I have obtained :VRMS= 0.40792Which is very close to the 0.5 mentioned in the specs.I guess I can use peak detect to start a counter and then get data points for until it hits 0 or close to that and calculate the RMS from there.Hmmm.... wonder what the RMS will be for the second graph... back to calculation!EDIT: Arbitrary waveform similar to the second graph gives RMS of 0.7514 which is "over spec" so I guess that settles that

author: Ammar
author_rank: Regular Contributor
timestamp: December 16, 2016, 01:20:27 pm
content: Assuming a time constant of 0.5 I get 0.789.http://www.wolframalpha.com/input/?i=sqrt(integral+of+2(1.2e%5E(-2x))%5E2+from+0+to+1%2F2)

author: orolo
author_rank: Frequent Contributor
timestamp: December 16, 2016, 03:28:43 pm
content: Looking carefully to the picture, the 0.5 volt point is reached 1/6 of the way to 0.5 seconds, that is, at 1/12 of a second. A bit of math after, the voltage is 1.2*exp(-10.5*t).Integrating the square of that function from 0 to 0.5 is easy and gives (1.44/10.5)*(1-exp(-10.5)) = 0.137.Doubling it, we get 0.274, taking the square root, it is 0.523 volts RMS.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: December 16, 2016, 04:27:22 pm
content: As a practical matter, almost all true rms meters are ac coupled and don't give the correct results for waveforms with a dc offset. In those cases you need to take a meter reading for ac and a meter reading for dc. The root sum of the squares of the two readings gives you the correct rms value, assuming you used a true rms meter. There are a few meters that can be switched to do this automatically, even my ancient HP 3455A.

author: Ian.M
author_rank: Super Contributor
timestamp: December 16, 2016, 05:05:59 pm
content: Why suppose its RMS? Average is far more likely.

author: orolo
author_rank: Frequent Contributor
timestamp: December 16, 2016, 05:30:34 pm
content: I'd say they want the user to detect the 1.2V pulse and make sure that, after half a second, it is well below 0.5 volts. The time constant is not 0.5 seconds, if it were so the voltage would not be almost 0V in the end, as in the graphic. So perhaps a comparator to check if the signal gets below 0.5 volts is all that is needed to confirm good operation. Something like a missed pulse detector: if the wave doesn't get under 0.5 volts each half second, there is a malfunction. A checker could be done with acomparatornon-retriggerable monostable (to detect each 1.2V pulse and decay) and then a venerable 555 missed pulse detector.Could this be right?

author: Luminax
author_rank: Regular Contributor
timestamp: December 17, 2016, 12:05:22 pm
content: The problem with customer's "Engineering" specs is that... sometimes THEY don't know what they want...I have had one unit to test and it was indeed that same particular waveform and roughly 2Hz, so I'm guessing it might not be just a simple RC circuit(Or maybe it is?).My DSOX2014A gives me 450~460mV RMS full scale (standard deviation) and a rough graphical calculation gives me under 410mV, but then again, it might not be RMS, it might be average, it might not be... I've checked with the group responsible for the model transfer to confirm with our customer but waiting on that is like watching paint dry or to be more precise, waiting on a blue moon...

author: hans
author_rank: Super Contributor
timestamp: December 17, 2016, 12:48:51 pm
content: Quote from: Ammar on December 16, 2016, 01:20:27 pmI think you should take 5 * time constant for it to drop to zero in a satisfactory manner.With 1* time constant of 0.5, it only drops to about 1.2 * 1/e = 0.441V, not to zero.With e^(-10x) I get 0.379 Vrms.