How to use findpeaks to only find specific peaks and time where they occurred?

I'm attaching a plot of voltage versus time for a signal that I averaged over time. I want to use the findpeaks function to exclude the major peak seen at approx 40 ms, as well as the repetitive downward inflections that repeat. I just want the peakfinder function to automatically locate the inflection I've circled in yellow. Is there a way to find the magnitude of this peak and the time where it occurred, using the findpeaks function? Any other tips to automate this would be welcome as well!

[Star Strider]

That is going to be difficult.

First, the downward deflection needs to be converted into a peak, so use the negative of the signal to do that.

Second, it will be necessary to use the MinPeakHeight name-value pair (and perhaps some others, such as MinPrakDistance) to detect the peaks that you do not want.

After that, run findpeaks with no restrictions except MinPeakHeight (to avoid ths high-frequency noise, although you may want to filter that out first), then use setdiff or other functions as appropriate to isolate the locs output to the peak you want. (The filter step will require taking the Fourier transform of the signal to see the best frequency for the high-frequency cutoff. A lowpass filter will do this, however a bandpass filter can also eliminate the DC offset, making further analyses easier.)

The height of the peak you want (and all the others) will be the negatives of the peak heights that are returned, because the signal was negated first.

Automating that will require that all the signals have similar characteristics. You will have to experiment to see if that is true.

[Zuha Yousuf]

Thank you so much for your answer! I am a little confused about where you explained detecting the peaks I dont want. The code I wrote is as follows:

[peaks_noise,locs_noise,w_noise,proms_noise]=findpeaks(-signal,'MinPeakHeight',2.42e-5,'MinPeakDistance',16.7);

This outputs all the info about the peaks that I don't want.

You next said that I need to run findpeaks with MinPeakHeight to find the peak that I do want. Is there a way to eliminate the unwanted peaks I previously found from this signal, and then run findpeaks on a new cleaned signal to just locate this peak that I want? Since the findpeaks only outputs the locations and magnitudes of the peaks.

[Image Analyst]

And what was wrong about simply finding and erasing the big two peaks that you don't want before finding the smaller ones, like I showed you in my Answer below? What you just suggested was just what I had shown you an hour before you posted.

Answer 1

[Image Analyst]

Why can't you just erase everything before 50 ms and then find peaks?

index = find(t < 50, 1, 'last');
amplitude(1:index = 0);
[peakValues, locations] = findpeaks(amplitude);

Or if it's not always there at 50 or so, find the min point and go up until it hits the mean (about -2)

[~, index] = min(amplitude);
meanAmp = mean(amplitude)
while amplitude(index) < meanAmp
    index = index + 1;
end
amplitude(1:index = 0);
[peakValues, locations] = findpeaks(amplitude);