There is no events generated in the center of the PWM signal that can be used to trigger the SAADC sampling. If you know the timing from the edge until the center, you could use a TIMER/RTC to trigger the sampling, but this depends on your application. Are you using the PWM peripheral? What is the configuration of the output channels?
It is possible to create a pseudo event in the center of the PWM signal by using a very narrow pulse on one of the spare PWM channels in waveform mode, assuming only a single (or two) PWM outputs are being used, that is. For ADC sampling that would probably work well.
