*****NOTE: This is NOT a medical device*****
Using a diode and capacitor pair, the peak detector captures the peak AC amplitude of the pulse waveform and holds the voltage (because of the capacitor). When we place a resistor in parallel with the capacitor, the resistor allows the capacitor to discharge at a rate equal to the time constant of the RC circuit. If we tune the time constant correctly, the peak detector circuit will hold the peak long enough to distinguish the peak pulse amplitude from secondary peaks due to the dicrotic notch. Also, because of the discharge of the capacitor through the resistor, the peak detector does not hold the voltage indefinitely, but it adjusts to the changing peak AC amplitudes of the pulse waveform due to stronger or weaker pulses.