The aperture time (limitation 1) is the time required for the sampling gate
to close after a sample is acquired. In an ideal situation, the aperture
time is zero and the output signal of the S&H is proportional to the signal
value immediately before the sampling instant. However, in a nonideal
situation, the sampling gate requires a finite time to switch. During this
time, the input signal of the S&H is changed, leading to sampling error.