How it works?

In practice, analog signals are sampled using zero - order - hold (ZOH) devices that hold a sample value constant until the next sample is acquired. This is also called as flat - top sampling. This operation is equivalent to ideal sampling followed by a system whose impulse response is a pulse of unit height and duration  T_s ( to stretch the incoming pulses ). This is illustrated in Figure below :

Reconstruction of signal in Zero Order Hold Filter

The analog Signal (continuous - time signal) is multiplied with a periodic impulse train, referred to as Sampling Function. A sampled signal is then obtained as shown in figure below.

The ideally sampled signal x_p(t) is the product of the impulse train p(t) and the analog signal x_c(t) and is written as 

The ZOH Sampled Signal x_ZOH(t) can be regarded as the convolution of h_o(t) and a sampled signal x_p(t)