CMOS Analog Integrated Circuits

8.6    Track-and-hold (T/H) and sample-and-hold (S/H) circuits

A sample-and-hold circuit is commonly used at the interface between analog and digital systems to hold a sample of the time-varying signal for a period of time so that the high-frequency operation is facilitated. It is commonly realized by combining switching devices, such as MOS transistors, and capacitors.

Ideally, a sample-and-hold circuit takes a sample of the input signal in zero time and holds the sample value during a period T. The output signal obtained in response to a continuous-time unit impulse signal, δ(t), is known as the impulse response, which can be represented in terms of shifted unit step functions. Thus,

$h\left(t\right)=u_s\left(t\right)-u_s\left(t-T\right),$

(8.30)

where u_s(t) is the unit ...