The analog switch is an important device found in analog-to-digital interfaces.

Figure 2–5

(a) and

(b) shows the two basic versions of the device. In either type, the switch is actuated by applying a voltage to the terminal labeled gate. The switch in Figure 2–5

(a) is said to be normally open because it is open when no voltage is applied to the gate terminal and closes when voltage is applied. The switch in Figure 2–5

(b) is said to be normally closed because it is closed when no voltage is applied to the controlling gate and opens when voltage is applied.

Figure 2–5

(c) shows an application in which complementary analog switches are controlled by the same gate. When gate voltage is applied, the upper switch closes and the lower opens so that point A is connected to point C. Conversely, when no gate voltage is applied, the upper switch opens and the lower switch closes to connect point B to point C. In the analog world, this arrangement is called a double throw switch since point C can be connected to two other points. In the digital world, it is called a two-to-one multiplexer (or MUX) because it allows you to select the analog input at point A or point B under control of the digital signal applied to the gate.
In many applications, an analog switch can be treated as an ideal switch. In other cases, it may be necessary to account for its nonideal characteristics. When the switch is open, an analog switch acts like a very large resistance ðROFFÞ, as suggested in Figure 2–5(d). This resistance is negligible because it ranges from perhaps 109 to 1011 Ω.When the switch is closed it acts like a small resistor ðRONÞ, as suggested in Figure 2–5(e). Depending on other circuit resistances, it may be necessary to account for RON, because it ranges from perhaps a few mΩ to as high as 100 Ω