Analogue switch - Definition and Overview

The analogue (or analog) switch is an electronic component that behaves in a similar way to a relay, but has no moving parts. The switching element is normally a MOSFET, which is a type of transistor. The control input to the switch is a standard CMOS or TTL logic input, which is shifted by internal circuitry to a suitable voltage for switching the MOSFET. The result is that a logic 0 on the control input causes the MOSFET to have a high resistance, so that the switch is off, and a logic 1 on the input causes the MOSFET to have a low resistance, so that the switch is on. Analogue switches are usually manufactured as integrated circuits in packages containing multiple switches (typically two, four or eight).

Unlike a relay, however, the analogue switch does not provide electrical isolation between the analogue signal and the control signal. This means that it should not be used in high-voltage circuits where such isolation is desired. Also, since there is no metallic path between the input and output, the maximum current allowed to flow through the switch may be smaller than that in a typical relay. The lack of a metallic path also means there are some constraints on the polarity, range of voltages, and the like, of the signal being switched.

Important parameters of an analogue switch are:

• on-resistance: the resistance of the MOSFET when switched on. This commonly ranges from 5 ohms to a few hundred ohms.
• off-resistance: the resistance of the MOSFET when switched off. This is typically a number of megohms or gigohms.
• signal range: the minimum and maximum voltages allowed for the signal to be passed through. If these are exceeded, the switch may be destroyed by excessive currents. Older types of switch can even latch up, which means that they continue to conduct excessive currents even after the faulty signal is removed.
• charge injection. This effect causes the switch to inject a small electrical charge into the signal when it switches on, causing a small spike or glitch. The charge injection is specified in coulombs.