Analog Switches

An Analog Switch is a solid-state semiconductor device that has one or more channels that can transmit analog signals when they're in the 'on' state or block them when they're in the 'off' state. The turning 'on' and 'off' of an analog switch is controlled by a digital gating signal applied to its control gate. Applications of analog switches include data acquisition, process control, instrumentation, video systems, and communication systems.

An ideal analog switch has zero resistance when 'on' (or closed), and infinite resistance when 'off' (or open). It also has a perfectly linear volt-ampere characteristic when transmitting an analog signal. Of course, analog switches of the real world are not 'ideal'. Being solid-state semiconductor devices, real analog switches exhibit non-zero 'on' resistance, a finite 'off' resistance, and a non-linear volt-ampere characteristic.

Just like mechanical switches, analog switches come in a variety of forms, depending on the number of poles and throws they offer. Thus, terms such as 'SPST' and 'SPDT' (single-pole single throw and single-pole double-throw, respectively) which are commonly used to describe mechanical switches are also applicable to analog switches. A single IC package can also have multiple switches in it, each of which corresponds to an analog channel.

There are many circuit configurations that can be used as gates for analog switches, some of which are very simple, e.g., consisting of just a single diode and several resistors. Most commercially available analog switches though employ well-engineered bipolar transistors, field-effect transistors (FET's), or a combination of both in their channels for the transmission or blocking of analog signals. FET's are widely used in analog switches because of their high 'off' resistance and low 'on' resistance. Figure 1 shows a simplified CMOS analog switch circuit.

Figure 1. A simple CMOS analog switch

The circuit in Figure 1 employs complementary MOSFETs (CMOS) consisting of an n-channel MOSFET and a p-channel MOSFET, both of which are connected such that their source terminals are on opposite sides of the circuit (i.e., one is on the input side and the other is on the output side). It then follows that their drain terminals are also on opposite sides of the circuit. Also, note that the control voltages at the gates of the transistors are digital (in this case, '1' means +5V and '0' means -5V) and complementary.

The effect of this entire configuration is that one value of Vc will turn both transistors 'off' and the other value of Vc will turn at least one transistor 'on'. In the latter case, which transistor is conducting depends on the current value of analog input Vin. The analog switch is 'off' if both transistors are 'off', and it is 'on' if at least one of the transistors is 'on.'

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