Analog switches

Nov. 9, 2006
Analog switches primarily serve as a means of letting electronic logic turn low-level analog signals on and off.

Edited by Leland Teschler

primarily serve as a means of letting electronic logic turn low-level analog signals on and off. They typically come in the form of integrated circuits that hold multiple switches. Most analog switch ICs are CMOS technology. Switch action takes place with the application of a logic level of the right polarity on the input of a CMOS differential pair. This turns on a set of FETs which let the analog signal applied to the input pass to the output.

The FETs in the analog path consist of an n-channel and a p-channel device in parallel. This configuration lets the switch handle analog currents flowing in either direction through the switch.

A point to note about analog switches is that there are parasitic circuit elements between the control and the analog signal paths. Circuit designers must allow for these effects. One such parameter is called charge injection. It results from capacitive coupling between the control signal and the analog path. The result can be a rapid shift in the voltage at the analog input.

The degree of charge injection is related to a parameter of the analog switch called Ron. This is the combined on-resistance of the FETs handling the analog signal. Switches with high Ron have less charge injection because they have less parasitic input capacitance. This also lets them switch faster than devices with low Ron.

Manufacturers try to minimize Ron for analog switches handling higher voltages and currents. But Ron varies with the analog input voltage, power-supply voltage, and temperature. A parameter called Ron flatness gives the difference between the maximum and minimum resistance over the prescribed operating conditions.

Two analog switches ganged together can form a double-throw switch. In this case the turn-on and turn-off times of the two switches are important. A turn-on time greater than a turn-off time yields a break-before-make action. Similarly, a turn-off time exceeding a turn-on time gives a make-before-break. The distinction becomes important when, say, there is a possibility of shorting two input channels if the switching action is not break-before-make.

handling frequencies of 10 MHz and above employ a special circuit configuration called a T switch. It consists of two analog switches in series with their connecting point attached to ground through a third switch. The third switch basically makes sure the switch turns off. It grounds out signals that might otherwise couple through the off-capacitance of the two series-connected FETs.

Analog Devices Inc.( and Maxim Integrated Products ( provided information for this article.

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