Analogue switch

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The analog (or analogue) switch, also called the bilateral 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 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). These include the 4016 and 4066 from the 4000 series.

The switch generally provides good isolation between the control signal and the input/output signals. However this has some limit so they may be not appropriate for some high-voltage or safety-critical circuits. For such cases, an opto-isolator may be used. Also the current-handling capability of smaller devices may be less than typical mechanical relays. Units are available that can reliably switch hundreds of volts and hundreds of amperes, up to the thyristors found in HVDC systems. There are also some constraints on the polarity and range of voltages 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 gigaohms.
• 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 switches 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 electric charge into the signal when it switches on, causing a small spike or glitch. The charge injection is specified in coulombs.

See also

• Analog-to-digital converter
• Transmission Gate