Switch Analog Signals#

A logic gate cannot pass an analog signal, and a relay is bulky and slow for routing a small voltage. The need here is to steer an analog signal — an audio line, a sensor voltage, a reference — on and off, or from one path to another, under digital control.

The Part#

The classic answer is the 4066, a quad bilateral (analog) switch. Each switch is a CMOS transmission gate — an NMOS and a PMOS transistor in parallel — that, when its control input is high, conducts in either direction and passes whatever analog voltage sits between the supply rails. When the control is low, the path is open. It is a genuine switch for signals, controlled by a logic level.

What to Watch#

Analog switches come with constraints that logic gates do not have:

• Signals must stay within the supply rails — the switch can only pass voltages between its Vss and Vdd; anything outside is clipped or conducts through the protection diodes.
• On-resistance is not zero — a closed switch has tens to a couple hundred ohms of Ron, which forms a divider with the load and varies a little with signal level; feed high-impedance loads to keep it negligible.
• Charge injection — switching couples a small charge spike onto the signal, which matters for sample-and-hold and precision paths.

For selecting one of several analog sources rather than just gating one, use an analog multiplexer such as the 4051. And for switching real power rather than signals, this is the wrong tool — that is a job for a MOSFET, a relay, or a solid-state relay.