Analog switch vs. digital switch ICs

Analog switch integrated chips (ICs), when turned on, will conduct both analog and digital signals from the input pin to the output pin. Digital switches can only accept digital signals and duplicate the logic level on the input pin at the output pin. When the digital switch is turned off, it returns to a default logic state.

The long version

An analog switch acts like a solid-state relay (i.e., with no moving parts). Analog switches can also isolate devices at their terminals when the analog switch is OFF. When ON, they conduct both analog and digital signals, regardless of the direction in which the signal is traveling. The switch control input for both kinds of switches is usually a digital signal input (called an input select), although other control triggers can be used to make specific applications easier to implement.

Analog switches are able to pass or isolate both analog and digital signals, but digital switches can only pass or isolate digital signal lines. Both are used instead of mechanical switches for convenience, reliability, and their small size as compared to mechanical switches.

There are some limitations on the signals that can be carried by both types of switches. Analog switches have a frequency response limitation due to channel capacitance. (A signal-level change can be caused by parasitic capacitance at high frequencies, for example.) For digital switches, there is a maximum frequency that can be fed into the input of the digital switch, after which the switch’s output state will no longer reliably follow the input. Calculate the transmission rate of your digital signal by noting the rate of the digital signal’s rise and fall times. You must also account for any delay from when the digital switch’s control signal (which activates the switch) changes, and when it subsequently enables the output. (Expect to see a change in rise and fall times based on varying conditions.) With both types of switches, the switch’s datasheet will reveal the limitations.

Both types of switches are typically manufactured as integrated circuits, often in packages with multiple (individual) switches. Multiplexors are also switches, but act more like a train station with multiple trains feeding via railroad switches onto one track, and is a separate topic.

Analog switches

The remainder of this article concerns analog switches. For analog switches, the switching portion of the device is made up of a couple of transistors. In Figure 1, the transistors are MOSFETs; one is a P-channel MOSFET and the other is N-channel. MOSFETs make perfect switching devices and are often used in power applications.

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Solid-state switches are much smaller than mechanical devices. They are fast, easy-to-use, and use less power than relays or other electrically controlled switches. However, no switch is perfect in real life. The ideal switch would transmit a signal without no changes to the signal whatsoever. However, actual analog switches will generate ground bounce noise and demonstrate propagation delay. All of the issues with common parameters are going to be present with solid-state analog switches; for instance, the on-state drain-to-source resistance that is a common parameter when switching MOSFETs will be present in switching in an analog switch. One downside to MOSFETs is that they are not actually perfect; one imperfection is some resistance associated with turning on the MOSFET. R_ON, or more specifically RDS(on), is the total resistance in the path from source-to-drain and is made up of a series of resistances that traverses the path of current flow through the MOSFET switch. RDS(on) is the basis for a maximum current rating of the MOSFET and is also associated with current loss, thus a lower RDS(on) is preferred.

Issues like input capacitance and on-resistance can cause insertion loss (loss of power due to insertion in a transmission line) and harmonic distortion. Such issues are quite typical in electronic circuits, however, and analog switches are not immune.

Applications for analog switches

Analog switches are most often used in switching audio and video (A/V) signals, A/V data transmission, telecom routing, and interface isolation/protection.

Selecting an analog switch

When selecting an analog switch, you will want to know a few things to narrow down your selection per the specifications:

• What is that supply voltage range that is available to power the analog switch IC? (R_ON decreases with increasing supply voltage.[i])
• What are the control signal levels for switching the analog switch ON and off? In many applications, a digital output from another area controls the switch. The voltage levels of the switch (for on/off operation) need to be compatible with the switch control (or input select).
• What is the maximum distortion that can be tolerated in the signal? Total Harmonic Distortion (THD) is a measurement of the switch’s linearity. R_ON is the largest contributor to THD.
• What are the maximum and minimum amplitudes of the signal that will pass through the analog switch?
• If you have a single supply (voltage) system, then choose a single supply analog switch, if at all possible. (It will have only one positive voltage supply pin, not positive and negative supply voltage pins.)
• Turn-on (t_ON) and turn-off (t_OFF) times specify how long it takes to actuate. These times should be in the low milliseconds or less if you are to avoid audible clicks when switching audio signals. The magnitudes of the switching times, t_ON and t_OFF, are also important with respect to each other.
  • Break-Before-Make (BBM): If t_ON > t_OFF, then the analog switch will produce a break-before-make switching action. That is, the first set of contacts will break (open) before the new contacts are engaged (closed), preventing a momentary connection of the old and new signal paths. This could be critical in some applications, especially where it’s necessary to avoid even a momentary short circuit. BBM ensures that two paths do not get electrically connected when the select input changes the signal’s pass-through path.
  • Make-Before-Break (MBB): MBB (when t_ON < t_OFF) describes a condition to avoid opening both switches at the same time. Make-before-break ensures that there’s not an open circuit; signal paths are not open when the select input changes state.
• A lower ON-State Resistance (R_ON) is more desirable, as R_ON contributes to signal degradation.
• OFF Isolation is a measure of the switch’s OFF-state impedance.
• Feedthrough refers to the switch’s ability to block signals when it’s in the OFF state. At high frequencies, parasitic capacitance can cause signal coupling through the switch, such that it appears to be ON.

Several semiconductor manufacturers offer a variety of analog switches and have selection guides that can help narrow down to the best choice. Major manufacturers who market analog switches are Maxim Integrated, Analog Devices, Microchip, Texas Instruments, IDT, ON Semiconductor, Vishay, ST Microelectronics, Intersil, NXP, ROHM Semiconductor, and others.