Selecting and Using a Switch IC for Analog and Digital Applications

Many electronics accept user input with manual switching, but this is not always the best way to trigger a logical condition in every system. What if we have an embedded system that interfaces with some web API and there is no mechanical switch? What if the board is buried in a larger system and will never be manually accessible?

A switch IC is an excellent option for activating peripherals based on logical conditions or user input that does not necessarily involve some mechanical component. A switch IC is activated electrically, sometimes with an MCU or an analog signal level. When you need to implement this type of switching in your system, here are some guidelines you can follow for selecting and using switch ICs.

Important Switch IC Specifications

A switch IC is a simple integrated circuit that is the electrical analog of a mechanical switch. These components provide a convenient switching mechanism based on user input, logical conditions, or even a sensor level. Switching is triggered with a low-speed digital interface or DC applied to the enable pin on the component (e.g., as supplied with a GPIO). Switch ICs can provide the same electrical function as a typical mechanical switch (SPST, SPDT, etc.), making them easy to implement in an electronics system.

Pay attention to these specifications when you’re selecting a switch IC for your system:

• Poles, throws, and channels. Switch ICs are typical SPST, SPDT, or DPDT types. Multiple switches (i.e., channels) can appear in the same package.

• Analog vs. digital switch. There are two types of switch that are available, and they are not always interchangeable. These two types of switches will be discussed below. 

• Bandwidth and resistance/impedance. This only becomes important at high frequencies, where impedance matching may be needed to ensure there is no signal reflection at the switch. The die and switching circuit structure will also limit the bandwidth of the switch. For high frequencies, the resistance should be chosen so that the input impedance looking into the switch matches the line impedance. Note that there are specialty RF switches available for these applications. 

• Data rate (for digital switches). Digital switches have some rise time that determines the bandwidth of the switch. This will limit the data rate that can be supplied when the device is switched on. For applications where switching between different high-speed digital interfaces is needed, such as in a backplane, pay attention to this specification. 

• Single vs. dual supply system. If you have a single supply voltage in your layout, then you should use a single supply switch if possible as it makes the layout much more convenient. Note that some analog switches require connecting a negative supply pin, which will limit the negative polarity output from the component. 

When working with high-speed digital data, a specialized crosspoint switch is used to switch data. These components are built using high-speed logic and are intended to support specific protocols, modulation types (e.g., NRZ), or signaling standards.

Analog Switch ICs

Switch ICs come in digital and analog varieties. Both types of switches have their advantages, but they function differently at the die level due to how the output is enabled in the IC. You can use an analog IC with digital signals in some situations, but the converse is not true.

An analog switch IC can conduct digital or analog signals when turned on. In essence, they act like a relay, where the input control trigger signal drives the analog switch into a high conductivity state. Analog switches are also bidirectional, as one would expect in order for the switch to replicate the analog signal on the output. However, the output can saturate, either when the load is too small or when the input voltage level exceeds the supply voltage. The same idea applies to digital signals, although with digital signals we only need to ensure fanout is not disturbed when the switch is used on a bus.

One example of an analog SPDT switch is the NLAS4157 from ON Semiconductor. This device has low on-state resistance of ~0.8 Ohm, making it an excellent choice for DC or low-frequency analog applications. It also supports reasonably high continuous current through each output of up to 300 mA. Each output is also highly isolated with crosstalk rated at -57 dB (typical value) at 1 MHz for a 50 Ohm load. Total harmonic distortion is rated at 0.012% for 0.5 Vp-p, and the -3 dB bandwidth is rated at 8 MHz, making this component an excellent choice for audio applications.

Digital Switch ICs

Digital switches cannot be used with analog signals. When used with a digital signal, a digital switch IC attempts to replicate the logic level of the input signal. Obviously, the simplest implementation of an SPST switch is with an AND gate, where the logic families of the driver and switch IC are the same. Fanout is also an important point here when using a single driver with multiple switches, although there are bus switch ICs available with multiple outputs and independent enable triggers.

An example of a 2-bit digital bus switch IC is the SN74CBTD3306 from Texas Instruments. This dual FET switch IC includes 2 independent output enable pins and 2 independent inputs, allowing the switch to be wired up as two SPST switches or a flip-flop style configuration. One useful application of this component is in level shifting 5 V signals (TTL) down to 3.3 V thanks to an internal diode on the VCC pin. Although not designed for high-speed data transmission, it does provide fast switching at ~5 ns enable and disable times with only 250 ps propagation delay.

When you need to place a switch in your new system, there are many options on the market, but you’ll likely be using other components to generate and accept signals. Some other components that are commonly found in systems with switch ICs are:

• HMI for accepting user input

• Specialty amplifiers

• ADCs for mixed signal capabilities

If you’re looking for a switch IC or any other components for your new design, use the advanced search and filtration features in Octopart to find the parts you need. The electronics search engine features in Octopart give you access to updated distributor pricing data, parts inventory, parts specifications, and CAD data, and it’s all freely accessible in a user-friendly interface. Take a look at our integrated circuits page to find the components you need.

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