[FAQ] What is the Difference between an Analog Signal Chain Multiplexer and a Relay?

Expert 3680 points

Replies: 0

Views: 56

At the surface a relay and an analog signal multiplexer seem like they are interchangeable terms as both are devices that can switch a signal by the use of a control signal on  a different terminal; however due to architecture differences these parts cannot be used interchangeably in all  applications.

Relays can be generalized into two large groups, ones with moving parts and ones without moving parts. They are also categorized using the following notation:

x Form {} - where x is the number of channels and forms refer to the configuration common ones include   A = NO 1:1 switch ; B = NC 1:1 switch ; C = 2:1 switch. 

Electromechanical Relays use a low power control signal to physically induce a metal switch to move either breaking or making a connection for a higher power circuit. The control signal is electrically isolated from the signal inputs of the relay. They have the benefit of having extremely low resistance due to the fact that it essentially just a metal wire, but the downside is that due to the mechanical nature of the switch its lifetime is shorter than compared to switches without moving parts.

Solid State Relays use the same principle that a electromechanical relay uses, except there is are no moving parts. The most common architecture is the use of a MOSFET based switch that is activated when the control signal activates an optocoupler. This provides electronic isolation as the signal is transferred via light to activate the switch. These switches have the benefit of longer lifetimes, but have higher resistances due to the material properties of semiconductors. 

Relays are typically used to switch higher current loads, where a typical rating will get you in the 1 - 2 A range.

Analog Signal Multiplexers have a few differences. The first is that they are not limited to the 3 common configurations seen with relays; common multiplexer configurations can range from 1:1 configurations up to 16:1 configurations. Second, most signal multiplexers directly apply a voltage to a CMOS inverter when switching occurs instead of using a more isolated method such as a using a coil+solenoid / opto-coupler. Finally the use cases typically vary based on what is being is being switched; signal switches and multiplexers are used for lower current applications maxing out around 500mA, with most being less than or equal to 64mA while relays have applications in single digit A range.

In conclusion when looking at Analog Signal Multiplexer IC's versus a relay they can not be used interchangeably in every application. The system designer must take into effect what is being switched, what is the current flowing through the switch, do they need more isolation between the control signal and I/O signal, and what configurations are needed. When keeping these ideas in mind the designer can adequately choose the right type of switch for their system.