Matrix Switches vs. Multiplexers in Automated Test Systems
Filed under Switches
Designing an automated test system (ATE) requires choosing the optimal signal routing topology to balance flexibility and signal integrity. This guide compares matrix switches, multiplexers, and general-purpose actuator switches, outlining how each path layout affects test fixture performance. While multiplexers connect multiple channels to a single instrument sequentially, matrix switches allow simultaneous connections between multiple instruments and multiple test points. However, this high degree of routing flexibility introduces design tradeoffs in signal degradation, bandwidth limits, and crosstalk. This article reviews these topologic differences, referencing parameters from common switch modules like the Keysight Technologies (Agilent HP) 34904A, Keysight Technologies (Agilent HP) 44473A, and Keysight Technologies (Agilent HP) 34903A, configured within mainframes like the Keysight Technologies (Agilent HP) 3488A.
- Matrix Topologies
- 4x4 (16 crosspoints) or 4x8 (32 crosspoints)
- Actuator Channels
- Up to 20 independent SPDT relays
- Typical Matrix AC Bandwidth
- 10 MHz (into 50 Ohms)
- Typical Matrix Crosstalk
- < -75 dB at 100 kHz
Routing Topologies: Flexibility vs. Simplicity
The fundamental difference between routing topologies lies in their connection paths. A multiplexer acts as a selector switch, routing multiple input channels to a single shared bus or instrument. This structure is highly efficient for sequential testing, such as scanning multiple voltage points with a digital multimeter.
In contrast, a matrix switch allows any row to connect to any column, providing any-to-any connection paths. For example, the Keysight Technologies (Agilent HP) 44473A features a 4x4 two-wire matrix with 16 crosspoints, while the Keysight Technologies (Agilent HP) 34904A provides a 4x8 two-wire matrix with 32 crosspoints. This matrix topology allows multiple instruments—such as function generators, power supplies, and oscilloscopes—to connect to different nodes on a Device Under Test (DUT) simultaneously.
For simple on/off or routing applications that do not require a shared bus, actuator switches are used. The Keysight Technologies (Agilent HP) 34903A is a 20-channel general-purpose switch featuring independent SPDT (Form C) latching relays, providing isolated, point-to-point routing.
Signal Degradation and Bandwidth Limits
While matrix switches offer high routing flexibility, the physical layout of rows and columns introduces parasitic capacitance and longer stub lines. Each unused column or row in a matrix acts as an open stub, which can reflect high-frequency signals and degrade the overall bandwidth.
Both the Keysight Technologies (Agilent HP) 34904A matrix switch and the Keysight Technologies (Agilent HP) 34903A actuator switch offer a typical -3 dB AC bandwidth of 10 MHz into a 50 ̢ load. However, as matrices are expanded by chaining multiple modules together—such as installing multiple modules in a Keysight Technologies (Agilent HP) 3488A mainframe—the cumulative capacitance increases, further degrading high-frequency signal performance compared to simpler, dedicated point-to-point switches.
Crosstalk and Isolation Performance
Crosstalk occurs when signals from one active channel couple into an adjacent channel. In matrix configurations, row and column traces run parallel and cross over each other, making them more susceptible to capacitive and inductive coupling than isolated channels. For instance, the Keysight Technologies (Agilent HP) 34904A has a typical crosstalk specification of less than -75 dB at 100 kHz.
To minimize signal degradation, test engineers must monitor system contact resistance. Relay contacts degrade over time due to switching cycles and electrical loads. The Keysight Technologies (Agilent HP) 34904A has an initial contact resistance of less than 1.0 ̢, which can rise to less than 3.0 ̢ at its end-of-life. Similarly, the Keysight Technologies (Agilent HP) 44473A starts with closed-channel resistance below 1.0 ̢, increasing to less than 2.0 ̢ at its end-of-life. General-purpose actuator switches, such as the Keysight Technologies (Agilent HP) 34903A, avoid the complex path lengths of a matrix, making them better suited for switching higher power loads up to 1 A / 50 W without degrading delicate measurement lines.
Example instruments
Frequently asked questions
- When should a matrix switch be chosen instead of a multiplexer?
- A matrix switch should be chosen when a test system requires connecting multiple instruments to multiple points on a DUT simultaneously. If the application only requires scanning multiple signals one-by-one to a single measurement device, a multiplexer is preferred due to its lower cost, lower path resistance, and superior signal isolation.
- What is the function of an actuator switch module?
- An actuator switch module, such as the Keysight Technologies (Agilent HP) 34903A, consists of independent, uncommitted relays (such as SPDT Form C). These are designed to turn on power to a DUT, actuate external devices, or control isolated signal paths without sharing a common internal backplane or matrix structure.
- How does matrix expansion affect test system signal integrity?
- Expanding a matrix by connecting multiple modules increases the overall capacitance and path lengths of the rows and columns. This expansion increases crosstalk, rises contact resistance, and lowers the usable AC bandwidth of the test system.