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Controlled Impedance Routing Ensures Signal Integrity in Circuit Boards

Key Takeaways

  • In circuit boards, controlled impedance routing configures the dimension of the routes to match a specified characteristic impedance. 

  • This approach aims to control the impedance of the routes by changing their dimensions and environment so the signal traveling through them remains within range.

  • Controlled impedance matching has two steps: 

  1. Impedance matching of components

  2. Impedance matching of traces

PCB board

PCB boards with controlled impedance routing are assured signal integrity

To achieve signal integrity in circuits, we follow different design best practices such as avoiding right-angle bends, separating clock signals and power signals, and maintaining the shortest distance between components. When end-users are concerned about signals and their quality, they are often not aware of the back-end design modifications made to protect signal integrity. 

Controlled impedance routing in circuit boards configures the dimension of the routes to match a specified characteristic impedance. This approach is used to control the impedance of the routes by changing their dimensions and environment so that the signal traveling through them retains the proper signal integrity characteristics. 

As the speed of a signal increases—even from GHz range with a smaller footprint area—signal integrity is often a major consideration. Controlled impedance routing is a method to assure signal integrity and help preserve the signal strength until it reaches the destination point from the source. 

The Importance of Controlled Impedance Matching

In a circuit, there will be a lot of components other than the source and the load. The energy from the output pins of components flows towards the load through traces, but with distortions and losses. In certain cases, the energy that is not absorbed by the load reflects back to the source side and causes additive or subtractive effects and results in ringing. The signal integrity of the circuit is compromised in all these conditions. 

When the signal integrity is compromised, the circuit may start to behave in unexpected ways.  The reflection of the signals back to the source side can be prevented by matching the impedances of the traces. 

Within circuit boards, power is propagated along the length of the trace. The power transferred is maximum when the source and load impedances are matched. The impedance matching couples the energy from the source to the routing and from routing to load. 

Considering the criteria of impedance matching during the routing of a circuit is called controlled impedance routing. This technique prevents distortions, ringing, and loss of signal in circuit boards. 

Designing Controlled Impedance Routing

Controlled impedance routing in circuit boards is done in two steps:

  1. Impedance matching of components

  2. Impedance matching of traces 

Impedance Matching of Components

The first step of controlled impedance matching is to match the impedances of the components in the circuit. The high impedance of input pins and the low impedance of output pins leads to impedance mismatch. Including termination components between the input and output pins matches the impedance of the components.

Impedance Matching of Traces

In this step, the routing of the circuit traces is done to achieve the required impedance. The characteristic impedance of the traces is dependent on the resistance, inductive reactance, capacitive reactance, and conductance. The geometry of the length, width, and thickness of the trace determines the impedance, and changing the dimension is one method to achieve impedance matching. 

If the board is designed with controlled impedance routing, the impedance remains constant throughout the trace from source to load, even when it passes through different layers. Typical impedance values of the traces range from 25 to 125 Ω.

The PCB material dielectric constant and dielectric thickness also influences the impedance of the traces. The selection of PCB material is a criterion to match the impedance.  For high-speed board designs, the laminate material of low dielectric constant and low loss tangent  is preferable. This enhances the signal performance and minimizes the signal distortion and phase jitters. A PCB fabricated in low loss material improves the signal integrity of high-frequency designs. 

Examples of Controlled Impedances in PCBs

In multilayer circuit boards, the traces are between the layers, so the thickness of the laminate material on either side of the trace is considered for impedance matching. The following are some examples of controlled impedances in multilayer PCBs: 

Embedded Microstrip

In this microstrip, the trace is sandwiched between the planes in such a way that there is a plane on one side and laminate on either side, followed by air. 

Embedded microstrip

Offset Stripline

In this routing, the trace is covered by laminate on both sides.

Offset stripline

Edge Coupled Offset Stripline

Edge coupled offset stripline consists of two controlled impedance traces placed between two planes. 

Edge coupled offset stripline

Apply Controlled Impedance Routing for Improved Signal Integrity

Controlled impedance routing is necessary for handling and maintaining high-frequency signals and data integrity in circuit boards. Boards with controlled impedance routing provide repeatable high-frequency performance. 

High-speed digital circuits in telecommunications, signal processing, RF communications, real-time graphic processing, and process controls are the main applications where controlled impedance routing is implemented. In household applications, controlled impedance circuit boards are found in modems, printers, personal computers, and cameras. 

When you are working on high-speed, high-frequency circuits handling critical signals or data, signal integrity needs to be maintained throughout the operation. Controlled impedance routing is one of the best practices followed by PCB designers to ensure that signals reach the load from the source, without any loss or distortions. When circuit traces are impedance matched, the probability of signal integrity failure is reduced to a minimum. 

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