登录查看更多内容

How do you implement differential signaling in mixed-signal PCB design to isolate analog and digital signals?

由人工智能和领英社区提供技术支持

Differential signaling is a technique that uses two complementary signals to transmit data or power over a PCB. It can improve the performance and reliability of mixed-signal circuits by reducing noise, interference, and distortion. In this article, you will learn how to implement differential signaling in mixed-signal PCB design to isolate analog and digital signals.

此文章中的业界达人

由社区从 9 条内容中精选。了解更多

Zachariah Peterson

Owner, NWES | PCB Design for RF, Mil-Aero, Data Center, AI/ML | That guy in the Altium videos

1 Why use differential signaling?

Differential signaling offers several advantages over single-ended signaling, which uses one signal referenced to a common ground. Differential signaling can cancel out common-mode noise, which is noise that affects both signals equally, by subtracting the two signals at the receiver. It can also reject electromagnetic interference (EMI), which is external noise that couples into the signal traces, by using twisted pairs, differential drivers, and receivers. Differential signaling can also achieve higher data rates and lower voltage levels than single-ended signaling, which reduces power consumption and heat generation.

添加您的观点

Zachariah Peterson

Owner, NWES | PCB Design for RF, Mil-Aero, Data Center, AI/ML | That guy in the Altium videos
举报内容
Differential pairs do not "reject EMI", this is a myth. If this were true then differenital pairs would be immune to differential noise and differential crosstalk. Differential pairs only provide total EMI rejection when the two traces exist right on top of each other. This is exactly why twisted pairs provide high EMI rejection.

已翻译

赞

加载更多内容

2 How to design differential pairs?

A differential pair is a pair of traces that carry the differential signals. To ensure proper signal integrity, you need to design the differential pairs with some guidelines in mind. First, you need to match the length and impedance of the two traces, so that they have the same propagation delay and voltage drop. Second, you need to maintain a constant spacing between the two traces, so that they have a consistent coupling and differential impedance. Third, you need to avoid sharp bends, vias, and stubs, which can introduce reflections, crosstalk, and impedance discontinuities.

添加您的观点

Zachariah Peterson

Owner, NWES | PCB Design for RF, Mil-Aero, Data Center, AI/ML | That guy in the Altium videos
举报内容
You do not need to match the length of the two traces in the pair, you need to match the time required for the signal on each trace to travel to the receiver. Technically you do not need to maintain constant spacing as long as mode conversion stays below acceptable limits up to the interface's Nyquist frequency. Vias are just fine on differential pairs, they only start to matter above 3-5 GHz, something which I have shown in multiple simulations and others have confirmed in simulation and measurement.

已翻译

赞

加载更多内容

3 How to route differential pairs?

Routing differential pairs can be challenging, especially in mixed-signal PCBs, where you need to isolate analog and digital signals. You can use some strategies to simplify the routing process and avoid signal degradation. One strategy is to use a dedicated layer for differential pairs, and avoid crossing other signal layers or planes. Another strategy is to use a ground plane as a reference layer for differential pairs, and keep it as continuous and close as possible. A third strategy is to use shielding traces or guard traces around differential pairs, which can provide additional isolation and protection from EMI.

添加您的观点

Zachariah Peterson

Owner, NWES | PCB Design for RF, Mil-Aero, Data Center, AI/ML | That guy in the Altium videos
(已编辑)
举报内容
Do not use guard traces around differential pairs. If you just use a GND reference on the PCB then you don't need a guard trace if you don't know how to terminate them. Guard traces can increase crosstalk depending on how the guard trace is terminated and depending on whether we are dealing with differential striplines vs. differential microstrips. This has been illustrated by Simonovich and Bogatin in an older Signal Integrity Journal article. Eric Bogatin also showed similar results with copper pour between single-ended traces. Guard traces are also tough to route in your PCB design software, so given the time required and the risk, it is much better to just use a ground plane and you will get the desired benefits.

已翻译

赞
Pavel Kolinko

Research and Development (R&D) - RF System Design Integration and Validation - Complete RF Product Design Cycle
举报内容
Agree with not using guard traces around diff pairs. The guard traces will often require extensive vias to the ground plane below, which is a pain to route.

已翻译

赞

加载更多内容

4 How to test differential pairs?

Testing differential pairs is important to verify their performance and functionality. You can use some tools and methods to measure and analyze the differential signals. One tool is a differential probe, which is a device that connects to an oscilloscope and allows you to measure the differential voltage and current of a pair of signals. Another tool is a network analyzer, which is a device that measures the frequency response and impedance of a circuit. A third tool is a time-domain reflectometer (TDR), which is a device that sends a pulse along a trace and detects any reflections caused by impedance mismatches.

添加您的观点

加载更多内容

5 How to optimize differential pairs?

Optimizing differential pairs can improve the quality and efficiency of your mixed-signal PCB design. You can use some techniques to enhance the performance and reliability of your differential signals. One technique is to use termination resistors, which are resistors that connect the differential pair to a voltage source or ground. Termination resistors can reduce reflections, ringing, and overshoot by matching the impedance of the source, load, and trace. Another technique is to use common-mode chokes, which are inductors that filter out common-mode noise and EMI by creating a high impedance for common-mode signals and a low impedance for differential signals. A third technique is to use differential amplifiers, which are amplifiers that amplify the difference between two signals and reject the common-mode signal.

添加您的观点

Zachariah Peterson

Owner, NWES | PCB Design for RF, Mil-Aero, Data Center, AI/ML | That guy in the Altium videos
(已编辑)
举报内容
This is another example of overgeneralization by AI. Terminating components may not be required to be placed as discrete components in differential interfaces on modern components. The reason is that the termination circuit is often applied on-die, it already exists in the component that has the interface. Ethernet is an example of an exception as it uses a magnetics termination circuit, but Magjack connectors make this a non-issue. But many serdes components and standardized interfaces will have on-die termination. In some components this may be required as an external discrete, but you would have to look at the application example in the datasheet, if it is needed as an external component then it should be obvious from the datasheet.

已翻译

赞

6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?

添加您的观点

PCB Design

+ 关注

给文章评分

我们借助人工智能创建了此文章。您认为这篇文章怎么样？

很棒不太好

举报此文章

查看全部

How do you implement differential signaling in mixed-signal PCB design to isolate analog and digital signals?

1

2

3

4

5

6

1 Why use differential signaling?

2 How to design differential pairs?

3 How to route differential pairs?

4 How to test differential pairs?

5 How to optimize differential pairs?

6 Here’s what else to consider

PCB Design

给文章评分

感谢您的反馈

更多PCB Design相关文章

更多相关阅读内容