Routing Different Types of Signals
So far, what we’ve presented about electrical signals and routing between components focuses on a specific type of signal and trace arrangement. In particular, we only looked at an individual trace that gets routed on its own; the only other important element needed to define the voltage carried by a signal is the ground plane.
In this lesson, we’ll explore these different trace routing styles (single-ended vs. differential), as well as the signals that they carry. We’ll also look at some of the important points to watch in your board to ensure signal integrity.
Single-ended Traces vs. Differential Pairs
Before discussing anything with impedance, we need to look at the two main types of signaling in PCB design: single-ended signals and differential signals. These are routed on single-ended traces and differential pairs, respectively.
Differential pairs are often discussed in terms of high-speed design, something which we will look at later in this lesson. Most of the standard computer interfaces (like USB, Ethernet, HDMI, and many others) are routed on a PCB using differential pairs. However, there are other digital signal protocols that are low speed but are still routed as differential pairs. Meanwhile, some buses that are at the very edge of being considered high speed are still routed as single-ended signals.
Single-ended traces are simple: this just refers to a trace routed along a specific layer, ideally above a ground plane. The voltage carried by the signal in this case is measured between the trace and a ground plane. A differential pair uses two traces routed side-by-side to carry a digital or analog signal between two components. The signal is carried as two equal and opposite signals routed on the traces (V+ and V- in the above image), and the voltage carried by the signal is measured between the two traces.
The image below shows an example to illustrate how differential pairs and differential signaling works. Suppose our digital signal carries V1 = 350 mV. In a differential pair, there is an equal and opposite copy of the signal on the 2nd trace, which has voltage V2 = -350 mV. When the differential signal is collected by the receiving component, it is measured as 350 mV - (-350 mV) = 700 mV.
Differential pairs can be microstrips, as shown above, or they can be routed on an internal layer as striplines. When planning out routing for either type of signal, you’ll need to decide whether you’ll route on the surface as microstrips, or an internal signal layer as striplines, for both types of signals. We’ll look more at some design rules for differential pairs later; for now we want to look more about incorporating these into our routing strategy.