Low-speed digital interfaces, some moderately high speed components, and RF signal lines will use single-ended signals.
In contrast, high speed digital interfaces, such as networking protocols and computer peripheral interfaces, use differential signaling.
Differential signals are length-matched and impedance controlled to ensure common-mode noise cancellation at a receiver.
A complex board like this may use differential signaling to send data between components
In days past, if you used a telephone that plugged into the wall, you could hear conversations from others bleed into your phone line. Today, we combat this and other signal integrity problems in high speed PCB design using differential pairs. It’s not as if all components will use differential signaling, but differential pairs are the primary routing style used with high speed digital signaling protocols, including standard computing interfaces like USB and networking interfaces like Ethernet.
The differences between single-ended and differential signals are simple at the physical layout level, but they can be complex at the signal level and in terms of driver/receiver component functions. If you’re making the first jump into using differential signaling, read our guide to learn more about this signaling method. Once you’ve mastered differential pair routing, you’ll have the foundational knowledge needed to design with some of the highest speed computing interfaces, such as PCIe, SerDes channels, gigabit Ethernet, and much more.
What Are Single-Ended and Differential Signals?
Single-ended and differential signaling are two methods for transferring data between components. Today, digital interfaces are standardized to use one of these methods of signaling: low-speed protocols use single-ended signaling, while high-speed protocols use differential signaling, although some low-speed protocols still use differential signaling. These two types of signaling and routing can be used with various topologies.
Single-ended signals are simple to understand: the HIGH level is brought up to a logic level (5 V, 3.3 V, etc.), and the LOW level is defined as zero. Differential pairs are different; each side of a differential pair carries the same magnitude signal, but with opposite polarity. At the receiver, the signal is recovered by taking the difference between the signal levels on each line.
Differential signal recovery at a receiver component
Successful readout and signal recovery requires the lengths of the pairs to be precisely matched within some small tolerance. The advantage here is that common-mode noise will be canceled when reading the signal, as shown in the conceptual diagram above. This requires following some basic routing requirements for all differential signals.
Single-Ended and Differential Pair Routing
The general requirements for differential pair routing and layout are shown in the table below. Single-ended signaling requirements are also shown for comparison.
The signal readout and recovery process for differential pairs accounts for the fact that the two pairs carry equal and opposite polarity signals. This simple idea and parallel routing in a differential pair solves some important signal integrity problems in high speed PCB design.
What Differential Signals Can Solve
If you look through high speed signaling standards, you’ll see that differential pair routing is predominantly used. Differential pairs are useful for two major reasons:
Common-mode noise. A differential pair is read out as a difference in signal level between the two pairs at a receiver. In other words, any common-mode noise will be subtracted at the receiver and will not interfere with the received signal. This includes common-mode crosstalk that might be received from a single-ended signal. We’ll explain this in more detail below.
Lack of consistent reference. Differential pairs don’t need a uniform ground plane to provide controlled differential impedance. Instead, the pairs reference each other. Interestingly, if you calculate the Z-parameters for a differential pair, you’ll find that the self-impedance and coupling impedance both diverge to infinity, but the difference between these is a constant.
The image below shows a driver and receiver stage used in low-voltage differential signaling (LVDS). Here, there is no ground plane surrounding the pair. Because the signals have equal magnitude and opposite polarity, the electric field terminates on each side of the differential pair. This particular diagram is nice because it illustrates the situation seen in a standard like Ethernet over UTP cable (e.g., Cat5), which may not have any grounding in a significant portion of the interconnect.
Example differential channel with LVDS
In this example with LVDS, the receiver end is terminated with 100 Ohm impedance, which is equal to the pair’s differential impedance. This eliminates reflection in the differential signal at the receiver end. Because differential receivers have high input impedance, the terminator is placed in parallel with the inputs, converting the injected current into a voltage that can be recovered at the receiver.
What Differential Signals Don’t Solve
It’s important to note that differential pairs are not a cure-all for every signal integrity problem. A differential pair can experience some of the same signal integrity problems as single-ended traces, but they are manifested in different ways. Here are the broad signal integrity problems experienced by differential pairs.
Differential-mode noise and differential crosstalk. Contrary to popular belief, differential pairs produce crosstalk and are vulnerable to crosstalk. In particular, differential crosstalk can be induced between two differential pairs, which will then interfere with signal recovery at the receiver.
EMI reception. It’s another myth that differential signals are immune to all EMI. This is incorrect. Differential signals can still receive radiated EMI from an external source, but only differential-mode noise will affect the receiver.
EMI emission. Similar to the previous point, differential pairs do emit electromagnetic radiation that can be received as common-mode noise in other interconnects. However, the field polarities from each trace in the pair are opposite, so the emitted radiation is weaker when the pairs are denser.
Signal distortion. As a signal travels along an interconnect, it will experience losses and dispersion, both of which will create signal distortion. Differential signals will suffer from signal distortion, just like single-ended signals.
In both types of signaling, a reference plane near the traces helps solve one problem—it provides shielding against EMI. It also provides a place for some return current around a trace by allowing field lines to terminate into the reference plane. The reference plane also defines the single-ended impedance (characteristic impedance) for each trace in a differential pair, as well as in a single-ended signal. Routing tools can help you enforce length matching in your differential pairs while maintaining controlled impedance as you route your PCB layout.
Once you understand the difference between single-ended and differential signals, you can create the highest quality layout with PCB design and analysis software from Cadence. OrCAD’s advanced routing tools help you follow best practices for layout and routing, and you’ll have everything needed to create high-quality designs in a single application.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.
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