1. Single-ended Signal vs Differential Signal
1.1 Single-ended and Differential Signals
Single-ended signals use one signal conductor referenced to a ground plane. In other words, a single-ended signal is the potential difference between a single conductor and the ground plane. This requires that the reference ground potentials at source and receiver be essentially the same.
Differential signals use two conductors: one carries the positive polarity and the other the negative polarity. The receiver determines the signal by comparing the difference between the positive and negative signals. This allows correct reception even if the source and receiver reference ground potentials differ.
1.2 Transmission Differences
Single-ended signals are referenced to the ground plane. When DC flows through the reference ground plane, there is almost no potential difference between the source and receiver. For AC currents, large currents, or especially high-frequency signals, parasitic inductance in the reference ground plane can create a potential difference between source and receiver. The magnitude of this potential difference depends on signal frequency, edge rates, operating current, and the parasitic inductance of the reference ground plane.
Although both lines of a differential pair are referenced to the ground plane, when the ground plane moves both lines move together (ideally), so the differential voltage between them remains nearly constant. Since the receiver senses the difference between the two lines, differential transmission is much less sensitive to the quality of the reference ground than single-ended transmission.
When a signal traverses a magnetic field, an induced electromotive force appears on the conductor. For single-ended signals this induced voltage is added directly to the signal. For differential pairs, induced voltages on both conductors are equal, so their difference is zero and the useful differential signal is unaffected. This is why differential signaling generally has better noise immunity than single-ended signaling.
2. Advantages and Disadvantages of Differential Signaling
2.1 Advantages
One advantage of differential signaling is that the receiver uses the difference between the two signals for detection, so precise reference ground potential is not critical. A second advantage is stronger interference immunity and lower EMI emission. A third advantage is the ability to handle bipolar signals accurately in single-supply systems.
2.2 Disadvantages
Differential signaling requires the two lines to have equal amplitude, 180 degree phase difference, opposite polarity, and matched lengths. Because the receiver compares two signals, phase and delay become very important; this is not an issue for single-ended signals.
3. Differential Pair Design Rules
3.1 Tight Coupling
When the pair is tightly coupled, currents in the two lines are equal in magnitude and opposite in polarity. Their generated magnetic fields are equal and opposite and cancel each other. Another benefit of tight coupling is that external interference appears as equal common-mode noise on both lines. Since the receiver is sensitive only to the differential signal and rejects common-mode noise, the receiver suppresses common-mode noise.
3.2 Matched Lengths and Spacing
Differential pairs should maintain equal electrical length and constant spacing between the two traces along their entire length. Variations in spacing cause unbalanced magnetic coupling and reduce the effectiveness of field cancellation. In addition to increased EMI, changes in spacing can alter the characteristic impedance, causing impedance discontinuities and reflections that degrade signal integrity.
Equal electrical length ensures signals arrive at the receiver at the same time. For matched-length differential pairs, the two signals are equal and opposite, so their sum is ideally zero. If the electrical lengths differ, the shorter trace will change state earlier than the longer one. In severe cases, there can be points where both traces are driven with currents in the same direction. The summed signal during transitions moves away from zero, and under high-frequency conditions this can cause return currents to flow through the reference plane back to the source, forming a loop antenna that radiates.
3.3 Controlled Impedance
Differential impedance is determined by the physical geometry of the trace pair, their relationship to nearby reference layers, and the PCB dielectric. These geometries must remain consistent along the whole trace length. Discontinuities occur when the differential impedance deviates from its nominal value (for example 100 ± 15%). Discontinuities can cause reflections from impedance mismatch and thereby degrade signal integrity.
3.4 Return Path Integrity
For high-frequency circuits, providing a relatively continuous reference plane on adjacent layers gives the return current a low-impedance path. This minimizes the loop area between the signal current and the return current, causing the magnetic fields to cancel and reducing EMI. Layer transitions or splits can cause uncontrolled return path area; then the magnetic fields from the signal and the return current cannot effectively cancel and EMI increases.
Because differential pairs have different differential-mode loop shapes, induced noise levels can differ and common-mode noise may not be fully cancelled at the receiver, degrading performance.
4. In-depth Analysis of Differential Signal Return Paths
4.1 Return Path Analysis
Common misconception about differential return paths:
Many electronic engineers believe that differential pairs are immune to interference and have low radiated emission mainly because the positive signal travels along one trace to the load and returns on the negative trace back to the source. They assume differential currents circulate in a closed loop confined to the pair, canceling magnetic fields so no return currents flow through the reference plane.
Actual idealized return path:
In reality, the return paths for differential signals are similar to single-ended signals. For example, D+ current flows from the source along the transmission line to the load and returns to the source via the reference plane; D- current flows from the load to the source along its transmission line and returns to the load via the reference plane.
In the ideal case, because differential currents are equal in magnitude and opposite in polarity, their magnetic fields cancel; return currents are also equal and opposite and cancel as well, resulting in very low radiated emission.
Under high-frequency conditions, if the reference ground potentials at source and receiver are not equal, the differential pair may rise or fall together relative to the reference plane, while their differential remains unchanged, so reception is unaffected. Similarly, when a differential pair traverses a magnetic field, equal induced voltages on both lines keep the differential unchanged. These factors contribute to the stronger interference immunity of differential signaling.
4.2 Two Return Path Design Approaches
Multi-layer board design:
On multi-layer boards, differential pairs typically place the pair adjacent to a continuous ground or power plane used as the return path, minimizing loop area so the signal and return currents generate magnetic fields that cancel, yielding minimal radiated emission.
Two-layer board design:
On two-layer boards it is hard to reserve a whole layer solely for a reference plane due to routing density. A common approach is to route the differential pair on one layer and run grounded traces or pours on either side to serve as return paths. Signal and return currents largely cancel, keeping radiated emission low.
Top-layer routing with bottom reference plane:
When routing entirely on the top layer with a bottom reference plane, it appears similar to a multi-layer approach but has key differences. The design requires the differential pair to be routed entirely on the top layer while the corresponding bottom layer beneath the top traces maintains a relatively continuous reference plane to provide a low-impedance return path.
Design challenges:
- It is possible on a two-layer board to keep a differential pair on a single layer, but keeping the corresponding bottom-layer reference plane intact is difficult, especially while preserving cost effectiveness and generality.
- For a two-layer design that relies on the bottom layer as the return path, the distance from the top-layer pair to the side grounded traces must be less than the board thickness between top and bottom. Otherwise the return path may prefer the side grounded traces instead of the bottom plane.
- Even if the bottom reference plane under the pair is preserved, the bottom plane must have low-impedance connections to the source and receiver reference grounds. This is particularly challenging for BGA components.
Cross-routing on top and bottom layers with side ground traces as the reference:
To balance cost and performance, two-layer boards often use cross-routing across top and bottom layers with grounded traces or pours on both sides serving as the reference plane. This approach often leads to reference plane transitions and sometimes simultaneous transitions of the reference plane and the differential pair. This design requires careful control of reference plane integrity, the equipotential relationship between side grounded traces and the bottom reference plane, and handling of plane splits.
Design considerations:
- Keep the side grounded traces continuous from source to receiver so that the return path magnetic fields largely cancel and radiation is minimized.
- When the differential pair transitions layers, the side grounded traces should also transition so that the signal return path transitions synchronously, minimizing radiation.
- Even if the differential pair itself does not change layers, side ground traces may need to change layers due to layout constraints or chip PIN routing; layer transitions for ground traces should use the smallest-area paths back to the main chip.
- Side grounded traces should be connected to the system reference plane via ground vias to form an equipotential body. The placement and number of ground vias require special attention.
- Length differences between the differential pair and between the side grounded traces are important factors for magnetic field cancellation. Keep these length differences as small as possible.
