When the net current in differential pairs equals zero, the signal traveling through one trace returns through the other trace, which serves as the differential signal return path.
In high-frequency systems using differential pairs, the unbalanced currents or ground currents take the path with the least impedance.
Placing the ground plane underneath the differential pairs is one technique to reduce the loop area and impedance.
In high-speed circuits and communication schemes, such as HDMI and USB, differential signals are utilized for transmitting information. The differential signaling system transmits signals using two wires, traces, or cables. In PCBs, each trace of a differential pair carries one signal and the signals are complementary to each other. The load or receiver circuit considers the electrical difference between the two complementary signals for further processing. Usually, the differential signals, which are equal and opposite, do not require a ground as the return path. When the net current in differential pairs equals zero, the signal traveling through one trace returns through the other trace, which serves as the differential signal return path.
In high-frequency circuits or high-speed digital systems, a differential signaling scheme is employed. As we discussed, this scheme uses a pair of lines that are identical and uniform. The distance between the two lines or traces remains the same throughout the length of the differential pairs.
An Example Using Differential Pairs
Let’s consider a differential pair where the traces are trace 1 and trace 2, respectively. The differential pair is carrying signals which are equal and opposite. The current flowing through trace 1 will induce some interferences into the trace 2 signal and vice versa. If the currents are equal and opposite, the interferences induced will also be equal and opposite. When the difference between the trace 1 signal and the trace 2 signal is taken at the receiver end, the interferences induced cancel each other. Therefore, differential signaling is the best signaling scheme for offsetting electromagnetic interference, common-mode noise, and induced noise in order to maintain signal integrity in circuits.
Differential Signal Return Paths in Balanced and Unbalanced Differential Pairs
When differential pair signals are equal and opposite, the net current flowing through the traces is equal to zero and the differential pair is considered balanced. There is no requirement for a ground return path in a balanced differential pair. The current flowing through trace 1 will return back to the source through trace 2. Here, trace 2 serves as a differential signal return path when the differential pair is carrying balanced currents. However, in certain circuits, the net current in differential pairs can be something other than zero, and such differential pairs are called unbalanced differential pairs. In unbalanced differential pairs, a ground reference plane is needed to serve as the differential signal return path.
The Two Return Paths
We can summarize that, theoretically, in differential pairs, there are two return paths: the adjacent trace (trace 2) running parallel to trace 1 and the ground plane. In high-frequency systems using differential pairs, unbalanced currents, or ground currents, take the path with the least impedance. To be precise, the current flows through the least inductance path. To keep the inductance small, the loop area should be kept as small as possible. Placing the ground plane underneath the differential pairs is the best technique to reduce the loop area and the impedance.
The Geometry of Differential Pairs and Discontinuities in Reference Planes
The geometry of differential traces plays an important role in bringing the majority of the return current onto the ground reference plane. The two identical, parallel uniform wires, along with a reference plane placed underneath the traces, form a unit for differential signals. For example, consider a differential pair in a pure microstrip configuration, where the ground current takes the least impedance path. Differential microstrip trace geometry with a solid reference plane brings the entire return current into the reference plane running under the traces. Discontinuities in the reference plane can deviate the path of the differential signal return current. The split planes and layer transitions with different references are some of the common discontinuities encountered by differential signaling schemes.
The differential signal return path with and without the ground reference plane is a sensitive part of high-frequency circuit design. Cadence offers PCB design tools that can accentuate the design of differential traces and their reference planes.