Do You Need A Differential Pair Reference Plane

December 20, 2019

I have no idea why ads of VPNs are popping all over my Facebook feeds. I’m undecided on whether to have one installed on my device, as it is claimed that having a VPN prevents government agencies and hackers from spying on me.

Do I need a VPN after all, as I’m not working on anything sensitive in nature? While I’m in a dilemma on beefing up internet security, some PCB designers may be equally undecided on the use of a differential pair reference plane.

Is a differential pair reference plane mandatory, or is it’s left out of the PCB? Let’s clear some doubts on this often-debated topic.

What Is A Differential Pair Reference Plane

A differential pair reference plane refers to the ground copper polygon that is placed in adjacent layers to the differential pair signal. Theoretically, the reference plane is used as a ground return path for the signal and to minimize electromagnetic interference(EMI)

The reference plane shares the same ground connection as the transceiver chip that the differential pair traces are connected to. It is usually applied in designs involving differential signals like USB, SATA, HDMI, and PCIe.

Do You Need A Reference Plane For Differential Pairs?

There have always been questions raised on the need for a reference plane for differential signals. To understand the issue, let’s get back to the definition of a differential signal. Differential signal functions by transmitting data over two copper traces with opposite signal polarity.

For example, a logic ‘1’ is represented by +3.3V in the positive trace and -3.3V on the negative trace. The advantage of such signaling is that data integrity is unaffected by common-mode noise. The receiver only needs to compare the difference between both signals to determine the value of the data packet.

Theoretically, both signal traces of the signal pair cancel out each other, thus resulting in no return current. Does this mean that a reference plane is not necessary since there is no current flowing through the ground?

In an ideal case, you wouldn’t need a reference plane. However, you would have realized that the world isn’t ideal, and neither is electronics design.

A reference plane is mandatory for high-speed signals like USB

Unless both traces on the PCB are perfectly identical and similar in length, there will be a slight difference in the amplitude of the signals. This will result in a slight amount of current flowing through the ground. Regardless of whether you have a reference plane or not, the current will flow to complete a loop and through the path of least impedance. Without a reference plane specifically for the differential signal, you’ll have an increased risk of EMI from the return current.

You may get away for not having a ground plane for low-speed differential signals such as RS485. But when you’re dealing with high-speed differential signals like USB and PCIe, a reference plane becomes mandatory, but for a totally different reason.

When signal transmission happens at a high rate, electrical noise will be coupled into adjacent traces or planes. A ground plane placed underneath the differential pair is handy to mitigate the effect of EMI from the high-speed transmission. The ground plane should be limited to the area of the differential pair, which should be placed near to each other to limit the current loop area.

Differential Pair Reference Plane Guidelines

It isn’t worth the risk of omitting the reference plane when you’re routing differential pair signals. With that said, you’ll want to get the reference plane design right or introduce more issues into the circuit.

Proper placement of reference plane helps minimize EMI.

The reference plane should be a continuous copper plane that goes below the differential signal and connected to the same digital ground of the transceiver IC. Avoid having a discontinued plane as it will lead to an unpredictable return path and increase the likelihood of EMI. If that’s unavoidable, use an AC capacitor to connect between both planes below the signal pair. The AC capacitor will form a low impedance path for the return current of high-speed differential signals.

As for getting a precise area of the reference plane, using a PCB design software like OrCAD helps. So, don’t neglect the differential pair reference plane as it’s better to be safe than sorry.