In part 3 of our connectors series, we briefly examined signal integrity in connectors that need to carry high-speed signals between two pieces of equipment. For high-speed design, we often care about things like reflection and losses, which can be quantified using S-parameters or another parameter set. However, there is another design consideration when using connectors that applies to everything from high-speed signaling to DC power: including ground in your connector pinout.
Connectors that carry DC power, signals, or both will also need to carry a ground connection. There are multiple reasons for this, but whether you are connecting distant equipment through a cable or building a multi-board system, grounding across the connection will be a requirement in many cases. As we’ll see, this challenge of grounding is related to crosstalk and setting impedance for signal traversing a connector/cable, and a pinout design will be needed to help ensure signal integrity and low EMI.
Connector Grounding for Power
Whenever you route a power connection across a connector, you should ensure that the ground connection is also routed across the connector. How you connect the grounds on each side of a link is another matter, and this is an important part of EMI in cables and board-to-board connectors. However, regardless of how ground is connected across a connector/cable, there should be a connection between grounds on the receiving equipment.
Consider two modules that need to be linked with a connector; there are three ways to make a connection between grounds:
- The ground on the receiving module is floating compared to the driver side ground, so it can be bridged to the driving module
- The two grounds are on the same utility circuit with no ground offset, so they can be directly bridged (this is uncommon)
- The two grounds may have some ground offset, so they can be connected with a capacitor
Point #3 above is important on large installations with shielded cabling. In larger installations, it is not common to deliver power or ground through a connector, only data needs to be delivered through a 2-wire differential interface (such as CAN or Ethernet). This is because differential serial protocols can withstand large ground offsets that might exist between the two ends of the system. However, in cases where multiple signals are being routed through a smaller connection, like a board-to-board connector or a short cable, ground becomes an important element of EMI suppression and signal integrity.
Ground Pins for High-Speed Signals
The primary purpose of placing ground pins in a connector pinout for connecting two components is to provide a return path for any signals being routed across the connector. Where the ground pins are placed in the pinout will determine how strongly the return path in a cable is coupled to the signal line. Cables are intentionally designed to maintain an input impedance target for signals interesting the cable, as well as to ensure there is a clear return path across the cable if a ground connection must be included.
Note that some cables, like Ethernet cables, are totally isolated and will not include any ground connection. Other cables, like USB, carry all three types of pins (ground, power, and signal), and the ground connection is included to set impedance as well as provide a return path.
The standard pinout for USB-C includes ground, power, signal, and additional expansion pins. Note that this pinout is reversible.
With that in mind, where should ground pins be placed? For the USB-C connector pinout shown above, there is only one differential pair crossing the connector; the ground pins provide a return path and are used for power delivery across the connector.
When a connector contains multiple data signals, the common practice is to interleave ground between those signals. This is common on custom pinouts for interface connectors that you might build with a D-sub or a mezzanine connector. In older parallel single-ended bus architectures, ground would be interleaved between groups of parallel (even-mode) signals as shown below. The table below lists a pinout for a 160-pin board-to-board connector used in VME64x backplane-to-daughtercard connections.
VME64x 160-pin connector pinout used in older backplane architectures. There are multiple pin groups (e.g., A, IRQ, and D) that are part of parallel buses.
High-speed computing peripherals use serial differential signaling, and this has been mainstream for two decades. In this case, groups of differential pins are interleaved with corresponding ground pins. This is also standard in modular open architectures for embedded equipment, e.g., VITA standards for backplanes. In the backplane example, these designs use alternating pairs of ground pins and differential signal pins to route between the backplane and daughtercards (see below).
Example VPX connector pinout with multiple differential serial interfaces used in an Elma backplane. We can see how ground is interleaved between each of these. A separate connector section can be used to provide power at multiple voltages.
Interleaving ground in this way also helps to suppress crosstalk between groups of signal pins. The grounded pins are intended to also act as shielding between groups of differential signals. In your pinout, make sure to allocate more pins than you need for signal and power, and interleave these unused pins as ground connections between power and signal.
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