In part 4 of our connectors series, we looked at grounding for high-speed signals and power connections across connectors. These connections are very important in board-to-board connectors, as well as in connectors providing DC power and signal over short cables. There is one other aspect of grounding that may need to be considered in connectors: ground connections on the connector body and how these are used for shielding. A shielded connector will require this connection to be made to some ground point in the system.
The question then becomes: how should shielded connectors be integrated into the grounding strategy for an electronic assembly? Shielding needs to be connected to a uniform system reference to be effective and to eliminate the potential to act like a broadcasting antenna. In this iteration of our connectors series, we’ll look at the strategy for using shielded connectors.
How to Use Shielded Connectors
A shielded connector is constructed with a conductive enclosure that can help prevent reception of EMI. A shielded connector should be used with a shielded cable to get the maximum noise suppression benefit, otherwise an unshielded cable can still pick up EMI and the connector shielded will not provide any noise reduction. Shielded connectors also provide some shielded near the outlet of an enclosure, so they do block reception and emission of radiated EMI in that area.
Perhaps ironically, shielding connector datasheets may not provide any kind of shielding specification in their datasheets. These connectors will list a variety of specifications that vary by manufacturer, product, and target market for the connector., including:
- Insertion loss and return loss
- Contact resistance
- Maximum electrical ratings (current and/or voltage)
One other aspect of shielded connectors is the construction of the connector body. In some cases, the shielding is used to build the retention mechanism, and the connection from the connector body to ground comes through a board-mount mating connector. An example is the BNC connector shown below. This connector is the cable connector, while the mating connector sits on the PCB.
This BNC connector is a shielded connector that is often used in specialty measurements and scientific equipment.
The use of grounded shielding on a connector body makes sense from an EMI perspective; the pins in the connector assembly have some extra ground nearby, which can then block reception of external radiated EMI. There is another important form of EMI that should also be considered: protecting against electrostatic discharge (ESD).
One of the main reasons to connect a shielded connector is ESD suppression. Connectors will be exposed to the external environment during operation, so they can be a location where the user can create an ESD event. Therefore, it makes sense to take advantage of shielded connectors for ESD protection as well as low-level higher frequency EMI suppression.
Getting the most benefit out of a shielded connector in terms of ESD protection requires thinking about the grounding strategy. The theory here is simple and applies to shielded connectors in the same way as a metal enclosure. In a metal enclosure, the chassis is used to receive any ESD pulse and dump the current directly to earth. The same idea applies in shielded connectors; the connector shielding can send an ESD pulse directly back to a safety ground (chassis), which then sends the pulse back to earth.
Doing this properly requires mounting the connector such that its shielding connection connects directly to the chassis and NOT to the system ground. One way to do this is to connect the connector shielding to a large ground rail on an internal layer, and connect this rail to your mounting holes and the chassis.
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.
The above layout strategy is common in industrial embedded computing products, and specifically in industrial Ethernet systems. This strategy requires dedicating a portion of a layer specifically to this rail, and it is best implemented when interleaved between two plane layers. What if your layer count is too low and you can’t make room for this rail? Furthermore, what if there is no metal chassis and thus no frame ground?
What if There is No Frame Ground?
Some designs will not have any frame ground, either as a floating metal element in the chassis or as a metal enclosure connected to earth. Consider a small battery-powered device in a plastic enclosure; this device will have a floating ground in the internal layers of the PCB (assuming there is a ground plane). If there is a shielded connector in the assembly, you have two options:
- Leave that shielding floating
- Connect it to the main system ground
In this case, you should probably just connect it to the main system ground. In this case, it’s also important to apply some ESD suppression circuit as well. This will accomplish both objectives of suppressing EMI and protecting against ESD up to some high voltage level. If very high voltage ESD is a concern, then it is worth looking at a shielded enclosure that also connects to the connector body shielding. This can give much higher ESD protection than common ESD circuits.
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