EMC Question of the Week: November 21, 2022

Capacitors in series with differential signal traces on a circuit board

The purpose of the capacitors in the figure is to provide

  1. low-pass filtering
  2. high-pass filtering
  3. power bus decoupling
  4. propagation delay


The best answer is “b.” These are DC-blocking capacitors on high-speed differential signal traces. From the figure, it is clear that they are placed in series with length-matched trace pairs. Series capacitors in traces connected to a matched termination provide a high-pass filtering function.

Transformer coupled differential signals do not require DC-blocking capacitors, but pseudo-differential signals may utilize these capacitors to prevent the flow of DC current between circuit boards. Of course, the signal must be designed to have no DC component.   

Capacitors connected to ground or between the traces would provide low-pass filtering, but the capacitors in the figure are clearly connected in series with the traces. Power bus decoupling capacitors would be connected between power and power return traces, but the number of trace pairs and the precise nature of the routing make it clear that these are not power traces.

And while the inductance and length of the capacitor connections may introduce a tiny propagation delay, they are clearly not placed there for that purpose. The precise amount of delay is miniscule and difficult to predict. Also, the fact that the capacitors are placed in matched pairs indicates that they are not designed to delay one side of the signal relative to the other. 

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