Datasheet

HD3SS3412
www.ti.com SLAS828B –FEBRUARY 2012–REVISED NOVEMBER 2013
APPLICATION INFORMATION
AC Coupling Caps
Many interfaces require AC coupling between the transmitter and receiver. The 0402 capacitors are the preferred
option to provide AC coupling, and the 0603 size capacitors also work. The 0805 size capacitors and C-packs
should be avoided. When placing AC coupling capacitors symmetric placement is best. A capacitor value of 0.1
µF is best and the value should be match for the ± signal pair. The placement should be along the TX pairs on
the system board, which are usually routed on the top layer of the board.
There are several placement options for the AC coupling capacitors. Because the switch requires a bias voltage,
the capacitors must only be placed on one side of the switch. If they are placed on both sides of the switch, a
biasing voltage should be provided. A few placement options are shown below. In Figure 12, the coupling
capacitors are placed between the switch and endpoint. In this situation, the switch is biased by the system/host
controller.
Figure 12. AC Coupling Capacitors Between Switch TX and Endpoint TX
In Figure 13, the coupling capacitors are placed on the host transmit pair and endpoint transmit pair. In this
situation, the switch on the top is biased by the endpoint and the lower switch is biased by the host controller.
Figure 13. AC Coupling Capacitors on Host TX and Endpoint TX
If the common mode voltage in the system is higher than 2 V, the coupling capacitors are placed on both sides of
the switch (shown in Figure 14). A biasing voltage of less than 2 V is required in this case.
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