Are there any concerns when switching high-speed signals using AC coupling capacitors with a bus switch?

The use of AC coupling capacitors for high-speed signals allows communication between TX and RX with different common-mode voltages. AC coupling capacitors must be placed so that the bus switch is supplied with the common mode voltage at which it operates.

Fig. 1 Recommended layout of AC coupling capacitor
Fig. 1 Recommended layout of AC coupling capacitor

First, the reasons why AC coupling is needed for high-speed differential signals are given below.
The reason why AC coupling is needed for high-speed signals is to remove the DC components from the signal. Since the common-mode voltage of the signal is a DC component, using an AC coupling capacitor allows only the AC component to be transmitted between TX and RX, which have different common mode voltages. A specific circuit example is shown below.

First, an example of using an AC coupling capacitor without a bus switch is shown in Fig. 1 below.
In this example, TX and RX are placed on different boards and the signal lines are connected with connectors.
If TX and RX are placed on different boards and connected by connectors in this way, it is recommended that an AC coupling capacitor be placed between TX and the connector.
The AC coupling capacitor should allow common mode signals to be supplied from the TX before passing through the capacitor and from the RX after passing through the coupling capacitor.

Next, an example of using an AC coupling capacitor with a bus switch is shown in below.
In this case, the AC coupling capacitor must be placed so that the common mode voltage that satisfies the bus switch operating range is supplied to the bus switch.
For example, a common mode voltage of 0 to 2.0V must be applied to the TDS4A212MX and TDS4B212MX. (Table-1)

Table-1 Operating Ranges of the TDS4A212MX, TDS4B212MX (Note)
Characteristics Symbol Rating Unit
Supply voltage VCC 1.6 to 3.6 V
Input voltage (/OE, SEL) VIN 0 to 3.6 V
Signal pins differential voltage. VI/O (Diff) 0 to 1.8 V
Signal pins common mode voltage.  VI/O (Com) 0 to 2.0 V
Operating temperature Topr -40 to 85  °C
Input rise and fall times dt/dv 0 to 10 ns/V

Note: The operating ranges must be maintained to ensure the normal operation of the device.
         Unused control inputs must be tied to either VCC or GND. 

Fig. 2 Example of AC coupling capacitor is placed between the bus switch (Mux/De-Mux) and the device board.
Fig. 2 Example of AC coupling capacitor is placed between the bus switch (Mux/De-Mux) and the device board.

The example in Fig. 2 shows an AC coupling capacitor placed between the bus switch (Mux/De-Mux) and the device board.

Fig. 3 Example of AC coupling capacitor is placed between the bus switch (Mux/De-Mux) and TX.
Fig. 3 Example of AC coupling capacitor is placed between the bus switch (Mux/De-Mux) and TX.

In this arrangement, common mode signals are supplied to the bus switch (Mux/De-Mux) from the host IC, not from the device board. Therefore, even if the common mode voltage on the device board side is outside the operating range of the bus switch (Mux/De-Mux), the connection can still be made, and the device board can be more freely connected to the host board.

The example in Fig. 3 shows an AC coupling capacitor placed between the bus switch (Mux/De-Mux) and TX.

Fig. 4 Example of AC coupling capacitors placed on both sides of a bus switch (Mux/De-Mux)
Fig. 4 Example of AC coupling capacitors placed on both sides of a bus switch (Mux/De-Mux)

In this arrangement, the path (blue line) passing through the above De-Mux is supplied with the common signal voltage from the device board (RX), and the path (red line) passing through the Mux is supplied with the common signal voltage from the host IC (RX). The advantage of this arrangement is that the number of capacitors used is smaller compared to the first example.

The example in Fig. 4 shows capacitors placed on both sides of a bus switch (Mux/De-Mux).

In this arrangement, the AC coupling capacitors are placed on both sides of the bus switch (Mux/De-Mux), so the common signal voltage is not supplied from either the host IC or the device board. Therefore, the common signal voltage must be supplied separately. If the common signal voltage supplied from the host IC or device board is outside the operating range of the bus switch (Mux/De-Mux), it is recommended that this arrangement be used. Therefore, apply a common signal voltage of 0 to 2.0V with a resistor of 10kΩ or more as shown in the figure.

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