What should I consider when selecting TVS diodes (ESD protection diodes) for high-speed signal lines?

You need to consider the normal operating state in which a circuit is not being exposed to static electricity and other excessive voltage (in this state, the TVS diode is off).

A simplified equivalent circuit of the TVS diode
Figure 1. Simplified equivalent circuit of TVS diode

A simplified equivalent circuit of the TVS diode is shown below. When the TVS diode is off, the TVS diode acts in the same way as a capacitor by junction capacitance. This capacity is called the total capacitance CT. Since this capacitance is placed between a signal line (data path) and GND, the signal integrity of a high-speed signal is subject to degradation. To maintain signal integrity, it is necessary to select TVS diodes with adequate total capacitance CT.

The following illustrates an example of TVS diode evaluation on a high-speed signal line.
When TVS diode with a capacitance of 5.0 pF is used for a signal operating at 20 Gbps, signal waveform appears as closure of the eye pattern* , resulting in degraded signal integrity (Figure 3, right). In contrast, when using our low-capacitance TVS diode, the eye pattern remains open, enabling transmission without signal degradation (Figure 3, center). Therefore, for high-speed signal inputs, it is critical to carefully consider the capacitance value when selecting a TVS diode.
*Eye pattern: A visual representation created by superimposing multiple digital signal waveforms, making signal timing and voltage characteristics observable. It is commonly used to verify whether high-speed signals are transmitted correctly.

Example of TVS diode placement on differential signal lines (Rx and Tx lines such as USB3.1)
Figure 2. Example of TVS diode placement on differential signal lines (Rx and Tx lines such as USB3.1)
Figure 3. On differential signal lines, eye pattern data example (Rx and Tx lines such as USB3.1)
Figure 3. On differential signal lines, eye pattern data example (Rx and Tx lines such as USB3.1)
figure as a guide when selecting TVS diodes
Figure 4. Rough total capacitance vs signal frequency

Use the following figure as a guide when selecting TVS diodes.

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