3-1 Key characteristics for normal operation (in the absence of an ESD event)(3)

3-1(3) Signal polarity (i.e., whether the signal voltage crosses the GND potential like an analog signal)

It is necessary to select either unidirectional or bidirectional ESD protection diodes, considering the polarity of signal lines to be protected. Different types of diodes are used for unmodulated digital signals that swing only in the positive direction (e.g., between 0 V (logic Low) and 5 V (logic High)) and unbiased analog signals whose voltage can be positive and negative. Bidirectional ESD protection diodes should be used for signals that range above and below GND as shown below. (Both unidirectional and bidirectional diodes can be used for signals whose voltage can only be positive or negative.)