It is important to evaluate circumstances when an input signal is present while the ADC is powered and unpowered. When the input signal exceeds the power-supply voltage, it is possible to back drive the analog power-supply voltage with the input signal through a conduction path of the internal ESD diodes. Back driving the ADC power supply can also occur when the power-supply voltage is on. The back-drive, fault-current path is shown in Figure 11-2. Depending on the external power-supply components, it is possible that the maximum rating of the ADC power-supply voltage can be exceeded if back-driven. ADC power supply overvoltage must be prevented in all cases. One solution is to clamp the AVDD to AVSS voltage with an external 6-V Zener diode.