The AD719x is seeing overvoltages in my application. How can I protect the ADC against these overvoltages?
The analog input pins can tolerate a voltage between GND - .3V and AVDD + 0.3V. There are ESD diodes connected from each analog input to AVDD and from each analog input to GND. If an overvoltage is seen on the analog input (a voltage with is in excess of 500 mV outside the power supply rails), these diodes will turn on. These ESD diodes provide a low resistance path so the overvoltage is directed away from the ADC’s modulator. However, the ESD diodes can only tolerate a continuous current less than 10 mA. So, to prevent damage of the ADC, some external resistance is required so that the current is limited to less than 10 mA.
Using external limiting resistors prevents the ADC from getting damaged. However, the external overvoltage may cause the on-chip registers to be corrupted.
To prevent the internal registers from being corrupted, the overvoltage needs to be directed away from the ADC. Connecting schottky diodes from each analog input to AVDD and GND is one method that can be used. The schottky diode must have a turn-on voltage of 300mV. This ensures that the external diode turns on before the ADC’s internal diodes so that the overvoltage is directed away from the ADC. Since the ADC does not experience any overvoltage, the on-chip registers will remain intact.
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