ABCs of ADCs - Analog-to-Digital Converter Basics (PDF)

Overdriving any InputV DDevice CircuitryDevice Pin46The ESD protection diodes on the die will conduct when the input goes far enough above thesupply voltage or far enough below device ground. Never assume that these diodes do notconduct until there is 600mV across them. Some of them conduct enough to be a problem withas little as 50 to 100 mV across them! As the current through these diodes become significantenough for that particular device, other parasitic diodes and transistors can be turned on. Theresult can be as dramatic as turning on a very low impedance path between the supply pin andground. This is known as CMOS latchup and is destructive to the device.A more common effect is to charge or discharge nodes within the ADC such that conversionaccuracy is lost or the device may not function at all.NEVER allow the pins of a device to go beyond the supply rails, not even on a transient basis,unless the device data sheet clearly states that it is o.k.ABCs of ADCs - Rev 3, June 2006Authored by: Nicholas “Nick” Gray46Copyright © 2003, 2004, 2006 National SemiconductorCorporationAll rights reserved