AD7821: Overvoltage protection at the analog inputs

Question

Accepted Answer

Question How long can the AD7821 ADC withstand a voltage of 12V on its VIn pin? Has the 
 device got internal clamping diodes, or can you reccommend some diodes to put 
 in front of the device? Answer According to the absolute maximum ratings on page 4 of the datasheet, the 
 maximum tolerable input voltage at the analog input pins is Vdd + 0.3V, and as 
 the maximum allowable Vdd is 7V, +10V at the input will very likely damage the 
 AD7821.
 
 In general we suggest you add external Schottky diodes and current limiting 
 resistor to the input that is likely to exceed the abs max ratings. Note the 
 diodes need to be mounted physically close to the AD7821. Ideally, you should 
 design your system to ensure that the AD7821 is powered up fully before any 
 analog or digital signals are applied. 
 
 Any other semiconductor IC has basic ESD protection diodes that protect the 
 device from possible ESD hits due to handling and production. It is the 
 designers responsibility to provide external protection circuitry if the input 
 is likely to exceed the supplies at any time. 
 
 These ESD diodes can protect the IC from ESD hits up to about 4kV. These ESD 
 protection diodes will act to clamp the voltage at any pin to within 0.3V of 
 the supplies. (So that's the problem solved right? No not quite.) ESD 
 protection diodes can carry quite high currents but only for a short period of 
 time so they can protect the IC from large pulses of short duration (the total 
 energy is still quite low). The maximum DC current that these protection diodes 
 can carry is 10mA. Therefore unless you can guarantee that the current into in 
 pin will me less then 10mA you need some kind of external protection. External 
 protection could be as simple as a series resistor to limit the current into a 
 pin. For example if the maximum overvoltage voltage applied to a pin will be 5V 
 you need to add a 500Ohm series resistor in each line to limit the current to 
 
 A high series resistance in an IO line can cause other problems such as slowing 
 the rise and fall time of high-speed signals. You might also want to protect 
 against higher overvoltages but you don't want to add any more series 
 resistance, so what can you do? Well, you can do this by adding external 
 Schottky diodes between each onput line and the supply lines. A schottky diode 
 will clamp applied voltages to within ~0.3V of the supply so the majority of 
 the current will be diverted via the external diodes (which can carry higher 
 current) and not through the internal ESD protection diodes. There are other 
 protection techniques that include use of spark gaps, large capacitors to earth 
 ground, small choke inductors and more. One the best structures I have seen for 
 protecting against both overvoltage and ESD is a small series resistor followed 
 by Schottky diodes to the supplies followed by another small series resistor.

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AD7821: FAQ