AD7328: Overoltage protection

Question How must I protect input terminals of AD7328 from overvoltage (like +o- 40V). Answer Applying +/-40V to the analog input channel of the AD7328 exceeds the absolute max ratings for the part. We have not included any extra over voltage protection circuitry on this part other than standard ESD protection diodes. Exceeding the max ratings of the IC will cause stress and possibly latch-up of the chip. A recommended i/p structure to protect against overvoltage is to use 2 shottky diodes connected from the i/p pin to the supplies such that in the event of an overvoltage condition the i/p pins will remain within their max ratings. A series resistor should also be used to limit the current that flows through the diodes. For more details on overvoltage protection refer to the attached seminar chapter on overvoltage protection. Also: Some general notes on Overvoltage protection: Any other semiconductor IC has basic ESD protection diodes which 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 1.5kV. These ESD protection diodes will act to clamp the voltage at any pin to within 0.5V of the supplies. 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 which 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 digital line to limit the current to can make this series resistance the better. A high series resistance in a digital IO line can cause other problems such as slowing the rise and fall time of high speed digital signals. Since finger connections of your video card will be accessible by the user, you'll also need to consider ESD protection too much higher levels than +-1.5kV. 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 digital IO 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 which 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. You can see that designing suitable protection circuitry is not a trivial matter. You need to decide how much protection you need, how much abuse you expect the card to be subjected to, how much board space and component cost you can allow, and what test levels you need to meet.

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AD7328 Production
The AD7328 is an 8-channel, 12-bit plus sign, successive approximation ADC designed on theÂ iCMOSâ„¢ (industrial CMOS) process.Â iCMOS is a process combining...
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