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AD7328: Overoltage protection

How must I protect input terminals of AD7328 from overvoltage (like +o- 40V).


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.


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 <10mA.  The higher you
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

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.