AD7892: Overvoltage protection

Document created by analog-archivist Employee on Feb 23, 2016
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Customer would like to have a technical confirmation concerning the min and max
input voltage of the AD7893-2...
Do you confirm that digital info is locked at 111...111 when input voltage is
between +FSR (2.5Volt) and +10Volt ?
Same confirmation for digital info locked at 000...000 when input voltage is
between AGnd (0Volt) and -5Volt ?

 

The output of the AD7893-2 will not roll over when the analog input voltage
exceeds the positive or negative full-scale. However, the max rating for the
analog input of the –2 grade is –0.3V to Vdd. If your application requires that
the AD7893-2 withstand a voltage of +10V and –5V, you will have to provide
external protection circuitry to ensure that the max ratings (page 3 of
datasheet) are not violated.

A few 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. (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 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 to 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. Check app notes AN202 and
AN397 for more info.

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