Q
How should I protect from Overvoltage?
A
In general we suggest you add external Schottky diodes and current limitingresistor to any input which is likely to exceed the abs max ratings. Note the
diodes need to be mounted physically close to the AD8646. Ideally, you should
design your system to ensure that the AD8646 is powered up fully before any
analog or digital signals are applied. I've also included a few general notes
on overvoltage.
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 line to limit
the current to <10mA. The higher you can make this series resistance the
better.
A high series resistance in a 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 input 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 AN-202 and AN-397 on Analog Devices website for more
information on overvoltage protection. Also the following web address points
you to a relevant chapter in one if our seminar books (overvoltage effects on
analog IC's)