AD7821: Overvoltage protection at the analog inputs

Document created by analog-archivist Employee on Feb 23, 2016
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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?

 

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
<10mA.  The higher you can make this series resistance the better.

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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