Question
The problem I have is as follows: I am having to apply a signal of
approximately +/-1 volt to the -ve input of one amplifier in an AD822, while
the device is unpowered. This event occurs only once, during our system
proving, and the signal lasts for about 2 milliseconds in each direction.
The supplies are decoupled by 22uF // 100nF, and these capacitors will
therefore charge via the parasitic diodes in the amplifier. My question is
will this damage the parasitic diodes? Having just had the second
conversation with you, I must stress this is a ONE OFF event which only
occurs during the explosive shock trial on our equipment. There is no
specific resistance in series, though wiring resistance will be about one
ohm. Please call again if I can give any more detail to help with answering
my question.
We do not want to put any resistance in series if we can avoid it, as the
AD822 is the front end of a low noise amplifier in a sonar system.
Answer
The AD822 allows the input range to extend below the negative supply. The
AD822 remains functional with input 0.2V below the negative supply and the
inputs can extend 20V below the negative supply without damage. However, the
AD822 provides no protection against input voltage which exceed the positive
supply by more than 0.2V.
The abs max ratings must be respected at all times even during transient
conditions. The +1V input excursion beyond the positive supply (0V when
unpowered) will cause permanent damage to the AD822. The damage may manifest as
a catastrophic failure or may be cumulative in nature showing up as a change in
input offset voltage, input bias current, and increase in noise, or an early
life failure.
Assuming the positive excursion can be guaranteed to be 1V max, limiting the
input current to less than 5mA will prevent damage. Alternatively a Schottky
diode between the –in pin and the positive supply would be suitable protection.
Included below are some general notes on Overvoltage issue.
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 typically 1.5kV. These
ESD protection diodes will act to clamp the voltage at any pin to within 0.4V
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 5mA. Therefore unless you can guarantee that the current
into in pin will me less then 5mA 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 1000Ohm series resistor in each digital line
to limit the current to
the better.