Question
What is the maximum input current and overvoltage when the device is powered
off?
How to protect it against overvoltage when powered off? Are clamping Diodes
(Shotky) enough?
Are there internal diodes to the substrate present?
Answer
To provide a complete answer, it is necessary to know what the power supplies
look like when they are "off". This can range from a short circuit to ground,
or a fully open circuit.
The worst case, of course, is a short circuit. All inputs and outputs of the
AD8185 (and virtually all of our other parts) have ESD diodes that connect to
each supply. These diodes are oriented such that they are back-biased when the
part is in normal operation. Thus, if the output is a short circuit to ground,
the ESD diodes will begin to conduct when the input (or output) goes outside
the range of ~ +/-0.7V.
We do not provide a firm spec as to the current-handling capability of these
diodes. This is because their main function is to provide ESD protection. This
parameter is also a function of the duration of the overdrive and its duty
cycle. Informally, I have heard that these diodes should be able to withstand
greater than 100 mA on a continuous basis. This creates a power dissipation of
.1A x .7V = 70 mW.
If the over-voltage is assumed to be +/-5V (the supply voltage of the AD8185),
then a 75-ohm resistor at the output will limit the current to (5V - 0.7V) / 75
ohms = 57 mA. The ESD diodes should be able to handle this on a continuous
basis.
The inputs to the AD8185 are high-impedance. Therefore, it is possible to
provide some series resistance at the input and not affect the performance. A
resistor of up to 100 ohms should be OK. If this resistor is made much bigger,
then the bandwidth will be affected because of the low-pass filter formed by
the series input resistor and the parasitic capacitance of the input.