Designing circuit inputs for high voltage

When a voltage beyond the supply rail is applied to an amplifier current typically flows through the ESD protection diodes to the supply.  If this current is not limited, the input structure of the amplifier can be damaged, causing large bias currents, large voltage offsets, or total failure in functionality.  Damage is cumulative:  an amplifier that shows no apparent ill effects after an overvoltage event may still be weakened and more susceptible to future overvoltage events.  Increasing the temperature of the amplifier makes the amplifier more vulnerable by lowering the current threshold at which it can be damaged.

The typical way to protect against high voltages on the inputs is to use resistors in series with the inputs of the amplifier.  This resistance limits the current into the amplifier during the overvoltage condition, preventing damage.  The applications section of our instrumentation amplifier datasheets typically describes the resistance needed.  If no guidance is given, limiting the current to 5 mA is a pretty good rule of thumb.  The resistance used for overvoltage protection can be the same resistance used in the RFI filter circuit.

Adding resistance at the inputs for protection has some downsides:  resistors add noise and may interact with the input offset current of the in amp to create a larger offset voltage.  The protection resistors also need to have a large enough physical size to handle the heat dissipation during the overvoltage event.

We do sell a few amplifiers with internal overvoltage protection.  An example of this type of amplifiers is the AD8226/AD8227, which has voltage protection of 40V from the opposite supply.  For example, if the AD8226 were powered from +Vs = 5V and -Vs = 0V, then the part can withstand -35V to 40V.