The absorption device is an internal clamping device, which limits the voltage across the op amp supply when an input ESD event is directed onto the supply through the input diodes. Figure 3-3 shows the typical high-level structure of an absorption device.

The absorption device is designed to clamp the supply voltage during an out-of-circuit ESD event to prevent damage. Once the event ends, typically after a few nanoseconds, the absorption device turns off since there is no power supply connected to the device. If the absorption device turns on for an in-circuit ESD event, it can remain on and in a low impedance state until power is removed from the circuit. Thus, it is imperative that the absorption device is not allowed to turn on during in-circuit electrical overstress events.

Figure 3-4 is an example of an ESD pulse being applied to the non-inverting node of an op amp. In the real world, this can happen by improper handling of the device; for example, assembling a PCB on a non-static safe work surface. Notice that diode D3 becomes forward biased and steers the ESD pulse to the absorption device. The absorption device is designed to limit the voltage and absorb the energy of the ESD pulse. Note that if the ESD pulse were applied to different pins, different diodes turn on and steer the pulse to the absorption device. Also, notice that the input resistance R2 limits the input current from the ESD pulse. A larger value of R2 makes the circuit more robust, since this limits the current going into the device. However, errors can be introduced from bias current and noise, and can also affect the frequency response of the circuit.