An EOS event can also occur if the op amp has a transient triggered protection scheme. However, the protection method for an edge triggered op amp is different than a dual diode protection scheme. In the edge triggered protection, the ESD cell is only triggered at a certain dv/dt level.

Here, Schottky diodes are more appropriate to use for protection. Schottky diodes have very fast switching characteristics which are preferred for ESD/EOS surges. Since the edge triggered diode protection structure does not have a set trigger voltage, the Schottky diodes help to detect the surge event, directing most, if not all, of the surge through the diodes. Schottky diodes have a low forward bias voltage, around 0.3V. Ideally, the Schottky diodes added must have a lower forward bias voltage drop than the internal diodes. This allows the majority of the EOS current to flow through the external diodes, decreasing the likelihood of damage to the op amp.

Figure 5-5 shows an op amp in a noninverting configuration. R_p has been added to the input, as well as two Schottky diodes. Adding R_p in series with the Schottky diodes further limits the current the op amp sees from the surge event.

However, Schottky diodes have a high leakage current. Thus, if this is an important design factor, other diodes must be considered. In this example, we only have input protection because only the input side is connected externally. External connections are much more prone to EOS events. Examples of this include large induced voltages, or long sensor leads often seen in factory automation.

Sometimes, there is a need to protect the output of the op amp (see Figure 5-6). The figure below shows a similar circuit that can be used. In this case, select R_p such that R_p does not limit the output swing of the op amp. Generally, choosing a resistance between 10 - 20Ω allows for good protection and functionality. Also note that R_p is inside the feedback loop. This allows an accurate output voltage to be maintained despite the voltage dropped across R_p . Finally, notice that very low current flows through R_F to the input of the circuit as the value of R_F is generally much larger than R_p .

Like all circuits, there are trade-offs. Adding protection to the circuit also introduces noise to the system. Consider noise, component space, and so on when designing the circuit. However, such considerations are beyond the scope of this paper. For more details on how minimize noise, see this paper on minimizing noise while protecting your op amp.