As mentioned in the previous section, if the input voltage continues to rise beyond the absolute maximum ratings, a deep snap-back effect occurs at the input. The effect causes the voltage of the protection circuitry to suddenly fall and clamp the input voltage at a much lower level while raising the input current to higher levels and damaging the input structure itself. Furthermore, the device can become stuck in this state, a phenomenon known as latch-up.

While ESD protection structures without input diodes to the V+ rail can work well in applications where the amplifier must be turned off while an input signal is present, the possibility of a deep snap-back effect needs to be considered when the input voltages can potentially go beyond the absolute maximum ratings. The best practice is to avoid entering the high-current conduction regions altogether. Add input resistors that limit the input current to ±10 mA, or even ±1 mA.

Figure 3-3 shows an example of this effect. In this example, the OPA348 amplifier has the V– pin at ground and the V+ pin connected to a supply at 0 V. The output and inverting input are left open, while the non-inverting input is driven with a positive voltage. The corresponding input ESD structure current is then measured. The test configuration is shown in Figure 3-2.

From an input voltage of 0 V to about 10 V, there is low current passing through the ESD structure. In this region, the alternate ESD protection structure prevents an input signal from back-powering the amplifier or creating significant current. After about 10.5 V, the input current begins to rise rapidly. When the input is driven even further, deep snap-back occurs. The voltage suddenly drops and is clamped at a lower level. Simultaneously, the input current rises to very high levels. Avoid the operating regions of higher current to prevent damage. For the purpose of demonstration, the input current in this experiment was allowed to exceed the data sheet limit of 10 mA. Do not exceed the absolute maximum ratings in your design.

Effectiveness of the alternative input structure depends on the amplifier circuit configuration, even when the amplifier is turned off. For example, if a TLV341 amplifier is placed in a non-inverting configuration while the supplies are grounded and the device turned off, the input is able to handle some positive voltage without leaking current to the V+ rail or output (Figure 3-4 left). However, if the same amplifier under the same scenario is put into an inverting configuration, current does not pass through the input pin but is passed to the output through the feedback loop (Figure 3-4 right). Furthermore, the diodes at the output pin can turn on and pass into the V+ pin. So, care must be given not just to the op amp, but also to the circuit setup.