OPA2228: Integrated ESD protection

It appears that there are diodes, because the absolute maximum ratings forbid you going 0.7 V beyond the rails.

Hi Maynard,

I found the original OPA227/OPA228 design review documentation from nearly 25 years ago. The schematics go pretty deep and show the typical ESD current steering diodes from the inputs and output to each of the supply rails supporting Clemens' assessment of their presence.

Additionally, the schematic shows a Zener diode symbol connected from the V+ pin to the V- pin which serves as the supply ESD clamp. This function is seen as the Power Supply ESD cell depicted in Figure 2-2 of the TI Application Report you reference.

I didn't find any details about that Zener diode, but when I have researched these clamp structures in the past they are almost always a NPN transistor with the collector connected to V+ and the emitter to V-. The base terminates back to V- through a resistor or a simple return circuit. The transistor is forced into to avalanche breakdown during a positive polarity ESD event clamping the V+ to V- rails and protecting the op amps circuitry. Should a negative polarity ESD event occur I believe it is the base-collector diode of that transistor that provides the clamping by becoming forward biased.

My understanding is the transistor functioning in avalanche breakdown mode turns on much faster than a conventional Zener diode. ESD protection needs to be first on if it is going to protect the op amp's internal components. 

Regards, Thomas

Precision Amplifiers Applications Engineering