Opamp input protection schemes

Hi
Anyone have a paper discussing the various methods of protecting opamp inputs?? I know there are a couple of ways, but uncertain as to which works best where.

diy didi said:

Hi
Anyone have a paper discussing the various methods of protecting opamp inputs?? I know there are a couple of ways, but uncertain as to which works best where.

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hi,
It depends upon the hazard you want to protect them from, also the required design specification of the amplifier .

Do you have an example of what the application may be.??

Schottky diodes up to the supply is the usual way. Put a schottky diode from the op amp input up to the supply and one from ground up to the input. The op amp has internal ESD diodes behind the input pins. One pointing from the input up to the supply and one from ground up to the input. If these start to conduct, you will leak energy into the substrate. This is where the ABS MAX conditions are derived from. The max voltage on the input is normally 0.6V above the supply voltage. This is when the diode to the positive supply starts to conduct. If you put schottky diodes at the inputs, they will bypass the internal ESD diodes and save the amp. Obviously limit the current into the Schottkys with an external resistance. Google it and it should point you in the right direction

Hi,

For a low millivolt input application i once did i used two sets of back to back 1N4148 diodes. Two diodes in series wont start conducting too much until about 0.8v or more, so two of those sets in antiparallel also with an input resistor protected the input in case of input voltages higher than 0.9v or so.
The diodes provide a clamping action but a series input resistor is also required to limit the current through the diodes in case of an over voltage.

There's also single diode clamping to ground and single diode clamping to +Vcc such as might be used with a microcontroller input. The catch here is that the diode to +Vcc could actually drive the +Vcc line higher than it is regulated to be so this solution might also require a shunt resistor from +Vcc to ground to absorb the extra current. The drawback to this is that the resistor always draws current and so isnt too good for battery operated equipment. A zener clamp might be better here.