OPA111: Rating on input pin

Hi Siddharth,

I don't think that the OPA111 has clamp diodes at the inputs. in order to keep the input bias current ultra low, they were omitted in the old days. The chip does also have no specified ESD rating, which might be the reason why the OPA111 is not recommended for new designs.

Read the chapter "input protection" and the "absolute maximum ratings" to see, what the OPA111 can withstand or not.

If the ultra low input bias current is not important for you, you could protect the inputs by the help of external diodes, shunting ESD and overvoltages to the supply lines. BAV99 or BAV199 (low leakage) could be used. Or you can take a newer OPAmp for your application.

Kai

thanks for the reply, i read the input protection section and from there i felt that the datasheet specifies that a 10K resistor should be used in case the +/-15v supplies are lost from which i tend to conclude that the input pin current should be limited to 1.5ma?

Also what is the limit to which the inputs can go beyond the supplies for this opamp ?
thanks

Hi Siddharth,

The OPA111 is a legacy op amp. My search for the internal schematic indicates that it was introduced about 35 year ago. The schematic does not show the any ESD cells in the design. That is not unusual for a precision op amp from that era. Thus, it does not have any clamps from the inputs to the internal supply rails.

The Absolute Maximum Ratings table lists the Input Voltage Range (note 2) .... ±18VDC. Note 2 - For supply voltages less than ±18VDC, the absolute maximum input voltage is equal to +18V > VIN > –VCC – 6V. See Figure 2. Here is Figure 2 from the datasheet:

The figure indicates the maximum safe input current is +/-1 mA for the conditions shown. I suggest not exceeding the Input Voltage Range limits in the Absolute Maximum Ratings table if you want to avoid damaging the amplifier.

Regards, Thomas

Precision Amplifiers Applications Engineering