Op Amp

Core

R

F

R

I

R

L

V

IN

(See

Note 1)

I

D

IN

OUT

+IN

ESD Current-

Steering Diodes

+V

S

V+

R

S

(See Note 3)

TVS

(See Note 2)

Edge-Triggered ESD

Absorption Circuit

V±

V

S

TVS

(See Note 2)

Copyright © 2016, Texas Instruments Incorporated

19

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SBOS807 – DECEMBER 2016

Product Folder Links: OPA187 OPA2187 OPA4187

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Copyright © 2016, Texas Instruments Incorporated

Another common question involves what happens to the amplifier if an input signal is applied to the input while

or at a level below the input signal amplitude. If the supplies appear as high impedance, then the operational

amplifier supply current may be supplied by the input source via the current-steering diodes. This state is not a

normal bias condition; the amplifier most likely will not operate normally. If the supplies are low impedance, then

the current through the steering diodes can become quite high. The current level depends on the ability of the

input source to deliver current, and any resistance in the input path.

If there is any uncertainty about the ability of the supply to absorb this current, external TVS (Transient Voltage

Suppressor) diodes may be added to the supply pins, as shown in Figure 40. The TVS voltage must be selected

such that the diode does not turn on during normal operation. However, the TVS voltage should be low enough

so that the TVS diode conducts if the supply pin begins to rise above the safe operating supply voltage level.

NOTE 3: Suggested value is approximately 1 kΩ.

Figure 40. Equivalent Internal ESD Circuitry Relative to a Typical Circuit Application

The OPAx187 input terminals are protected from excessive differential voltage with back-to-back diodes, as

shown in Figure 40. In most circuit applications, the input protection circuitry has no consequence. However, in

low-gain or G = 1 circuits, fast-ramping input signals can forward-bias these diodes because the output of the

amplifier cannot respond rapidly enough to the input ramp. If the input signal is fast enough to create this

forward-bias condition, the input signal current must be limited to 10 mA or less. If the input signal current is not

inherently limited, an input series resistor can be used to limit the signal input current. This input series resistor

degrades the low-noise performance of the OPAx187. Figure 40 shows an example configuration that

implements a current-limiting feedback resistor.

7.4 Device Functional Modes

The OPAx187 has a single functional mode, and is operational when the power-supply voltage is greater than

4.5 V (±2.25 V). The maximum power supply voltage for the OPAx187 is 36 V (±18 V).