OPA187_17 Datasheet, PDF (19/35 Pages) Texas Instruments – Drift, Low Power, Rail-to-Rail Output 36-V Operational Amplifiers Zero-Drift Series

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OPA187, OPA2187, OPA4187
SBOS807 – DECEMBER 2016
Another common question involves what happens to the amplifier if an input signal is applied to the input while
the power supplies +V_S or –V_S are at 0 V. Again, this question depends on the supply characteristic while at 0 V,
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.
(See Note 2)
TVS
R_F
+V_S
V+
I_D
V_IN
(See
Note 1)
R_I
IN
(See Note 3)
R_S
+IN
Op Amp
Core
ESD Current-
Steering Diodes
Edge-Triggered ESD
Absorption Circuit
OUT
R_L
V±
V_S
TVS
(See Note 2)
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NOTE 1: V_IN = +V_S + 500 mV.
NOTE 2: TVS: +V_S(max) > V_{TVSBR (min)} > +V_S.
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).
Copyright © 2016, Texas Instruments Incorporated
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