OPA188_13 Datasheet, PDF (19/33 Pages) Texas Instruments – Precision, Low-Noise, Rail-to-Rail Output,36-V, Zero-Drift Operational Amplifiers

OPA188
www.ti.com
SBOS642A – MARCH 2013 – REVISED MARCH 2013
Figure 43 shows a specific example where the input voltage, V_IN, exceeds the positive supply voltage (+V_S) by
500mV or more. Much of what happens in the circuit depends on the supply characteristics. If +V_S can sink the
current, one of the upper input steering diodes conducts and directs current to +V_S. Excessively high current
levels can flow with increasingly higher V_IN. As a result, the data sheet specifications recommend that
applications limit the input current to 10 mA.
If the supply is not capable of sinking the current, V_IN may begin sourcing current to the operational amplifier, and
then take over as the source of positive supply voltage. The danger in this case is that the voltage can rise to
levels that exceed the operational amplifier absolute maximum ratings.
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 zener diodes may be
added to the supply pins, as shown in Figure 43. The zener voltage must be selected such that the diode does
not turn on during normal operation. However, the zener voltage should be low enough so that the zener diode
conducts if the supply pin begins to rise above the safe operating supply voltage level.
(2)
TVS
R_F
R_I
IN
(3)
R_S
+IN
I_D
V_IN(1)
+V_S
V+
Op Amp
Core
OPA188
ESD Current-
Steering Diodes
OUT
Edge-Triggered ESD
Absorption Circuit
R_L
V-
V_S
(2)
TVS
(1) V_IN = +V_S + 500 mV.
(2) TVS: +V_S(max) > V_{TVSBR (min)} > +V_S.
(3) Suggested value is approximately 1 kΩ.
Figure 43. Equivalent Internal ESD Circuitry Relative to a Typical Circuit Application
The OPA188 input terminals are protected from excessive differential voltage with back-to-back diodes, as
shown in Figure 43. 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 OPA188. Figure 43 shows an example configuration that implements
a current-limiting feedback resistor.
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