OPA2140 Datasheet, PDF (17/25 Pages) Texas Instruments – High-Precision, Low-Noise, Rail-to-Rail Output 11MHz JFET Op Amp

OPA140
OPA2140, OPA4140
www.ti.com
Figure 38 depicts 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 datasheet
specifications recommend that applications limit the
input current to 10mA.
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 and/or –V_S are at 0V.
SBOS498A – JULY 2010 – REVISED AUGUST 2010
Again, it depends on the supply characteristic while at
0V, 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 an 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 38. The zener voltage must be selected such
that the diode does not turn on during normal
operation.
However, its 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
I_D
V_IN(1)
R_I
-In
(3)
R_S
+In
+V_S
+V
OPA140
Op Amp
Core
ESD Current-
Steering Diodes
Out
Edge-Triggered ESD
Absorption Circuit
R_L
-V
-V_S
(2)
TVS
(1) V_IN = +V_S + 500mV.
(2) TVS: +V_S(max) > V_{TVSBR (Min)} > +V_S
(3) Suggested value approximately 1kΩ.
Figure 38. Equivalent Internal ESD Circuitry and Its Relation to a Typical Circuit Application
Copyright © 2010, Texas Instruments Incorporated
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