OPA1688 Datasheet, PDF (23/41 Pages) Texas Instruments – OPA168x SoundPlus 36-V, Single-Supply, 10-MHz, Rail-to-Rail Output Operational Amplifiers

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OPA1688, OPA1689
SBOS724 – SEPTEMBER 2015
Another common question involves what happens to the amplifier if an input signal is applied to the input when
the power supplies (+V_S or –V_S) are at 0 V. Again, this question depends on the supply characteristic when at
0 V, or at a level below the input-signal amplitude. If the supplies appear as high impedance, then the input
source supplies the operational amplifier current through the current-steering diodes. This state is not a normal
bias condition; most likely, the amplifier 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, add external zener diodes to the
supply pins; see Figure 48. Select the zener voltage so that the diode does not turn on during normal operation.
However, the zener voltage must be low enough so that the zener diode conducts if the supply pin begins to rise
above the safe-operating, supply-voltage level.
The OPA168x input pins are protected from excessive differential voltage with back-to-back diodes; see
Figure 48. In most circuit applications, the input protection circuitry has no effect. 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,
limit the input signal current to 10 mA or less. If the input signal current is not inherently limited, an input series
resistor can be used to limit the input signal current. This input series resistor degrades the low-noise
performance of the OPA168x. Figure 48 illustrates an example configuration that implements a current-limiting
feedback resistor.
9.4.3 Overload Recovery
Overload recovery is defined as the time required for the op amp output to recover from the saturated state to
the linear state. The output devices of the op amp enter the saturation region when the output voltage exceeds
the rated operating voltage, either resulting from the high input voltage or the high gain. After the device enters
the saturation region, the charge carriers in the output devices need time to return back to the normal state. After
the charge carriers return back to the equilibrium state, the device begins to slew at the normal slew rate. Thus,
the propagation delay in case of an overload condition is the sum of the overload recovery time and the slew
time. The overload recovery time for the OPA168x is approximately 200 ns.
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