
OP191/OP291/OP491
REV. 0
–13–
2mA
1mA
–1mA
–2mA
–5V
–10V
10V
5V
I
IN
V
IN
Figure 56. Input Overvoltage Characteristics
T his input current is not inherently damaging to the device as
long as it is limited to 5 mA or less. In the case shown, for an
input of 10 V over the supply, the current is limited to 1.8 mA.
If the voltage is large enough to cause more than 5 mA of
current to flow, then an external series resistor should be added.
T he size of this resistor is calculated by dividing the maximum
overvoltage by 5 mA and subtracting the internal 5 k
resistor.
For example, if theinput voltage could reach 100 V, the external
resistor should be (100 V/5 mA) –5 k = 15 k
. T his resistance
should be placed in series with either or both inputs if they are
subjected to the overvoltages. For more information on general
overvoltagecharacteristicsof amplifiersrefer to the
1993 System
Applications Guide
, available from the Analog Devices Literature
Center.
Output Voltage Phase Reversal
Some operational amplifiers designed for single-supply
operation exhibit an output voltage phase reversal when their
inputs are driven beyond their useful common-mode range.
T ypically for single-supply bipolar op amps, the negative supply
1/2
OP291
–5V
+5V
V
OUT
V
IN
20Vp-p
3
2
8
4
1
10
90
100
0%
TIME – 200
μ
s/DIV
V
I
10
90
100
0%
20mV
5
μ
s
20mV
5
μ
s
V
O
TIME – 200
μ
s/DIV
Figure 57. Output Voltage Phase Reversal Behavior
determines the lower limit of their common-mode range. With
these devices, external clamping diodes, with the anode
connected to ground and the cathode to the inputs, prevent
input signal excursions from exceeding the device’s negative
supply (i.e., GND), preventing a condition which could cause
the output voltage to change phase. JFET -input amplifiers may
also exhibit phase reversal, and, if so, a series input resistor is
usually required to prevent it.
T he OP191 family is free from reasonable input voltage range
restrictions due to its novel input structure. In fact, the input
signal can exceed the supply voltage by a significant amount
without causing damage to the device. As illustrated in Figure
57, the OP191 family can safely handle a 20 V p-p input signal
on
±
5 V supplies without exhibiting any sign of output voltage
phase reversal or other anomalous behavior. T hus no external
clamping diodes are required.
Overdrive Recovery
T he overdrive recovery time of an operational amplifier is the
time required for the output voltage to recover to its linear
region from a saturated condition. T his recovery time is
important in applications where the amplifier must recover
quickly after a large transient event, such as a comparator. T he
circuit shown in Figure 58 was used to evaluate the OP191
family’s overload recovery time. T he OP191 family takes
approximately 8
μ
s to recover from positive saturation and
approximately 6.5
μ
s to recover from negative saturation.
1/2
OP291
V
OUT
3
2
1
R1
9k
R2
10k
R3
10k
V
S
=
±
5V
V
IN
10V STEP
Figure 58. Overdrive Recovery Time Test Circuit