OP491_15 Datasheet, PDF (18/24 Pages) Analog Devices – Micropower Single-Supply Rail-to-Rail Input/Output Op Amps

OP191/OP291/OP491
INPUT OVERVOLTAGE PROTECTION
As with any semiconductor device, whenever the condition
exists for the input to exceed either supply voltage, check the
input overvoltage characteristic. When an overvoltage occurs,
the amplifier could be damaged depending on the voltage level
and the magnitude of the fault current. Figure 62 shows the
characteristics for the OP191 family. This graph was generated
with the power supplies at ground and a curve tracer connected
to the input. When the input voltage exceeds either supply by
more than 0.6 V, internal PN junctions energize, allowing
current to flow from the input to the supplies. As described, the
OP291/OP491 do have 5 kΩ resistors in series with each input
to help limit the current. Calculating the slope of the current vs.
voltage in the graph confirms the 5 kΩ resistor.
I_IN
+2mA
+1mA
–10V
–5V
+5V
+10V
V_IN
–1mA
–2mA
Figure 62. Input Overvoltage Characteristics
This input current is not inherently damaging to the device as
long as it is limited to 5 mA or less. 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. The size of this
resistor is calculated by dividing the maximum overvoltage by
5 mA and subtracting the internal 5 kΩ resistor. For example, if
the input voltage could reach 100 V, the external resistor should
be (100 V/5 mA) − 5 kΩ = 15 kΩ. This resistance should be
placed in series with either or both inputs if they are subjected
to the overvoltages.
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.
Typically, for single-supply bipolar op amps, the negative supply
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 (that is, GND), preventing a condition that could cause
the output voltage to change phase. JFET input amplifiers can
also exhibit phase reversal, and, if so, a series input resistor is
usually required to prevent it.
The OP191 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 shown in Figure 64,
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. Thus, no external
clamping diodes are required.
OVERDRIVE RECOVERY
The 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. This recovery time is
important in applications where the amplifier must recover
quickly after a large transient event, such as a comparator. The
circuit shown in Figure 63 was used to evaluate the OPx91
overdrive recovery time. The OPx91 takes approximately 8 μs to
recover from positive saturation and approximately 6.5 μs to
recover from negative saturation.
V_IN
10V STEP
V_S = ±5V
R1
9kΩ
R2
10kΩ
3+
1/2
OP291 ¹
2–
R3
10kΩ
V_OUT
Figure 63. Overdrive Recovery Time Test Circuit
V_IN
20V p-p
+5V
8
3₊
1/2
OP291 ¹
2–
4
–5V
5µs
5µs
100
100
90
90
V_OUT
10
10
0%
0%
TIME (200µs/DIV)
Figure 64. Output Voltage Phase Reversal Behavior
Rev. E | Page 18 of 24
20mV
20mV
TIME (200µs/DIV)