OP191/OP291/OP491

REV. 0

–13–

2mA

1mA

–1mA

–2mA

–5V

–10V

10V

5V

I

IN

V

IN

Figure 56. Input Overvoltage Characteristics

This 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.

The size of this resistor is calculated by dividing the maximum

overvoltage by 5 mA and subtracting the internal 5 k

Ω resistor.

For example, if the input voltage could reach 100 V, the external

resistor should be (100 V/5 mA) –5 k = 15 k

Ω. This resistance

should be placed in series with either or both inputs if they are

subjected to the overvoltages. For more information on general

overvoltage characteristics of amplifiers refer 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.

Typically 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

µs/DIV

10

90

100

0%

20mV

5

µs

20mV

5

µs

TIME – 200

µs/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.

The 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. 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 58 was used to evaluate the OP191

family’s overload recovery time. The OP191 family takes

approximately 8

µs to recover from positive saturation and

approximately 6.5

µs to recover from negative saturation.

1/2

OP291

V

OUT

3

2

1

R1

9k

Ω

R2

10k

Ω

R3

10k

Ω

V

V

IN

10V STEP

Figure 58. Overdrive Recovery Time Test Circuit