7

®

OPA137, 2137, 4137

APPLICATIONS INFORMATION

OPA137 series op amps are unity-gain stable and suitable

for a wide range of general-purpose applications. Power

supply pins should be bypassed with 10nF ceramic capaci-

tors or larger. All circuitry is completely independent in dual

and quad versions, assuring normal performance when one

amplifier in a package is overdriven or short circuited. Many

key parameters are guaranteed over the specified tempera-

ture range, –40

°C to +85°C.

OPERATING VOLTAGE

OPA137 op amps can be operated on power supplies as low

as

±2.25V. Performance remains excellent with power sup-

plies ranging from

±2.25V to ±18V (+4.5V to +36V single

supply). Most parameters vary only slightly throughout this

supply voltage range. Quiescent current and short-circuit

current vs supply voltage are shown in Typical Performance

Curves.

Operation at very low supply voltage (VS ≤ ±3V) requires

careful attention to ensure that the common-mode voltage

remains within the linear range, VCM = (V–)+3V to (V+).

Inputs may need to be biased above ground in accordance

with the common-mode voltage range restrictions for linear

operation.

INPUT VOLTAGE

The input common-mode voltage range of OPA137 series

op amps extends from (V–)+3V to the positive rail, V+. For

normal operation, inputs should be limited to this range. The

inputs may go beyond the power supplies without output

phase-reversal. Many FET-input op amps (such as TL061

types) exhibit phase-reversal of the output when the input

common-mode range is exceeded. This can occur in voltage-

follower circuits, causing serious problems in control loop

applications.

Input terminals are diode-clamped to the power supply rails

for ESD protection. If the input voltage can exceed the

negative supply by 500mV, input current should be limited

to 2mA (or less). If the input current is not adequately

limited, you may see unpredicatable behavior in the other

amplifiers in the package. This is easily accomplished with

an input resistor as shown in Figure 1. Many input signals

are inherently current-limited, therefore, a limiting resistor

may not be required.

FIGURE 1. Input Current Protection for Voltages Exceed-

ing the Supply Voltage.

HIGH-SIDE CURRENT SENSING

Many applications require the sensing of signals near the

positive supply. The common-mode input range of OPA137

op amps includes the positive rail, enabling them to be used

to sense power supply currents as shown in Figure 2.

FIGURE 2. High-Side Current Monitor.

INPUT BIAS CURRENT

The input bias current is approximately 5pA at room tem-

perature and increases with temperature as shown in the

typical performance curve “Input Bias Current vs Tempera-

ture.”

Input Bias current also varies with common-mode voltage

and power supply voltage. This variation is dependent on

the voltage between the negative power supply and the

common-mode input voltage. The effect is shown in the

typical performance curve “Input Bias Current vs Common-

Mode Voltage.”

FIGURE 3. Photodetector Amplifier.

R

F

1M

Ω

3.3pF

V

O = – RF ID

I

D

λ

OPA137

I

D is proportional to

light intensity (radiant power)

Photodiode

BPW34

C

D = 75pF

OPA137

V

IN

V

OUT

I

OVERLOAD

2mA max

V–

V+

Inputs are internally

clamped to V+ and V–

R

1

0.1

Ω

R

2

1k

Ω

R

3

10k

Ω

OPA241

V

O =

I

L

OPA137

20pF

Load

Ground-referred

output

V

O

V+

R

1 R3

R

2

Zetex

Darlington

ZTX712