8

®

OPA277, 2277, 4277

Op Amp

(a)

OPA277

(b)

No bias current

cancellation resistor

(see text)

Conventional op amp with external bias

current cancellation resistor.

OPA277 with no external bias current

cancellation resistor.

R

2

R

1

R

2

R

1

R

APPLICATIONS INFORMATION

The OPA277 series is unity-gain stable and free from unex-

pected output phase reversal, making it easy to use in a wide

range of applications. Applications with noisy or high im-

pedance power supplies may require decoupling capacitors

close to the device pins. In most cases 0.1

µF capacitors are

adequate.

The OPA277 series has very low offset voltage and drift. To

achieve highest performance, circuit layout and mechanical

conditions should be optimized. Offset voltage and drift can

be degraded by small thermoelectric potentials at the op amp

inputs. Connections of dissimilar metals will generate ther-

mal potential which can degrade the ultimate performance of

the OPA277 series. These thermal potentials can be made to

cancel by assuring that they are equal in both input termi-

nals.

• Keep thermal mass of the connections made to the two

input terminals similar.

• Locate heat sources as far as possible from the critical

input circuitry.

• Shield op amp and input circuitry from air currents such as

cooling fans.

OPERATING VOLTAGE

OPA277 series op amp operate from

±2V to ±18V supplies

with excellent performance. Unlike most op amps which are

specified at only one supply voltage, the OPA277 series is

specified for real-world applications; a single limit applies

over the

±5V to ±15V supply range. This allows a customer

mance as a customer using

±15V supplies. In addition, key

parameters are guaranteed over the specified temperature

range, –40

°C to +85°C. Most behavior remains unchanged

through the full operating voltage range (

±2V to ±18V).

Parameters which vary significantly with operating voltage

or temperature are shown in typical performance curves.

OFFSET VOLTAGE ADJUSTMENT

The OPA277 series is laser-trimmed for very low offset

voltage and drift so most circuits will not require external

adjustment. However, offset voltage trim connections are

provided on pins 1 and 8. Offset voltage can be adjusted by

connecting a potentiometer as shown in Figure 1. This

adjustment should be used only to null the offset of the op

amp. This adjustment should not be used to compensate for

offsets created elsewhere in a system since this can intro-

duce additional temperature drift.

INPUT PROTECTION

The inputs of the OPA277 series are protected with 1k

Ω

series input resistors and diode clamps. The inputs can

withstand

±30V differential inputs without damage. The

protection diodes will, of course, conduct current when the

inputs are over-driven. This may disturb the slewing behav-

ior of unity-gain follower applications, but will not damage

the op amp.

INPUT BIAS CURRENT CANCELLATION

The input stage base current of the OPA277 series is inter-

nally compensated with an equal and opposite cancellation

circuit. The resulting input bias current is the difference

between the input stage base current and the cancellation

current. This residual input bias current can be positive or

negative.

When the bias current is canceled in this manner, the input

bias current and input offset current are approximately the

same magnitude. As a result, it is not necessary to use a bias

current cancellation resistor as is often done with other op

amps (Figure 2). A resistor added to cancel input bias

current errors may actually increase offset voltage and noise.

FIGURE 2. Input Bias Current Cancellation.

V+

V–

20k

Ω

OPA277 single op amp only.

Use offset adjust pins only to null

offset voltage of op amp—see text.

Trim Range: Exceeds

Offset Voltage Specification

OPA277

6

7

8

4

3

2

1

0.1µF

0.1µF

FIGURE 1. OPA277 Offset Voltage Trim Circuit.