7

®

OPA177

APPLICATIONS INFORMATION

The OPA177 is unity-gain stable, making it easy to use and

free from oscillations in the widest range of circuitry. Ap-

plications with noisy or high impedance power supply lines

may require decoupling capacitors close to the device pins.

In most cases 0.1

µF ceramic capacitors are adequate.

The OPA177 has very low offset voltage and drift. To

achieve highest performance, circuit layout and mechanical

conditions must 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 mask the ultimate performance of

the OPA177. These thermal potentials can be made to cancel

by assuring that they are equal in both input terminals.

1. Keep connections made to the two input terminals close

together.

2. Locate heat sources as far as possible from the critical

input circuitry.

3. Shield the op amp and input circuitry from air currents

such as cooling fans.

OFFSET VOLTAGE ADJUSTMENT

The OPA177 has been laser-trimmed for low offset voltage

and drift so most circuits will not require external adjust-

ment. Figure 1 shows the optional connection of an external

potentiometer to adjust offset voltage. This adjustment should

not be used to compensate for offsets created elsewhere in a

system since this can introduce excessive temperature drift.

INPUT PROTECTION

The inputs of the OPA177 are protected with 500

Ω series

input resistors and diode clamps as shown in the simplified

circuit diagram. The inputs can withstand

±30V differential

inputs without damage. The protection diodes will, of course,

conduct current when the inputs are overdriven. This may

disturb the slewing behavior of unity-gain follower applica-

tions, but will not damage the op amp.

NOISE PERFORMANCE

The noise performance of the OPA177 is optimized for

circuit impedances in the range of 2k

Ω to 50kΩ. Total noise

in an application is a combination of the op amp’s input

voltage noise and input bias current noise reacting with

circuit impedances. For applications with higher source

impedance, the OPA627 FET-input op amp will generally

provide lower noise. For very low impedance applications,

the OPA27 will provide lower noise.

INPUT BIAS CURRENT CANCELLATION

The input stage base current of the OPA177 is internally

compensated with an equal and opposite cancellation cur-

rent. 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 cancelled in this manner, the input

bias current and input offset current are approximately the

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

the DC resistance seen at the two input terminals (Figure 2).

A resistor added to balance the input resistances may actu-

ally increase offset and noise.

FIGURE 1. Optional Offset Nulling Circuit.

OPA177

V

2

3

1

8

Trim Range is approximately ±3.0mV

V+

20k

Ω

OUT

V

IN

Op Amp

(a)

R

B

OPA177

(b)

No bias current

cancellation resistor needed

Conventional op amp with

external bias current

cancellation resistor.

OPA177 with no external

bias current cancellation

resistor.

= R

R

2

R

1

R

2

R

1

FIGURE 2. Input Bias Current Cancellation.