REV. A

OP285

–9–

+15V

+

2

3

8

1

4

–15V

10pF

+

10 F

0.1 F

4.99k

2k

0.1 F

10 F

2.49k

4.99k

1/2

OP285

Figure 9. Unity-Gain Inverter

In inverting and noninverting applications, the feedback resis-

tance forms a pole with the source resistance and capacitance

pole can create excess phase shift and even oscillation. A small

is completely removed.

Figure 10. Compensating the Feedback Pole

High-Speed, Low-Noise Differential Line Driver

The circuit of Figure 11 is a unique line driver widely used in

industrial applications. With

±18 V supplies, the line driver can

deliver a differential signal of 30 V p-p into a 2.5 k

Ω load. The

high slew rate and wide bandwidth of the OP285 combine to

yield a full power bandwidth of 130 kHz while the low noise

front end produces a referred-to-input noise voltage spectral

density of 10 nV/

√Hz. The design is a transformerless, balanced

transmission system where output common-mode rejection of

noise is of paramount importance. Like the transformer-based

design, either output can be shorted to ground for unbalanced

line driver applications without changing the circuit gain of 1.

Other circuit gains can be set according to the equation in the

diagram. This allows the design to be easily set to noninverting,

inverting, or differential operation.

2

3

A2

1

3

2

A1

5

6

7

A3

R2

2k

A1 = 1/2OP285

A2, A3 = 1/2 OP285

GAIN = SET R2, R4, R5 = R1 AND R, R7, R8 = R2

1

R1

2k

R3

2k

R9

50

R11

1k

P1

10k

R12

1k

R4

2k

R5

2k

R6

2k

R10

50

R8

2k

R7

2k

Figure 11. High-Speed, Low-Noise Differential Line Driver

Low Phase Error Amplifier

The simple amplifier configuration of Figure 12 uses the OP285

and resistors to reduce phase error substantially over a wide

frequency range when compared to conventional amplifier designs.

This technique relies on the matched frequency characteristics

of the two amplifiers in the OP285. Each amplifier in the circuit

has the same feedback network which produces a circuit gain of

10. Since the two amplifiers are set to the same gain and are

matched due to the monolithic construction of the OP285, they

will exhibit identical frequency response. Recall from feedback

theory that a pole of a feedback network becomes a zero in the

loop gain response. By using this technique, the dominant pole

of the amplifier in the feedback loop compensates for the domi-

nant pole of the main amplifier,

1

2

3

A1

7

A2

5

6

R1

549

R2

4.99k

R3

499

R5

549

R4

4.99

A1, A2 = 1/2 OP285

Figure 12. Cancellation of A2’s Dominant Pole by A1