OP295/OP495

Rev. E | Page 10 of 16

Finally, the potentiometer, R8, is needed to adjust the offset

voltage to null it to zero. Similar performance can be obtained

using an OP90 as the output amplifier with a savings of about

185 μA of supply current. However, the output swing does not

include the positive rail, and the bandwidth reduces to approxi-

mately 250 Hz.

Table 6. Single-Supply Low Noise Preamp Performance

R1

270 Ω

1.0 kΩ

R3, R4

200 Ω

910 Ω

3.15 nV/√Hz

8.6 nV/√Hz

4.2 nV/√Hz

10.2 nV/√Hz

4.0 mA

1.3 mA

11 μA

3 μA

Bandwidth

1 kHz

1 kHz

Closed-Loop Gain

1000

1000

DRIVING HEAVY LOADS

The OP295/OP495 are well suited to drive loads by using a

power transistor, Darlington, or FET to increase the current to

the load. The ability to swing to either rail can assure that the

device is turned on hard. This results in more power to the load

and an increase in efficiency over using standard op amps with

their limited output swing. Driving power FETs is also possible

with the OP295/OP495 because of their ability to drive capaci-

tive loads of several hundred picofarads without oscillating.

Without the addition of external transistors, the OP295/OP495

can drive loads in excess of ±15 mA with ±15 V or +30 V

supplies. This drive capability is somewhat decreased at lower

supply voltages. At ±5 V supplies, the drive current is ±11 mA.

Driving motors or actuators in two directions in a single-supply

application is often accomplished using an H bridge. The

principle is demonstrated in Figure 20. From a single 5 V

supply, this driver is capable of driving loads from 0.8 V to

4.2 V in both directions. Figure 21 shows the voltages at the

inverting and noninverting outputs of the driver. There is a

small crossover glitch that is frequency-dependent; it does not

cause problems unless used in low distortion applications, such

as audio. If this is used to drive inductive loads, diode clamps

should be added to protect the bridge from inductive kickback.

10kΩ

1.67V

2N2222

2N2222

OUTPUTS

2N2907

2N2907

5V

10kΩ

10kΩ

–

+

–

+

Figure 20. H Bridge

10

90

100

0%

2V

2V

1ms

Figure 21. H Bridge Outputs

DIRECT ACCESS ARRANGEMENT

The OP295/OP495 can be used in a single-supply direct access

arrangement (DAA), as shown in Figure 22. This figure shows

a portion of a typical DM capable of operating from a single 5 V

supply, and it may also work on 3 V supplies with minor modi-

fications. Amplifier A2 and Amplifier A3 are configured so that

the transmit signal, TxA, is inverted by A2 and is not inverted

by A3. This arrangement drives the transformer differentially so

the drive to the transformer is effectively doubled over a single

amplifier arrangement. This application takes advantage of the

ability of the OP295/OP495 to drive capacitive loads and to save

power in single-supply applications.

–

+

2.5V REF

A3

750pF

A1

37.4kΩ

390pF

RxA

TxA

A2

3.3kΩ

20kΩ

475Ω

22.1kΩ

20kΩ

20kΩ

20kΩ

20kΩ

0.033µF

1:1

–

+

0.0047µF

OP295/

OP495

OP295/

OP495

OP295/

OP495

0.1µF

0.1µF

Figure 22. Direct Access Arrangement

SINGLE-SUPPLY INSTRUMENTATION AMPLIFIER

The OP295/OP495 can be configured as a single-supply

instrumentation amplifier, as shown in Figure 23. For this

includes ground, and the output swings to both rails.