OP295/OP495

Rev. E | Page 9 of 16

APPLICATIONS

RAIL-TO-RAIL APPLICATION INFORMATION

The OP295/OP495 have a wide common-mode input range

extending from ground to within about 800 mV of the positive

supply. There is a tendency to use the OP295/OP495 in buffer

applications where the input voltage could exceed the common-

mode input range. This can initially appear to work because of

the high input range and rail-to-rail output range. But above the

common-mode input range, the amplifier is, of course, highly

nonlinear. For this reason, there must be some minimal amount

of gain when rail-to-rail output swing is desired. Based on the

input common-mode range, this gain should be at least 1.2.

LOW DROP-OUT REFERENCE

The OP295/OP495 can be used to gain up a 2.5 V or other low

voltage reference to 4.5 V for use with high resolution ADCs

that operate from 5 V only supplies. The circuit in Figure 18

supplies up to 10 mA. Its no-load drop-out voltage is only

20 mV. This circuit supplies over 3.5 mA with a 5 V supply.

4

2

6

–

+

+

1/2

OP295/OP495

5V

5V

16kΩ

10Ω

20kΩ

0.001µF

REF43

1µF TO

10µF

Figure 18. 4.5 V, Low Drop-Out Reference

LOW NOISE, SINGLE-SUPPLY PREAMPLIFIER

Most single-supply op amps are designed to draw low supply

current at the expense of having higher voltage noise. This tradeoff

may be necessary because the system must be powered by a

battery. However, this condition is worsened because all circuit

resistances tend to be higher; as a result, in addition to the op

amp’s voltage noise, Johnson noise (resistor thermal noise) is

also a significant contributor to the total noise of the system.

The choice of monolithic op amps that combine the character-

istics of low noise and single-supply operation is rather limited.

Most single-supply op amps have noise on the order of 30 nV/√Hz

to 60 nV/√Hz, and single-supply amplifiers with noise below

5 nV/√Hz do not exist.

To achieve both low noise and low supply voltage operation,

discrete designs may provide the best solution. The circuit in

Figure 19 uses the OP295/OP495 rail-to-rail amplifier and a

matched PNP transistor pair—the MAT03—to achieve zero-

in/zero-out single-supply operation with an input voltage noise

of 3.1 nV/√Hz at 100 Hz.

R5 and R6 set the gain of 1000, making this circuit ideal for

maximizing dynamic range when amplifying low level signals in

single-supply applications. The OP295/OP495 provide rail-to-

rail output swings, allowing this circuit to operate with 0 V to

5 V outputs. Only half of the OP295/OP495 is used, leaving the

other uncommitted op amp for use elsewhere.

1

2

3

4

8

–

+

+–

26

5

3

7

1

Q1

Q2

MAT03

0.1µF

R1

LED

R4

R3

OP295/OP495

10µF

R6

10Ω

C2

10µF

R5

10kΩ

Q2

2N3906

R7

510Ω

R2

27kΩ

R8

100Ω

C1

1500pF

Figure 19. Low Noise Single-Supply Preamplifier

The input noise is controlled by the MAT03 transistor pair and

the collector current level. Increasing the collector current

reduces the voltage noise. This particular circuit was tested with

1.85 mA and 0.5 mA of current. Under these two cases, the

input voltage noise was 3.1 nV/√Hz and 10 nV/√Hz, respectively.

The high collector currents do lead to a tradeoff in supply

current, bias current, and current noise. All of these parameters

increase with increasing collector current. For example,

currents of 11 μA and 3 μA, respectively. Based on the high bias

currents, this circuit is best suited for applications with low

source impedance such as magnetic pickups or low impedance

strain gauges. Furthermore, a high source impedance degrades

the noise performance. For example, a 1 kΩ resistor generates

4 nV/√Hz of broadband noise, which is already greater than the

noise of the preamp.

The collector current is set by R1 in combination with the LED

and Q2. The LED is a 1.6 V Zener diode that has a temperature

coefficient close to that of the Q2 base-emitter junction, which

provides a constant 1.0 V drop across R1. With R1 equal to

270 Ω, the tail current is 3.7 mA and the collector current is half

that, or 1.85 mA. The value of R1 can be altered to adjust the

collector current. When R1 is changed, R3 and R4 should also

be adjusted. To maintain a common-mode input range that

includes ground, the collectors of the Q1 and Q2 should not go

above 0.5 V; otherwise, they could saturate. Thus, R3 and R4

must be small enough to prevent this condition. Their values

and the overall performance for two different values of R1 are

summarized in Table 6.