OP295/OP495

REV. B

–7–

APPLICATIONS

Rail-to-Rail Applications Information

The OP295/OP495 has a wide common-mode input range ex-

tending 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 may 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 it is always required that there 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 volts for use with high resolution A/D

converters that operate from +5 volt only supplies. The circuit

in Figure 1 will supply up to 10 mA. Its no-load drop-out volt-

age is only 20 mV. This circuit will supply over 3.5 mA with a

+5 volt supply.

16k

V

1 TO 10

µF

10

Ω

0.001

µF

20k

REF43

2

6

4

+5V

+5V

1/2

OP295/

OP495

Figure 1. 4.5 Volt, 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 characteris-

tics 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.

In order to achieve both low noise and low supply voltage opera-

tion, discrete designs may provide the best solution. The circuit

on Figure 2 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

provides rail-to-rail output swings, allowing this circuit to oper-

ate with 0 to 5 volt outputs. Only half of the OP295/OP495 is

used, leaving the other uncommitted op amp for use elsewhere.

MAT- 03

5

6

3

2

1

7

R3

R4

Q1

Q2

R1

R6

10

Ω

V

OUT

LED

1

8

4

3

2

R5

10k

Ω

C2

10

µF

R7

510

Ω

C1

1500pF

R8

100

Ω

R2

27k

Ω

V

IN

0.1

µF

10

µF

Q2

2N3906

OP295/

OP495

Figure 2. Low Noise Single Supply Preamplifier

The input noise is controlled by the MAT03 transistor pair and

the collector current level. Increasing the collector current re-

duces the voltage noise. This particular circuit was tested with

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

put voltage noise was 3.1 nV/

√Hz and 10 nV/√Hz, respectively.

The high collector currents do lead to a tradeoff in supply cur-

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

increase with increasing collector current. For example, typically

11

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

this circuit is best suited for applications with low source imped-

ance such as magnetic pickups or low impedance strain gages.

Furthermore, a high source impedance will degrade the noise

performance. For example, a 1 k

Ω 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” that has a temperature co-

efficient close to that of Q2’s base-emitter junction, which pro-

vides 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 collec-

tor current. Whenever R1 is changed, R3 and R4 should also be

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

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

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

have to be small enough to prevent this condition. Their values

and the overall performance for two different values of R1 are

summarized in Table I. Lastly, the potentiometer, R8, is needed

to adjust the offset voltage to null it to zero. Similar perfor-

mance can be obtained using an OP90 as the output amplifier

with a savings of about 185

µA of supply current. However, the

output swing will not include the positive rail, and the band-

width will reduce to approximately 250 Hz.