REV. D
OP295/OP495
–7–
APPLICATIONS
Rail-to-Rail Application Information
The OP295/OP495 has 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 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 V for use with high resolution ADCs
that operate from 5 V only supplies. The circuit in Figure 1 will
supply up to 10 mA. Its no-load drop-out voltage is only 20 mV.
This circuit will supply over 3.5 mA with a 5 V supply.
16k⍀
1/2
OP295/OP495
V = 4.5V
OUT
1F TO
10 F
10⍀
0.001F
20k⍀
REF43
2
6
4
5V
5V
Figure 1. 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.
In order to achieve both low noise and low supply voltage opera-
tion, discrete designs may provide the best solution. The circuit
in 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 1,000, 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.
10F
0.1F
R1LED
V
IN
2
35
6
3
71
MAT-03
Q1 Q2
R7
510⍀
C1
1500pF R4
R8
100⍀
R3
R2
27k⍀
R5
10k⍀
C2
10F
V
OUT
OP295/OP495
R6
10⍀
28
4
1
Q2
2N3906
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
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, respec-
tively. 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, typi-
cally the MAT03 has an h
FE
= 165. This leads to bias currents
of 11 µA and 3 µA, respectively. Based on the high bias cur-
rents, this circuit is best suited for applications with low source
impedance such as magnetic pickups or low impedance strain
gages. 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 Q2’s 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. Whenever 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 summa-
rized in Table I. Lastly, 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 will not include the positive rail, and the bandwidth will
reduce to approximately 250 Hz.