OP295/OP495
Rev. G | 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 19
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
V
OUT
=4.5V
5V
5V
16kΩ
10Ω
20kΩ
0.001µF
REF43
1µF TO
10µF
00331-017
Figure 19. 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 20 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
53
71
Q1 Q2
MAT03
0.1µF
R1
LED
R4R3
OP295/OP495
10µF
R6
10Ω
V
OUT
C2
10µF
R5
10kΩ
Q2
2N3906
R7
510Ω
R2
27kΩ
R8
100Ω
C1
1500pF
V
IN
00331-018
Figure 20. 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, respect-
ively. 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, typically
the MAT03 has an h
FE
= 165. This leads to bias 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.
