OP295/OP495
APPLICATIONS
Rail-to-Rail Applications Information
0.1µF
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
LED
R1
Q2
2N3906
3
5
10µ F
V
IN
2
Q1
1
MAT- 03
Q2
7
6
R5
10kΩ
1
R6
10Ω
2
R7
510Ω
C1
1500pF
R4
R8
100Ω
3
8
C2
10µF
V
OUT
OP295/
OP495
4
R2
27kΩ
R3
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
+5V
0.001µF
+5V
20k
2
REF43
4
6
10Ω
V
OUT
= 4.5V
Figure 2. Low Noise Single Supply Preamplifier
1/2
OP295/
OP495
1 TO 10µF
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.
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
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 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 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” 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.
REV. B
–7–
AD [ ANALOG DEVICES ]
