OP496_15 Datasheet, PDF (13/19 Pages) Analog Devices – Micropower RRIO Operational Amplifiers

OP196/OP296/OP496
A Micropower False-Ground Generator
Some single supply circuits work best when inputs are biased
above ground, typically at 1/2 of the supply voltage. In these
cases, a false-ground can be created by using a voltage divider
buffered by an amplifier. One such circuit is shown in Figure 5.
This circuit will generate a false-ground reference at 1/2 of the
supply voltage, while drawing only about 55 µA from a 5 V
supply. The circuit includes compensation to allow for a 1 µF
bypass capacitor at the false-ground output. The benefit of a
large capacitor is that not only does the false-ground present a
very low dc resistance to the load, but its ac impedance is low as well.
240k⍀
5V OR 12V
10k⍀
0.022␮F
240k⍀
2
7
OP196 ⁶
3
4
1␮F
100⍀
2.5V OR 6V
1␮F
Figure 5. A Micropower False-Ground Generator
Single-Supply Half-Wave and Full-Wave Rectifiers
An OP296, configured as a voltage follower operating from a
single supply, can be used as a simple half-wave rectifier in low
frequency (<400 Hz) applications. A full-wave rectifier can be
configured with a pair of OP296s as illustrated in Figure 6.
R1
100k⍀
R2
100k⍀
2Vp-p
<500Hz
5V
2k⍀
3
8
A1 ¹
2 4 _1/2
OP296
6
A2 7
5
1/2
OP296
V_OUTA
FULL-WAVE
RECTIFIED
OUTPUT
V_OUTB
HALF-WAVE
RECTIFIED
OUTPUT
1V
100
INPUT 90
500mV
500µs
V_OUTB
(HALF-WAVE
OUTPUT)
10
V_OUTA 0%
(FULL-WAVE
OUTPUT)
500mV
f = 500Hz
Figure 6. Single-Supply Half-Wave and Full-Wave
Rectifiers Using an OP296
The circuit works as follows: When the input signal is above
0 V, the output of amplifier A1 follows the input signal. Since
the noninverting input of amplifier A2 is connected to A1’s
output, op amp loop control forces A2’s inverting input to the
same potential. The result is that both terminals of R1 are at the
same potential and no current flows in R1. Since there is no
current flow in R1, the same condition must exist in R2; thus,
the output of the circuit tracks the input signal. When the input
signal is below 0 V, the output voltage of A1 is forced to 0 V.
This condition now forces A2 to operate as an inverting voltage
follower because the noninverting terminal of A2 is also at 0 V.
The output voltage of V_OUTA is then a full-wave rectified
version of the input signal. A resistor in series with A1’s
noninverting input protects the ESD diodes when the input
signal goes below ground.
Square Wave Oscillator
The oscillator circuit in Figure 7 demonstrates how a rail-to-rail
output swing can reduce the effects of power supply variations
on the oscillator’s frequency. This feature is especially valuable
in battery powered applications, where voltage regulation may
not be available. The output frequency remains stable as the
supply voltage changes because the RC charging current, which
is derived from the rail-to-rail output, is proportional to the
supply voltage. Since the Schmitt trigger threshold level is also
proportional to supply voltage, the frequency remains relatively
independent of supply voltage. For a supply voltage change
from 9 V to 5 V, the output frequency only changes about 4 Hz.
The slew rate of the amplifier limits the oscillation frequency to
a maximum of about 200 Hz at a supply voltage of 5 V.
V+
100k⍀
59k⍀
100k⍀
C
38
1
2 4 _1/2
OP296/
R OP496
FREQ OUT
f_OSC
=
1
RC
<
200Hz
@
V+
=
5V
Figure 7. Square Wave Oscillator Has Stable Frequency
Regardless of Supply Voltage Changes
A 3 V Low Dropout, Linear Voltage Regulator
Figure 8 shows a simple 3 V voltage regulator design. The regu-
lator can deliver 50 mA load current while allowing a 0.2 V
dropout voltage. The OP296’s rail-to-rail output swing easily
drives the MJE350 pass transistor without requiring special
drive circuitry. With no load, its output can swing to less than
the pass transistor’s base-emitter voltage, turning the device
nearly off. At full load, and at low emitter-collector voltages, the
transistor beta tends to decrease. The additional base current is
easily handled by the OP296 output.
The AD589 provides a 1.235 V reference voltage for the regula-
tor. The OP296, operating with a noninverting gain of 2.43,
drives the base of the MJE350 to produce an output voltage of
3.0 V. Since the MJE350 operates in an inverting (common-
emitter) mode, the output feedback is applied to the OP296’s
noninverting input.
REV. E
–13–