OP281 Analog Devices, OP281 Datasheet - Page 14

• Manufacturer Part Number: OP281
• Description: Ultralow Power, Rail-to-Rail Output Operational Amplifier (Dual)
• Manufacturer: Analog Devices

Specifications of OP281

• -3db Bandwidth: 105kHz
• Slew Rate: 28mV/µs
• Vos: 100µV
• Ib: 3nA
• # Opamps Per Pkg: 2
• Input Noise (nv/rthz): 75nV/rtHz
• Vcc-vee: 2.7V to 12V
• Isy Per Amplifier: 5µA
• Packages: SOIC,SOP

OP281/OP481
CAPACITIVE LOADING
Most low supply current amplifiers have difficulty driving
capacitive loads due to the higher currents required from the
output stage for such loads. Higher capacitance at the output
will increase the amount of overshoot and ringing in the amplifier’s
step response and may affect the stability of the device. However,
through careful design of the output stage and its high phase
margin, the OPx81 family can tolerate some degree of capacitive
loading. Figure 39 shows the step response of a single channel
with a 10 nF capacitor connected at the output. Notice that the
overshoot of the output does not exceed more than 10% with
such a load, even with a supply voltage of only 3 V.
MICROPOWER REFERENCE VOLTAGE GENERATOR
Many single-supply circuits are configured with the circuit biased
to half of the supply voltage. In these cases, a false ground reference
can be created by using a voltage divider buffered by an amplifier.
Figure 40 shows the schematic for such a circuit.
The two 1 MΩ resistors generate the reference voltage while
drawing only 1.5 μA of current from a 3 V supply. A capacitor
connected from the inverting terminal to the output of the op amp
provides compensation to allow a bypass capacitor to be
connected at the reference output. This bypass capacitor helps
to establish an ac ground for the reference output. The entire
reference generator draws less than 5 μA from a 3 V supply source.
Figure 40. Single Channel Configured as a Micropower Bias Voltage Generator
100
Figure 39. Ringing and Overshoot of the Output of the Amplifier
0%
90
10
1MΩ
1MΩ
1µF
2
3
OP281
3V TO 12V
8
4
0.022µF
10kΩ
1
100Ω
1µF
V
1.5V TO 6V
REF
Rev. D | Page 14 of 20
WINDOW COMPARATOR
The extremely low power supply current demands of the OPx81
family make it ideal for use in long-life battery-powered
applications such as a monitoring system. Figure 41 shows a
circuit that uses the OP281 as a window comparator.
The threshold limits for the window are set by V
provided that V
stay at the negative rail, in this case ground, as long as the input
voltage is less than V
OP281 (A2) will stay at ground as long the input voltage is
higher than V
outputs of both op amps will be 0 V. With no current flowing in
either D1 or D2, the base of Q1 will stay at ground, putting the
transistor in cutoff and forcing V
If the input voltage rises above V
ground, but the output of A1 goes to the positive rail and D1
conducts current. This creates a base voltage that turns on Q1
and drives V
below V
current. Therefore, V
V
The R1 and R2 voltage divider sets the upper window voltage,
and the R3 and R4 voltage divider sets the lower voltage for the
window. For the window comparator to function properly, V
must be a greater voltage than V
The 2 kΩ resistor connects the input voltage of the input
terminals to the op amps. This protects the OP281 from
possible excess current flowing into the input stages of the
devices. D1 and D2 are small-signal switching diodes (1N4446
or equivalent), and Q1 is a 2N2222 or an equivalent NPN
transistor.
L
and V
V
V
V
IN
H
L
L
H
=
=
with A2’s output going high and D2 conducting
Figure 41. Using the OP281 as a Window Comparator
, but low if the input voltage moves outside of that range.
R3
R1
2kΩ
R1
R2
R3
R4
OUT
R4
L
R2
+
3V
3V
+
. As long as V
H
low. The same condition occurs if V
R4
R2
V
V
> V
H
L
H
L
OUT
A1
A2
. The output of the first OP281 (A1) will
. Similarly, the output of the second
3V
3V
is high if the input voltage is between
OP281-A
OP281-B
IN
D1
D2
remains between V
L
H
OUT
.
, the output of A2 stays at
to the positive supply rail.
10kΩ
5.1kΩ
5.1kΩ
3V
Q1
H
L
and V
and V
IN
V
OUT
falls
L
,
H
, the
H