OP281_15 Datasheet, PDF (14/20 Pages) Analog Devices – Ultralow Power, Rail-to-Rail Output Operational Amplifiers

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.
100
90
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.
3V
3V
3V
5.1kΩ
R1
V_H
D1
10kΩ
A1
Q1
R2
OP281-A _5.1kΩ
V_OUT
V_IN
2kΩ
3V
3V
R3 _{VL A2}
D2
OP281-B
R4
10
0%
Figure 39. Ringing and Overshoot of the Output of the Amplifier
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.
3V TO 12V
10kΩ
0.022µF
1MΩ
1MΩ
2
8
OP281 ¹
3
4
1µF
100Ω
1µF
V_REF
1.5V TO 6V
Figure 40. Single Channel Configured as a Micropower Bias Voltage Generator
Figure 41. Using the OP281 as a Window Comparator
The threshold limits for the window are set by VH and VL,
provided that VH > VL. The output of the first OP281 (A1) will
stay at the negative rail, in this case ground, as long as the input
voltage is less than VH. Similarly, the output of the second
OP281 (A2) will stay at ground as long the input voltage is
higher than VL. As long as VIN remains between VL and VH, the
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 VOUT to the positive supply rail.
If the input voltage rises above VH, the output of A2 stays at
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 VOUT low. The same condition occurs if VIN falls
below VL with A2’s output going high and D2 conducting
current. Therefore, VOUT is high if the input voltage is between
VL and VH, but low if the input voltage moves outside of that range.
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, VH
must be a greater voltage than VL.
V_H
=
R2
R1 + R2
V_L
=
R4
R3 + R4
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.
Rev. D | Page 14 of 20