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OP481 Datasheet(PDF) 11 Page - Analog Devices

Part # OP481
Description  Ultralow Power, Rail-to-Rail Output Operational Amplifiers
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OP481 Datasheet(HTML) 11 Page - Analog Devices

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REV. B
OP281/OP481
–11–
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 could even 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 4 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.
10
0%
100
90
Figure 4. Ringing and Overshoot of the Output
of the Amplifier
Micropower Reference Voltage Generator
Many single-supply circuits are configured with the circuit biased
to 1/2 of the supply voltage. In these cases, a false ground refer-
ence can be created by using a voltage divider buffered by an
amplifier. Figure 5 shows the schematic for such a circuit.
The two 1 M
W resistors generate the reference voltage while
drawing only 1.5
mA 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 for 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
mA from a 3 V supply source.
1
8
4
2
3
OP281
10k
0.022 F
VREF
1.5V TO 6V
1 F
1 F
1M
3V TO 12V
100
1M
Figure 5. Single Channel Configured as a Micropower
Bias Voltage Generator
Window Comparator
The extremely low power supply current demands of the OPx81
family make it ideal for use in long-life battery-powered applica-
tions such as a monitoring system. Figure 6 shows a circuit that
uses the OP281 as a window comparator.
A1
R1
R2
3V
OP281-A
VIN
2k
5.1k
3V
3V
VOUT
Q1
5.1k
VH
D1
10k
A2
R3
R4
3V
3V
VL
D2
OP281-B
Figure 6. Using the OP281 as a Window Comparator
The threshold limits for the window are set by VH and VL, pro-
vided that VH > VL. The output of 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 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 will go to the
positive rail, and D1 will conduct current. This creates a base
voltage that will turn on Q1 and drive VOUT low. The same
condition occurs if VIN falls below VL with A2’s output going
high, and D2 conducting current. Therefore, VOUT will be high
if the input voltage is between VL and VH, and VOUT will be 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.
VH
=
R2
R1
+ R2
VL
=
R4
R3
+ R4
The 2 k
W resistor connects the input voltage to 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 equivalent NPN transistor.


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