OP281/OP481
Rev. D | Page 13 of 20
APPLICATIONS
THEORY OF OPERATION
The OPx81 family of op amps is comprised of extremely low
powered, rail-to-rail output amplifiers, requiring less than 4 μA of
quiescent current per amplifier. Many other competitors’ devices
may be advertised as low supply current amplifiers but draw
significantly more current as the outputs of these devices are driven
to a supply rail. The supply current of the OPx81 remains under
4 μA even when the output is driven to either supply rail. Supply
currents should meet the specification as long as the inputs and
outputs remain within the range of the power supplies.
Figure 36 shows a simplified schematic of a single channel for
the OPx81. A bipolar differential pair is used in the input stage.
PNP transistors are used to allow the input stage to remain
linear with the common-mode range extending to ground. This
is an important consideration for single-supply applications.
The bipolar front end also contributes less noise than a MOS
front end with only nanoamps of bias currents. The output of
the op amp consists of a pair of CMOS transistors in a common
source configuration. This setup allows the output of the
amplifier to swing to within millivolts of either supply rail. The
headroom required by the output stage is limited by the amount
of current being driven into the load. The lower the output
current, the closer the output can go to either supply rail.
Figure 11, Figure 12, and Figure 13 show the output voltage
headroom vs. the load current. This behavior is typical of rail-
to-rail output amplifiers.
00291-036
+IN
–IN
V
EE
OUT
V
CC
Figure 36. Simplified Schematic of a Single OPx81 Channel
INPUT OVERVOLTAGE PROTECTION
The input stage to the OPx81 family of op amps consists of a
PNP differential pair. If the base voltage of either of these input
transistors drops to more than 0.6 V below the negative supply,
the input ESD protection diodes become forward-biased, and
large currents begin to flow. In addition to possibly damaging the
device, this creates a phase reversal effect at the output. To prevent
this, the input current should be limited to less than 0.5 mA.
This can be done by simply placing a resistor in series with the
input to the device. The size of the resistor should be proportional
to the lowest possible input signal excursion and can be found
using the following formula:
3
,
105.0
−
×
=
MININEE
VV
R
where:
V
EE
is the negative power supply for the amplifier.
V
IN, MIN
is the lowest input voltage excursion expected.
For example, a single channel of the OPx81 should be used with a
single-supply voltage of +5 V if the input signal may go as low as
−1 V. Because the amplifier is powered from a single supply, V
EE
is
the ground; therefore, the necessary series resistance should be 2 kΩ.
INPUT OFFSET VOLTAGE
The OPx81 family of op amps was designed for low offset
voltages (less than 1 mV).
00291-037
OP281
V
OUT
+3V
100kΩ
100kΩ
100kΩ
+
–
100kΩ
V
IN
= 1kHz AT
400mV p-p
–0.1V
–0.27V
Figure 37. Single OPx81 Channel Configured as a Difference Amplifier
Operating at V
CM
< 0 V
INPUT COMMON-MODE VOLTAGE RANGE
The OPx81 is rated with an input common-mode voltage range
from V
EE
to 1 V less than V
CC
. However, the op amp can operate
with a common-mode voltage that is slightly less than V
EE
.
Figure 37 shows a single OPx81 channel configured as a difference
amplifier with a single-supply voltage of 3 V. Negative dc voltages
are applied at both input terminals, creating a common-mode
voltage that is less than ground. A 400 mV p-p input signal is
then applied to the noninverting input. Figure 38 shows the
resulting input and output waves. Notice how the output of the
amplifier also drops slightly negative without distortion.
10
0%
100
90
0V
OUT
V
IN
0.1V
00291-038
0.2ms
Figure 38. Input and Output Signals with V
CM
< 0 V
