OP281 Datasheet, PDF (10/16 Pages) Analog Devices – Ultralow Power, Rail-to-Rail Output Operational Amplifiers

OP281/OP481
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 mA
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 OPx81’s supply current remains
under 4 mA even with the output 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 1 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 nano-amps 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. TPCs 7, 8, and 9
show the output voltage headroom versus load current. This
behavior is typical of rail-to-rail output amplifiers.
V_CC
OUT
+IN
–IN
V_EE
Figure 1. 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 will become forward biased,
and large currents will begin to flow. In addition to possibly
damaging the device, this will create a phase reversal effect at
the output. To prevent these effects from happening, the input
current should be limited to less than 0.5 mA.
This can be done quite easily by placing a resistor in series with
the input to the device. The size of the resistor should be pro-
portional to the lowest possible input signal excursion and can
be found using the following formula:
where:
R
=
V_EE - V_{IN , MIN}
0.5 ¥ 10^-3
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 is to be used with a
single-supply voltage of +5 V where the input signal could possibly
go as low as –1 V. Because the amplifier is powered from a single
supply, V_EE is ground, so the necessary series resistance should
be 2 kW.
Input Offset Voltage
The OPx81 family of op amps was designed for low offset
voltages less than 1 mV.
–0.27V
100k
+3V
100k
100k
V_IN = 1kHz AT
400mV p-p
100k
V_OUT
OP281
–0.1V
Figure 2. 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 under V_CC. However, the op amp can still oper-
ate even with a common-mode voltage that is slightly less than V_EE.
Figure 2 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 volt-
age that is less than ground. A 400 mV p-p input signal is then
applied to the noninverting input. Figure 3 shows the input and
output waves. Notice how the output of the amplifier also drops
slightly negative without distortion.
100
90
V_OUT
V_IN
10
0%
0.1V
0.2ms
0V
Figure 3. Input and Output Signals with V_CM < 0 V
–10–
REV. B