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OP481GRU View Datasheet(PDF) - Analog Devices

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OP481GRU Datasheet PDF : 16 Pages
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OP181/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 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 OPx81’s supply current remains
under 4 µA 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 32 shows a simplified schematic of the OP181. 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 consider-
ation 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. Figures 7, 8 and 9 show the
output voltage headroom versus load current. This behavior is
typical of rail-to-rail output amplifiers.
VCC
OUT
+IN
–IN
VEE
Figure 32. Simplified Schematic of the OP181
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:
R
=
VEE VIN , MIN
0.5 × 103
where: VEE is the negative power supply for the amplifier, and
VIN, MIN is the lowest input voltage excursion expected
For example, an OP181 is to be used with a single supply volt-
age of 5 V where the input signal could possibly go as low as
–1.0 V. Because the amplifier is powered from a single supply,
VEE is ground, so the necessary series resistance should be 2 k.
Input Offset Voltage Nulling
The OPx81 family of op amps was designed for low offset
voltages less than 1 mV. The single OP181 does provide two
offset adjust terminals, should the user require greater precision.
In general, these terminals should be used only to zero amplifier
offsets and should not be used to adjust system offset voltages.
A 20 kpotentiometer connected to the offset adjust terminals,
with the wiper connected to VEE, can be used to reduce the
offset voltage of the amplifier. The OP181 should be connected
in the unity-gain configuration (as shown in Figure 33) or in a
gain configuration. The potentiometer should be adjusted until
VOUT is minimized. The wiper of the potentiometer must be
connected to VEE; connecting it to the positive supply rail could
damage the device.
+5V
2
7
OP181 6
4
3
5
1
20k
POT.
VEE = –5V
VOUT
Figure 33. Offset Voltage Nulling Circuit
Input Common-Mode Voltage Range
The OPx81 is rated with an input common-mode voltage range
from VEE to 1 volt under VCC. However, the op amp can still
operate even with a common-mode voltage that is slightly less
than VEE. Figure 34 shows an OP181 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 35 shows
a picture of the input and output waves. Notice how the output
of the amplifier also drops slightly negative without distortion.
–0.27V
100k
100k
100k
VIN = 1kHz AT
400mV p-p
–0.1V
+3V
VOUT
OP181
100k
Figure 34. OP181 Configured as a Difference Amplifier
Operating at VCM < 0 V
–10–
REV. 0
 

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