OP281/OP481

Rev. C | 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.

+IN

–IN

OUT

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

,

10

5

.

0

−

×

−

=

MIN

IN

EE

V

V

R

where:

is the negative power supply for the amplifier.

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

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).

OP281

+3V

100kΩ

100kΩ

100kΩ

+

–

100kΩ

400mV p-p

–0.1V

–0.27V

Figure 37. Single OPx81 Channel Configured as a Difference Amplifier

INPUT COMMON-MODE VOLTAGE RANGE

The OPx81 is rated with an input common-mode voltage range

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

0.1V

0.2ms