OP181/OP281/OP481

–10–

REV. 0

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.

+IN

–IN

OUT

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

=

0.5

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,

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 k

Ω potentiometer connected to the offset adjust terminals,

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

damage the device.

6

5

7

4

1

2

3

+5V

OP181

20k

Ω

POT.

Figure 33. Offset Voltage Nulling Circuit

Input Common-Mode Voltage Range

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

operate even with a common-mode voltage that is slightly less

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.

OP181

+3V

100k

Ω

100k

Ω

100k

Ω

100k

Ω

400mV p-p

–0.1V

–0.27V

Figure 34. OP181 Configured as a Difference Amplifier