OP191GSZ datasheet(19/24 Pages) AD

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OP191/OP291/OP491

Rev. D | Page 19 of 24

APPLICATIONS

SINGLE 3 V SUPPLY, INSTRUMENTATION

AMPLIFIER

The OP291 low supply current and low voltage operation

make it ideal for battery-powered applications, such as the

instrumentation amplifier shown in Figure 65. The circuit uses

the classic two op amp instrumentation amplifier topology, with

four resistors to set the gain. The equation is simply that of a

noninverting amplifier, as shown in Figure 65. The two resistors

labeled R1 should be closely matched both to each other and to

the two resistors labeled R2 to ensure good common-mode

rejection performance. Resistor networks ensure the closest

matching as well as matched drifts for good temperature

stability. Capacitor C1 is included to limit the bandwidth and,

therefore, the noise in sensitive applications. The value of this

capacitor should be adjusted depending on the desired closed-

loop bandwidth of the instrumentation amplifier. The RC

combination creates a pole at a frequency equal to 1/(2π ×

R1C1). If AC-CMRR is critical, then a matched capacitor to C1

should be included across the second resistor labeled R1.

1/2

OP291

1/2

OP291

100pF

VIN

VOUT

VOUT = (1 +

) = VIN

–

Figure 65. Single 3 V Supply Instrumentation Amplifier

Because the OP291 accepts rail-to-rail inputs, the input

common-mode range includes both ground and the positive

supply of 3 V. Furthermore, the rail-to-rail output range ensures

the widest signal range possible and maximizes the dynamic

range of the system. Also, with its low supply current of

300 μA/device, this circuit consumes a quiescent current of

only 600 μA yet still exhibits a gain bandwidth of 3 MHz.

A question may arise about other instrumentation amplifier

topologies for single-supply applications. For example, a

variation on this topology adds a fifth resistor between the two

inverting inputs of the op amps for gain setting. While that

topology works well in dual-supply applications, it is inherently

inappropriate for single-supply circuits. The same could be said

for the traditional three op amp instrumentation amplifier. In

both cases, the circuits simply cannot work in single-supply

situations unless a false ground between the supplies is created.

SINGLE-SUPPLY RTD AMPLIFIER

The circuit in Figure 66 uses three op amps of the OP491 to

develop a bridge configuration for an RTD amplifier that

operates from a single 5 V supply. The circuit takes advantage of

the OP491 wide output swing range to generate a high bridge

excitation voltage of 3.9 V. In fact, because of the rail-to-rail

output swing, this circuit works with supplies as low as 4.0 V.

Amplifier A1 servos the bridge to create a constant excitation

current in conjunction with the AD589, a 1.235 V precision

reference. The op amp maintains the reference voltage across

the parallel combination of the 6.19 kΩ and 2.55 MΩ resistors,

which generate a 200 μA current source. This current splits

evenly and flows through both halves of the bridge. Thus,

100 μA flows through the RTD to generate an output voltage

based on its resistance. A 3-wire RTD is used to balance the line

resistance in both 100 Ω legs of the bridge to improve accuracy.

1/4

OP491

VOUT

365Ω

1/4

OP491

100kΩ

0.01pF

GAIN = 274

100kΩ

1/4

OP491

37.4kΩ

AD589

2.55MΩ

6.19kΩ

200Ω

10 TURNS

26.7kΩ

100Ω

RTD

ALL RESISTORS 1% OR BETTER

Figure 66. Single-Supply RTD Amplifier

Amplifier A2 and Amplifier A3 are configured in the two op

amp instrumentation amplifier topology described in the Single

3 V Supply, Instrumentation Amplifier section. The resistors are

chosen to produce a gain of 274, such that each 1°C increase in

temperature results in a 10 mV change in the output voltage, for

ease of measurement. A 0.01 μF capacitor is included in parallel

with the 100 kΩ resistor on Amplifier A3 to filter out any

unwanted noise from this high gain circuit. This particular RC

combination creates a pole at 1.6 kHz.

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