OP191/OP291/OP491

REV. 0

–14–

Single Supply RTD Amplifier

The circuit in Figure 60 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’s 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 will work 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 k

Ω and 2.55

M

Ω resistor, which generates 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

V

OUT

365

365

1/4

OP491

100k

Ω

0.01pF

A3

+5V

GAIN = 274

100k

Ω

1/4

OP491

37.4k

+5V

AD589

2.55M

6.19k

200

Ω

10-TURNS

26.7k

26.7k

A2

A1

100

Ω

100

Ω

RTD

NOTE:

ALL RESISTORS 1% OR BETTER

Figure 60. Single Supply RTD Amplifier

Amplifiers A2 and A3 are configured in the two op amp IA

discussed above. Their 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.

APPLICATIONS

Single +3 V Supply, Instrumentation Amplifier

The OP291’s low supply current and low voltage operation

make it ideal for battery powered applications such as the

instrumentation amplifier shown in Figure 59. The circuit

utilizes 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 the figure.

The two resistors labeled R1 should be closely matched to each

other as well as both 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/(2

π × R1C1). If AC-CMRR is critical, than a matched

capacitor to C1 should be included across the second resistor

labeled R1.

1/2

OP291

V

OUT

3

2

1

R1

R2

R2

1/2

OP291

5

6

7

R1

C1

100pF

V

IN

8

4

+3V

V

R1

R2

Figure 59. 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

not appropriate for single supply circuits. The same could be

said for the traditional three op amp instrumentation amplifier.

In both cases, the circuits simply will not work in single supply

situations unless a false ground between the supplies is created.