AD8551/AD8552/AD8554

Rev. C | Page 20 of 24

APPLICATIONS

5 V PRECISION STRAIN GAGE CIRCUIT

The extremely low offset voltage of the AD8552 makes it an

ideal amplifier for any application requiring accuracy with high

gains, such as a weigh scale or strain gage. Figure 65 shows a

configuration for a single-supply, precision, strain gage

measurement system.

A REF192 provides a 2.5 V precision reference voltage for A2.

The A2 amplifier boosts this voltage to provide a 4.0 V refer-

ence for the top of the strain gage resistor bridge. Q1 provides

the current drive for the 350 Ω bridge network. A1 is used to

amplify the output of the bridge with the full-scale output

voltage equal to

()

B

2

1

R

R

R +

×

2

(17)

B

Using the values given in Figure 65, the output voltage linearly

varies from 0 V with no strain to 4.0 V under full strain.

NOTES

1. USE 0.1% TOLERANCE RESISTORS.

AD8552-A

AD8552-B

REF192

5V

2.5V

6

4

3

2

4.0V

A2

A1

0V TO 4.0V

40mV

FULL-SCALE

Q1

2N2222

OR

EQUIVALENT

350Ω

LOAD

CELL

1kΩ

12.0kΩ

20kΩ

17.4kΩ

100Ω

17.4kΩ

100Ω

Figure 65. A 5 V Precision Strain Gage Amplifier

3 V INSTRUMENTATION AMPLIFIER

The high common-mode rejection, high open-loop gain, and

operation down to 3 V of supply voltage makes the AD855x an

excellent choice of op amp for discrete single-supply instrumen-

tation amplifiers. The common-mode rejection ratio of the

AD855x is greater than 120 dB, but the CMRR of the system is

also a function of the external resistor tolerances. The gain of

the difference amplifier shown in Figure 66 is given as

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

+

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

+

=

1

2

2

1

4

3

4

OUT

R

R

V

R

R

R

R

R

V

V

2

1

1

(18)

V2

V1

AD8551/

AD8552/

AD8554

IF

Figure 66. Using the AD855x as a Difference Amplifier

In an ideal difference amplifier, the ratio of the resistors are set

exactly equal to

3

4

1

2

V

R

R

R

R

A

=

=

(19)

Which sets the output voltage of the system to

(20)

Due to finite component tolerance, the ratio between the four

resistors is not exactly equal, and any mismatch results in a

reduction of common-mode rejection from the system.

Referring to Figure 66, the exact common-mode rejection ratio

can be expressed as

3

2

4

1

3

2

4

2

4

1

R

R

R

R

R

R

R

R

R

R

CMRR

2

2

2

−

+

+

=

(21)

In the three-op amp, instrumentation amplifier configuration

shown in Figure 67, the output difference amplifier is set to

unity gain with all four resistors equal in value. If the tolerance

of the resistors used in the circuit is given as δ, the worst-case

CMRR of the instrumentation amplifier is

δ

CMRR

MIN

2

1

=

(22)

2R

(V1 – V2)

R

R

R

R

AD8554-C

V2

R

R

V1

AD8554-B

AD8554-A

Figure 67. A Discrete Instrumentation Amplifier Configuration

Consequently, using 1% tolerance resistors results in a worst-

case system CMRR of 0.02, or 34 dB. Therefore, either high

precision resistors or an additional trimming resistor, as shown

in Figure 67, should be used to achieve high common-mode

rejection. The value of this trimming resistor should be equal

to the value of R multiplied by its tolerance. For example, using

10 kΩ resistors with 1% tolerance requires a series trimming

resistor equal to 100 Ω.