AD8571/AD8572/AD8574

–15–

REV. 0

100k

AD857x

100k

Figure 54b. AD857x Test Circuit for Turn-On Time

APPLICATIONS

A 5 V Precision Strain-Gage Circuit

The extremely low offset voltage of the AD8572 makes it an ideal

amplifier for any application requiring accuracy with high gains,

such as a weigh scale or strain-gage. Figure 55 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 reference

for the top of the strain-gage resistor bridge. Q1 provides the cur-

rent 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:

2

12

×+

()

RR

R

B

(17)

in Figure 55, the output voltage will linearly vary from 0 V with

no strain to 4 V under full strain.

350

LOAD

CELL

AD8572-A

17.4k

100

17.4k

100

0V TO 4V

NOTE:

USE 0.1% TOLERANCE RESISTORS.

20k

A1

AD8572-B

REF192

12k

1k

5V

2.5V

6

4

3

2

4.0V

40mV

FULL-SCALE

Q1

2N2222

OR

EQUIVALENT

A2

Figure 55. 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 AD857x an

excellent choice of op amp for discrete single supply instrumenta-

tion amplifiers. The common-mode rejection ratio of the AD857x

is greater than 120 dB, but the CMRR of the system is also a

function of the external resistor tolerances. The gain of the differ-

ence amplifier shown in Figure 56 is given as:

VV

R

RR

R

R

V

R

R

+









+









−









11

2

4

34

1

2

2

1

(18)

V2

V1

AD857x

IF

=

(V1

V2)

Figure 56. Using the AD857x as a Difference Amplifier

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

exactly equal to:

A

R

R

R

R

2

1

4

3

(19)

Which sets the output voltage of the system to:

VA V

V

OUT

V

=−

()

12

(20)

Due to finite component tolerance the ratio between the four

resistors will not be exactly equal, and any mismatch results in a

reduction of common-mode rejection from the system. Referring

to Figure 56, the exact common-mode rejection ratio can be

expressed as:

CMRR

R R

RR

RR

RR

R R

=

++

−

1

4

24

23

14

2

3

2

22

(21)

In the 3 op amp instrumentation amplifier configuration shown

in Figure 57, the output difference amplifier is set to unity gain

with all four resistors equal in value. If the tolerance of the resis-

tors used in the circuit is given as

δ, the worst-case CMRR of

the instrumentation amplifier will be:

CMRR

1

2

δ

(22)

R

R

R

R

AD8574-C

V2

R

R

V1

AD8574-B

AD8574-A

2R

(V1

V2)

Figure 57. A Discrete Instrumentation Amplifier

Configuration

Thus, using 1% tolerance resistors would result 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 57,

should be used to achieve high common-mode rejection. The value

of this trimming resistor should be equal to the value of R multi-

plied by its tolerance. For example, using 10 k

Ω resistors with 1%

tolerance would require a series trimming resistor equal to 100

Ω.