AD8571ARM-R2 datasheet(20/24 Pages) AD

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AD8571/AD8572/AD8574

Rev. C | Page 20 of 24

APPLICATIONS

VOUT

AD8571/

AD8572/

AD8574

, THEN VOUT =

(V1 – V2)

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 63 shows a configura-

tion for a single-supply, precision strain gage measurement system.

The 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

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

Figure 64. Using the AD857x as a Difference Amplifier

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

equal to

(

)

R +

(17)

(19)

Set the output voltage of the system to

where RB is the resistance of the load cell.

Using the values given in Figure 63, the output voltage linearly

varies from 0 V with no strain to 4 V under full strain.

VOUT

AD8572-A

17.4kΩ

100Ω

17.4kΩ

100Ω

0V TO 4V

NOTE:

USE 0.1% TOLERANCE RESISTORS.

20kΩ

AD8572-B

REF192

12kΩ

1kΩ

2.5V

4.0V

40mV

FULL-SCALE

350Ω

LOAD

CELL

2N2222

EQUIVALENT

Figure 63. 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 make the

AD857x family an excellent choice of op amp for discrete

single-supply instrumentation 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 difference amplifier shown in Figure 64

is given as

⎟

⎠

⎞

⎜

⎝

⎛

−

⎟

⎠

⎞

⎜

⎝

⎛ +

⎟

⎠

⎞

⎜

⎝

⎛

OUT

(18)

OUT

= AV (V1 − V2)

(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 64, the exact common-mode rejection ratio can be

expressed as

R2R3

R1R4

R2R3

R2R4

1R4

CMRR

−

(21)

In the 3-op amp instrumentation amplifier configuration shown

in Figure 65, 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

MIN

CMRR

(22)

VOUT

AD8574-C

AD8574-B

AD8574-A

RTRIM

VOUT = 1 +

(V1 – V2)

Figure 65. Discrete Instrumentation Amplifier Configuration

Therefore, using 1% tolerance resistors results in a worst-case

system CMRR of 0.02, or 34 dB. To achieve high common-

mode rejection, either high precision resistors or an additional

trimming resistor, as shown in Figure 65, should be used. 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 would require a series trimming resistor

equal to 100 Ω.

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