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AD8551/AD8552/AD8554

Data Sheet

Rev. F | Page 20 of 24

APPLICATIONS INFORMATION

A 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 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

(

)

R +

(17)

where RB is the resistance of the load cell.

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

2.5V

4.0V

VOUT

0V TO 4.0V

40mV

FULL-SCALE

2N2222

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 AD8551/

AD8552/AD8554 an excellent choice of op amp for discrete

single-supply instrumentation amplifiers. The common-mode

rejection ratio of the AD8551/AD8552/AD8554 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













−

























OUT

(18)

VOUT

AD8551/

AD8552/

AD8554

, THEN VOUT =

× (V1 – V2)

Figure 66. Using the AD8551/AD8552/AD8554 as a Difference Amplifier

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

exactly equal to

(19)

Which sets the output voltage of the system to

VOUT = 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 66, the exact common-mode rejection ratio can be

expressed as

CMRR

−

(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

CMRRMIN

(22)

VOUT = 1 +

(V1 – V2)

AD8554-C

AD8554-B

AD8554-A

RTRIM

VOUT

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,

must be used to achieve high common-mode rejection. The

value of this trimming resistor must 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.

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