AD8551/AD8552/AD8554
R
2
APPLICATIONS
R
1
V2
V1
5 V PRECISION STRAIN GAGE CIRCUIT
V
OUT
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.
AD8551/
AD8552/
AD8554
R
3
R
4
R
R
R
R
R
R
4
2
2
IF
=
, THEN V
=
× (V1 – V2)
OUT
3
1
1
Figure 66. Using the AD855x as a Difference Amplifier
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
In an ideal difference amplifier, the ratio of the resistors are set
exactly equal to
R2 R4
R1 R3
AV =
=
(19)
Which sets the output voltage of the system to
OUT = AV (V1 − V2)
2×
(
R1 + R2
RB
)
(17)
V
(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
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.
2
5V
R1R4 + 2R2R4 + R2R3
3
2.5V
Q1
2N2222
CMRR =
(21)
REF192
4
1kΩ
6
2R1R4 − 2R2R3
A2
OR
AD8552-B
EQUIVALENT
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
12.0kΩ
20kΩ
4.0V
R
R
1
2
17.4kΩ 100Ω
V
OUT
0V TO 4.0V
40mV
FULL-SCALE
A1
350Ω
LOAD
CELL
1
AD8552-A
CMRRMIN
=
(22)
R
R
3
4
2δ
17.4kΩ 100Ω
AD8554-A
V2
R
NOTES
1. USE 0.1% TOLERANCE RESISTORS.
Figure 65. A 5 V Precision Strain Gage Amplifier
R
R
R
R
R
V
OUT
G
3 V INSTRUMENTATION AMPLIFIER
AD8554-C
R
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
R
V1
TRIM
AD8554-B
2R
V
= 1 +
(V1 – V2)
OUT
R
G
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.
⎛
⎞⎛
⎟⎜
⎟⎜
⎠⎝
⎞
⎟
⎟
⎠
⎛
⎜
⎜
⎝
⎞
⎟
⎟
⎠
R4
R1
R2
R2
R1
⎜
VOUT =V1
1+
−V2
(18)
⎜
R3 + R4
⎝
Rev. C | Page 20 of 24
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