
AD8551/AD8552/AD8554
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.
Rev. C | Page 20 of 24
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
(
)
2
1
R
B
R
R
+
×
2
(17)
where
R
B
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.
Figure 65. A 5 V Precision Strain Gage Amplifier
AD8552-A
R
4
100
AD8552-B
REF192
5V
2.5V
6
4
3
2
4.0V
A2
A1
V
OUT
0V TO 4.0V
40mV
FULL-SCALE
Q1
2N2222
OR
EQUIVALENT
350
LOAD
CELL
1k
12.0k
20k
R
1
17.4k
R
2
100
R
3
17.4k
0
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
V
R
R
V
R
R
R
R
R
V
2
1
1
(18)
V2
V1
R
1
R
2
V
OUT
AD8551/
AD8552/
AD8554
R
3
R
4
IFR
4
R
3
R
2
R
1
R
2
R
1
=
, THEN V
OUT
=
× (V1 – V2)
0
Figure 66. Using the AD855x as a Difference Amplifier
In an ideal difference amplifier, the ratio of the resistors are set
exactly equal to
R
R
3
4
1
2
V
R
R
A
=
=
(19)
Which sets the output voltage of the system to
V
OUT
=
A
V
(
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
R
R
R
R
R
CMRR
2
2
3
2
4
1
3
2
4
R
2
4
1
R
R
R
R
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
1
=
δ
CMRR
MIN
2
(22)
V
OUT
= 1 +
2R
R
G
(V1 – V2)
R
R
R
R
AD8554-C
V2
R
R
V1
AD8554-B
AD8554-A
R
TRIM
R
G
V
OUT
0
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.