AD8551/AD8552/AD8554
Data Sheet
Rev. E | 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
(
)
B
2
1
R
R +
×
2
(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
5V
2.5V
6
4
3
2
4.0V
A2
A1
VOUT
0V TO 4.0V
40mV
FULL-SCALE
Q1
2N2222
OR
EQUIVALENT
350
LOAD
CELL
1k
12.0k
20k
R1
17.4k
R2
100
R3
17.4k
R4
100
01101-
065
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 AD855x
an excellent choice of op amp for discrete single-supply
instrumentation 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
+
+
=
1
2
1
4
3
4
OUT
R
V
R
V
2
1
(18)
V2
V1
R1
R2
VOUT
AD8551/
AD8552/
AD8554
R3
R4
IF
R4
R3
R2
R1
R2
R1
=
, THEN VOUT =
× (V1 – V2)
01101-
066
Figure 66. Using the AD855x as a Difference Amplifier
In an ideal difference amplifier, the ratio of the resistors are set
exactly equal to
3
4
1
2
V
R
A
=
(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
expressed as
3
2
4
1
3
2
4
2
4
1
R
CMRR
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
δ
CMRRMIN
2
1
=
(22)
VOUT = 1 +
2R
RG
(V1 – V2)
R
AD8554-C
V2
R
V1
AD8554-B
AD8554-A
RTRIM
RG
VOUT
01101-
067
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 i
n 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.