AD8551_12 Datasheet, PDF (20/24 Pages) Analog Devices – Zero-Drift, Single-Supply, Rail-to-Rail Input/Output Operational Amplifiers

AD8551/AD8552/AD8554
Data Sheet
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
2 × (R₁ + R₂ )
(17)
R_B
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.
Q1
2N2222
OR
EQUIVALENT
4.0V
5V
2
1kΩ
2.5V
3
REF192
A2
6
AD8552-B
4
12.0kΩ
20kΩ
R₁
R₂
17.4kΩ 100Ω
350Ω
LOAD
CELL
40mV
FULL-SCALE
A1
V_OUT
0V TO 4.0V
AD8552-A
R₃
R₄
17.4kΩ 100Ω
NOTES
1. USE 0.1% TOLERANCE RESISTORS.
Figure 65. A 5 V Precision Strain Gage Amplifier
R₂
R₁
V2
V1
R_{3 R4}
V_OUT
AD8551/
AD8552/
AD8554
IF
R₄
R₃
=
R₂
R₁
,
THEN
V_OUT
=
R₂
R₁
× (V1 – V2)
Figure 66. Using the AD855x as a Difference Amplifier
In an ideal difference amplifier, the ratio of the resistors are set
exactly equal to
A_V
=
R₂
R₁
=
R₄
R₃
(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
CMRR = ^{R1R4 + 2R2R4 + R2R3}
(21)
2R₁R₄ − 2R₂R₃
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
CMRR_MIN
=
1
2δ
(22)
AD8554-A
V2
R
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
the difference amplifier shown in Figure 66 is given as
R
R_G
R
R
R
R
V_OUT
AD8554-C
V1
R_TRIM
AD8554-B
2R
V_OUT = 1 + _RG (V1 – V2)
Figure 67. A Discrete Instrumentation Amplifier Configuration
V_OUT
=
V1_{}
R₃
R₄
+ R₄
^{1 +}
R₁
R₂

−
V
2_{}
R₂
R₁

(18)
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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