Datasheet

AD8571/AD8572/AD8574
Rev. E | Page 20 of 24
APPLICATIONS INFORMATION
5 V PRECISION STRAIN GAGE CIRCUIT
The extremely low offset voltage of the AD8572 makes it an ideal
amplifier for any application requiring accuracy with high gains,
such as a weigh scale or strain gage. Figure 63 shows a configura-
tion for a single-supply, precision strain gage measurement system.
The 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  bridge network. A1 is used to amplify
the output of the bridge with the full-scale output voltage equal to
(
)
B
R
2R1R +×2
(17)
where R
B
is the resistance of the load cell.
Using the values given in Figure 63, the output voltage linearly
varies from 0 V with no strain to 4 V under full strain.
V
OUT
AD8572-A
R3
17.4kΩ
R4
100Ω
R1
17.4kΩ
R2
100Ω
0V TO 4V
NOTE:
USE 0.1% TOLERANCE RESISTORS.
20kΩ
A1
AD8572-B
REF192
12kΩ
1kΩ
5V
2.5V
6
4
3
2
4.0V
40mV
FULL-SCALE
A2
350Ω
LOAD
CELL
Q1
2N2222
OR
EQUIVALENT
01104-063
Figure 63. 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 the supply voltage make the
AD857x family an excellent op amp choice for discrete single-
supply instrumentation amplifiers. The common-mode
rejection ratio of the AD857x 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 64
is given as
⎟
⎠
⎞
⎜
⎝
⎛
−
⎟
⎠
⎞
⎜
⎝
⎛
+
⎟
⎠
⎞
⎜
⎝
⎛
+
=
1R
2R
2V
2R
1R
4R3R
4R
1VV
OUT
1 (18)
V2
V1
V
OUT
R1
R1 R1
R3
R4
R4
R3
R2
R2 R2
AD8571/
AD8572/
AD8574
IF = , THEN V
OUT
=
(V1 – V2)
01104-064
Figure 64. Using the AD857x as a Difference Amplifier
In an ideal difference amplifier, the ratio of the resistors is set
equal to
3R
4R
1R
2R
A
V
== (19)
Set 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 64, the exact common-mode rejection ratio can be
expressed as
R2R3R1R4
R2R3R2R41R4R
CMRR
22
2
−
+
+
= (21)
In the 3-op amp instrumentation amplifier configuration shown
in Figure 65, 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
=
2
1
MIN
CMRR (22)
V
OUT
R
R
R
R
AD8574-C
V2
R
R
V1
R
G
AD8574-B
A
D8574-A
R
TRIM
V
OUT
= 1 +
2R
R
G
(V1 – V2)
01104-065
Figure 65. Discrete Instrumentation Amplifier Configuration
Therefore, using 1% tolerance resistors results in a worst-case
system CMRR of 0.02, or 34 dB. To achieve high common-
mode rejection, either high precision resistors or an additional
trimming resistor, as shown in Figure 65, should be used. The
value of this trimming resistor should be equal to the value of
multiplied by its tolerance. For example, using 10 k resistors
with 1% tolerance would require a series trimming resistor
equal to 100 .
R