AD8571ARM View Datasheet(PDF) - Analog Devices
Part Name
Description
Manufacturer
AD8571ARM Datasheet PDF : 19 Pages
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AD8571/AD8572/AD8574
100k⍀
100k⍀
VSY = 0V TO 5V
AD857x
VOUT
Figure 54b. AD857x Test Circuit for Turn-On Time
APPLICATIONS
A 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 55 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 cur-
rent 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)
RB
(17)
Where RB is the resistance of the load cell. Using the values given
in Figure 55, the output voltage will linearly vary from 0 V with
no strain to 4 V under full strain.
Q1
2N2222
OR
EQUIVALENT
4.0V
5V
1k⍀
A2
AD8572-B
12k⍀
2
2.5V 6 REF192 3
4
20k⍀
R1
R2
17.4k⍀ 100⍀
350⍀
LOAD
CELL
40mV
FULL-SCALE
A1
VOUT
AD8572-A 0V TO 4V
NOTE:
R3
R4
17.4k⍀ 100⍀
USE 0.1% TOLERANCE RESISTORS.
Figure 55. 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 AD857x an
excellent choice of op amp for discrete single supply instrumenta-
tion 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 differ-
ence amplifier shown in Figure 56 is given as:
VOUT
=
V
1
R4
R3 + R4
1
+
R1
R2
−V
2
R2
R1
(18)
R2
R1
V2
R3
V1
R4
AD857x
VOUT
IF
R4
R3
=
R2
R1
,
THEN
VOUT
=
R2
R1
؋
(V1
؊
V2)
Figure 56. Using the AD857x as a Difference Amplifier
In an ideal difference amplifier, the ratio of the resistors are set
exactly equal to:
AV
=
R2
R1
=
R4
R3
(19)
Which sets the output voltage of the system to:
( ) VOUT = AV V1 −V 2
(20)
Due to finite component tolerance the ratio between the four
resistors will not be exactly equal, and any mismatch results in a
reduction of common-mode rejection from the system. Referring
to Figure 56, the exact common-mode rejection ratio can be
expressed as:
CMRR = R1R4 + 2R2R4 + R2R3
2R1R4 − 2R2R3
(21)
In the 3 op amp instrumentation amplifier configuration shown
in Figure 57, the output difference amplifier is set to unity gain
with all four resistors equal in value. If the tolerance of the resis-
tors used in the circuit is given as δ, the worst-case CMRR of
the instrumentation amplifier will be:
CMRRMIN
=
1
2δ
(22)
V2
AD8574-A
R
R
RG
R
R
R
R
VOUT
AD8574-C
V1
RTRIM
AD8574-B
VOUT = 1 +
2R
RG
(V1 ؊ V2)
Figure 57. A Discrete Instrumentation Amplifier
Configuration
Thus, using 1% tolerance resistors would result 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 57,
should be used to achieve high common-mode rejection. The value
of this trimming resistor should be equal to the value of R multi-
plied by its tolerance. For example, using 10 kΩ resistors with 1%
tolerance would require a series trimming resistor equal to 100 Ω.
REV. 0
–15–
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