Datasheet AD8571, AD8572, AD8574 (Analog Devices) - 21

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Data Sheet AD8571/AD8572/AD8574 APPLICATIONS INFORMATION 5 V PRECISION STRAIN GAGE CIRCUIT R2
The extremely low offset voltage of the AD8572 makes it an ideal
R1 V2
amplifier for any application requiring accuracy with high gains,
V R3 OUT V1 AD8571/
such as a weigh scale or strain gage. Figure 63 shows a configura-
R4 AD8572/
tion for a single-supply, precision strain gage measurement system.
AD8574 R4 R2 R2
064
IF = , THEN VOUT = (V1 – V2)
The REF192 provides a 2.5 V precision reference voltage for A2.
R3 R1 R1
01104- The A2 amplifier boosts this voltage to provide a 4.0 V reference Figure 64. Using the AD8571/AD8572/AD8574 as a Difference Amplifier for the top of the strain gage resistor bridge. Q1 provides the In an ideal difference amplifier, the ratio of the resistors is set current drive for the 350 Ω bridge network. A1 is used to amplify equal to the output of the bridge with the ful -scale output voltage equal to R2 R4 2 × ( 1 R + R2) A = = (19) (17) V 1 R R3 RB Set the output voltage of the system to where RB is the resistance of the load cell. VOUT = AV (V1 − V2) (20) Using the values given in Figure 63, the output voltage linearly varies from 0 V with no strain to 4 V under ful strain. 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
5V 2
to Figure 64, the exact common-mode rejection ratio can be
2.5V 6 3 1k REF192 Ω Q1
expressed as
A2 2N2222 OR AD8572-B 4
1R4 R + 2R2R4 + R2R3
EQUIVALENT 12kΩ 20kΩ
CMRR = (21)
4.0V
2R1R4 − 2R2R3
R1 R2 17.4kΩ 100Ω
In the 3-op amp instrumentation amplifier configuration shown in Figure 65, the output difference amplifier is set to unity gain
40mV
with all four resistors equal in value. If the tolerance of the
A1 VOUT FULL-SCALE 350Ω 0V TO 4V
resistors used in the circuit is given as δ, the worst-case CMRR
LOAD AD8572-A CELL
of the instrumentation amplifier is
R3 R4
063 1
NOTE: 17.4kΩ 100Ω
CMRR = (22)
USE 0.1% TOLERANCE RESISTORS.
MIN 01104- 2δ Figure 63. 5 V Precision Strain Gage Amplifier
AD8574-A V2 3 V INSTRUMENTATION AMPLIFIER R
The high common-mode rejection, high open-loop gain,
R R
and operation down to 3 V of the supply voltage make the
V RG R R OUT
AD8571/AD8572/AD8574 an excellent op amp choice for
R AD8574-C
discrete single-supply instrumentation amplifiers. The common-mode rejection ratio of the AD8571/AD8572/
V1 RTRIM AD8574-B
AD8574 is greater than 120 dB, but the CMRR of the system 065 is also a function of the external resistor tolerances. The gain
2R V (V1 – V2) OUT = 1 + RG
of the difference amplifier shown in Figure 64 is given as 01104- Figure 65. Discrete Instrumentation Amplifier Configuration  R4  1 R   R2  V = OUT V1 1 +  − V 2  (18) Therefore, using 1% tolerance resistors results in a worst-case  R3 + R4  R2   1 R  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 R multiplied by its tolerance. For example, using 10 kΩ resistors with 1% tolerance would require a series trimming resistor equal to 100 Ω. Rev. F | Page 21 of 28 Document Outline FEATURES APPLICATIONS GENERAL DESCRIPTION PIN CONFIGURATIONS TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS 5 V ELECTRICAL CHARACTERISTICS 2.7 V ELECTRICAL CHARACTERISTICS ABSOLUTE MAXIMUM RATINGS THERMAL CHARACTERISTICS ESD CAUTION TYPICAL PERFORMANCE CHARACTERISTICS FUNCTIONAL DESCRIPTION AMPLIFIER ARCHITECTURE BASIC AUTO-ZERO AMPLIFIER THEORY AUTO-ZERO PHASE AMPLIFICATION PHASE HIGH GAIN, CMRR, AND PSRR MAXIMIZING PERFORMANCE THROUGH PROPER LAYOUT 1/f NOISE CHARACTERISTICS RANDOM AUTO-ZERO CORRECTION ELIMINATES INTERMODULATION DISTORTION BROADBAND AND EXTERNAL RESISTOR NOISE CONSIDERATIONS OUTPUT OVERDRIVE RECOVERY INPUT OVERVOLTAGE PROTECTION OUTPUT PHASE REVERSAL CAPACITIVE LOAD DRIVE POWER-UP BEHAVIOR APPLICATIONS INFORMATION 5 V PRECISION STRAIN GAGE CIRCUIT 3 V INSTRUMENTATION AMPLIFIER HIGH ACCURACY THERMOCOUPLE AMPLIFIER PRECISION CURRENT METER PRECISION VOLTAGE COMPARATOR OUTLINE DIMENSIONS ORDERING GUIDE