link to page 20 link to page 20 link to page 20 link to page 20 link to page 20 link to page 20 AD8551/AD8552/AD8554Data SheetAPPLICATIONS INFORMATION A 5 V PRECISION STRAIN GAGE CIRCUITR2 The extremely low offset voltage of the AD8552 makes it an R1 ideal amplifier for any application requiring accuracy with high V2VOUT gains, such as a weigh scale or strain gage. Figure 65 shows a V1AD8551/R3 configuration for a single-supply, precision, strain gage R4AD8552/ AD8554 measurement system. R4R2R2 066 IF=, THEN VOUT =× (V1 – V2) A REF192 provides a 2.5 V precision reference voltage for A2. R3R1R1 01101- The A2 amplifier boosts this voltage to provide a 4.0 V reference Figure 66. Using the AD8551/AD8552/AD8554 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 are set current drive for the 350 Ω bridge network. A1 is used to exactly equal to amplify the output of the bridge with the ful -scale output R R voltage equal to 2 4 = V A = (19) R R 2× (R + R ) 1 3 1 2 (17) R Which sets the output voltage of the system to B V where R OUT = AV (V1 − V2) (20) B is the resistance of the load cell. Due to finite component tolerance, the ratio between the four Using the values given in Figure 65, the output voltage linearly resistors is not exactly equal, and any mismatch results in a varies from 0 V with no strain to 4.0 V under full strain. reduction of common-mode rejection from the system. Referring to Figure 66, the exact common-mode rejection ratio can be 5V2 expressed as Q12.5V3REF1921kΩ2N22226A2OR + + 1 R 4 R 2R2 4 R R2 3 R = EQUIVALENTAD8552-B4 CMRR (21) 2R R − 2R R 12.0kΩ20kΩ 1 4 2 3 4.0VR In the three-op amp, instrumentation amplifier configuration 1R217.4kΩ 100Ω shown in Figure 67, the output difference amplifier is set to unity gain with all four resistors equal in value. If the tolerance V40mVA1OUT of the resistors used in the circuit is given as δ, the worst-case 0V TO 4.0VFULL-SCALE350ΩAD8552-ALOAD CMRR of the instrumentation amplifier is CELLR3R417.4kΩ 100Ω CMRR 1 = MIN (22) δ 2 NOTES 065 1. USE 0.1% TOLERANCE RESISTORS. 01101- AD8554-A Figure 65. A 5 V Precision Strain Gage Amplifier V2R3 V INSTRUMENTATION AMPLIFIERRR The high common-mode rejection, high open-loop gain, and RGVOUT operation down to 3 V of supply voltage makes the AD8551/ RRAD8554-CR AD8552/AD8554 an excel ent choice of op amp for discrete single-supply instrumentation amplifiers. The common-mode V1RTRIMAD8554-B rejection ratio of the AD8551/AD8552/AD8554 is greater than 120 dB, but the CMRR of the system is also a function of the 2R 067 VOUT = 1 +(V1 – V2)RG external resistor tolerances. The gain of the difference amplifier 01101- Figure 67. A Discrete Instrumentation Amplifier Configuration shown in Figure 66 is given as Consequently, using 1% tolerance resistors results in a worst-case R R R V = 4 V1 1+ 1 − system CMRR of 0.02, or 34 dB. Therefore, either high precision OUT V 2 2 (18) R + 3 R4 R2 1 R 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. Rev. F | Page 20 of 24 Document Outline FEATURES APPLICATIONS GENERAL DESCRIPTION PIN CONFIGURATIONS TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS 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, PSRR MAXIMIZING PERFORMANCE THROUGHPROPER LAYOUT 1/f NOISE CHARACTERISTICS 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 A 5 V PRECISION STRAIN GAGE CIRCUIT 3 V INSTRUMENTATION AMPLIFIER A HIGH ACCURACY THERMOCOUPLE AMPLIFIER PRECISION CURRENT METER PRECISION VOLTAGE COMPARATOR OUTLINE DIMENSIONS ORDERING GUIDE
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