Datasheet ADA4522-1, ADA4522-2, ADA4522-4 (Analog Devices) - 27

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Data Sheet ADA4522-1/ADA4522-2/ADA4522-4 APPLICATIONS INFORMATION SINGLE-SUPPLY INSTRUMENTATION AMPLIFIER
ent set of op amp requirements. Its input noise, referred to the The extremely low offset voltage and drift, high open-loop gain, overall instrumentation amplifier input, is divided by the first high common-mode rejection, and high power supply rejection of stage gain and is not as important. Note that the input offset the ADA4522-1/ADA4522-2/ADA4522-4 make them excellent op voltage and the input voltage noise of the amplifiers are also amp choices as discrete, single-supply instrumentation amplifiers. amplified by the overall noise gain. Figure 82 shows the classic 3-op-amp instrumentation amplifier Any unused channel of the ADA4522-1/ADA4522-2/ADA4522-4 using the ADA4522-1/ADA4522-2/ADA4522-4. The key to high must be configured in unity gain with the input common-mode CMRR for the instrumentation amplifier are resistors that are well voltage tied to the midpoint of the power supplies. matched for both the resistive ratio and relative drift. For true Understanding how noise impacts a discrete instrumentation difference amplification, matching of the resistor ratio is very amplifier or a difference amplifier (the second stage of a 3-op- important, where R5/R2 = R6/R4. The resistors are important amp instrumentation amplifier) is important, because they are in determining the performance over manufacturing tolerances, commonly used in many different applications. The Load time, and temperature. Assuming a perfect unity-gain difference Cell/Strain Gage Sensor Signal Conditioning section and the amplifier with infinite common-mode rejection, a 1% tolerance Precision Low-Side Current Shunt Sensor section show the resistor matching results in only 34 dB of common-mode rejection. ADA4522-1/ADA4522-2/ADA4522-4 used as a discrete Therefore, at least 0.01% or better resistors are recommended. instrumentation or difference amplifier in an application.
VIN1 LOAD CELL/STRAIN GAGE SENSOR SIGNAL A1 R5 CONDITIONING USING THE ADA4522-2
The ADA4522-2, with its ultralow offset, drift, and noise, is well
R R1 R2 G1 A3 V
suited to signal condition a low level sensor output with high gain
R OUT G2 R3 R4
and accuracy. A weigh scale/load cell is an example of an
R6
application with such requirements. Figure 83 shows a configura-
A2 V
tion for a single-supply, precision, weigh scale measurement
IN2
system. The ADA4522-2 is used at the front end for amplification
R
78
G1 = RG2, R1 = R3, R2 = R4, R5 = R6
0
V
of the low level signal from the load cell.
OUT = (VIN2 – VIN1) (1 + R1/RG1) (R5/R2)
3168- 1 Figure 82. Discrete 3-Op-Amp Instrumentation Amplifier Current flowing through a PCB trace produces an IR voltage To build a discrete instrumentation amplifier with external resis- drop; with longer traces, this voltage drop can be several tors without compromising on noise, pay close attention to the millivolts or more, introducing a considerable error. A 1 inch resistor values chosen. R long, 0.005 inch wide trace of 1 oz copper has a resistance of G1 and RG2 each have thermal noise that is amplified by the total noise gain of the instrumentation amplifier approximately 100 mΩ at room temperature. With a load and, therefore, a sufficiently low value must be chosen to reduce current of 10 mA, the resistance can introduce a 1 mV error. thermal noise contribution at the output while still providing an Therefore, a 6-wire load cell is used in the circuit. The load cell accurate measurement. Table 10 shows the external resistors noise has two sense pins, in addition to excitation, ground, and two contribution referred to the output (RTO). output connections. The sense pins are connected to the high side (excitation pin) and low side (ground pin) of the
Table 10. Thermal Noise Contribution Example
Wheatstone bridge. The voltage across the bridge can then be
Value Resistor Thermal Thermal Noise
accurately measured regardless of voltage drop due to wire
Resistor (kΩ) Noise (nV/√Hz) RTO (nV/√Hz)
resistance. The two sense pins are also connected to the analog- RG1 0.4 2.57 128.30 to-digital converter (ADC) reference inputs for a ratiometric RG2 0.4 2.57 128.30 configuration that is immune to low frequency changes in the R1 10 12.83 25.66 power supply excitation voltage. R2 10 12.83 25.66 R3 10 12.83 25.66 The ADA4522-2 is configured as the first stage of a 3-op-amp R4 10 12.83 25.66 instrumentation amplifier to amplify the low level amplitude R5 20 18.14 18.14 signal from the load cell by a factor of 1 + 2R1/RG. Capacitors R6 20 18.14 18.14 C1 and C2 are placed in the feedback loops of the amplifiers and interact with R1 and R2 to perform low-pass filtering. This Note that A1 and A2 have a high gain of 1 + R1/RG1. Therefore, filtering limits the amount of noise entering the Σ-Δ ADC. In use a high precision, low offset voltage and low noise amplifier addition, C3, C4, C5, R3, and R4 provide further common-mode for A1 and A2, such as the ADA4522-1/ADA4522-2/ADA4522-4. and differential mode filtering to reduce noise and unwanted Conversely, A3 operates at a much lower gain and has a differ- signals. Rev. F | Page 27 of 33 Document Outline FEATURES APPLICATIONS GENERAL DESCRIPTION PIN CONNECTION DIAGRAM TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS ELECTRICAL CHARACTERISTICS—5.0 V OPERATION ELECTRICAL CHARACTERISTICS—30 V OPERATION ELECTRICAL CHARACTERISTICS—55 V OPERATION ABSOLUTE MAXIMUM RATINGS THERMAL RESISTANCE POWER SEQUENCING ESD CAUTION PIN CONFIGURATIONS AND FUNCTION DESCRIPTIONS TYPICAL PERFORMANCE CHARACTERISTICS THEORY OF OPERATION ON-CHIP INPUT EMI FILTER AND CLAMP CIRCUIT THERMAL SHUTDOWN INPUT PROTECTION SINGLE-SUPPLY AND RAIL-TO-RAIL OUTPUT LARGE SIGNAL TRANSIENT RESPONSE Case 1 Case 2 NOISE CONSIDERATIONS 1/f Noise Source Resistance Residual Ripple Current Noise Density EMI REJECTION RATIO CAPACITIVE LOAD STABILITY APPLICATIONS INFORMATION SINGLE-SUPPLY INSTRUMENTATION AMPLIFIER LOAD CELL/STRAIN GAGE SENSOR SIGNAL CONDITIONING USING THE ADA4522-2 PRECISION LOW-SIDE CURRENT SHUNT SENSOR PRINTED CIRCUIT BOARD LAYOUT COMPARATOR OPERATION USE OF LARGE SOURCE RESISTANCE Unity Gain Follower with Large Source Resistance Workaround OUTLINE DIMENSIONS ORDERING GUIDE