Low Cost, Low Power, True RMS-to-DC Converter Data Sheet AD737 FEATURES FUNCTIONAL BLOCK DIAGRAM COM 8k Computes 8k C C True rms value C F Average rectified value ABSOLUTE OUTPUT SQUARER VALUE Absolute value DIVIDER V IN CIRCUIT Provides C AV 200 mV full-scale input range (larger inputs with input scaling) +V S C AV BIAS Direct interfacing with 3 digit CMOS analog-to-digital SECTION POWER V S DOWN converters (ADCs) 12 Figure 1. High input impedance: 10 Low input bias current: 25 pA maximum High accuracy: 0.2 mV 0.3% of reading RMS conversion with signal crest factors up to 5 Wide power supply range: 2.5 V to 16.5 V Low power: 25 A (typical) standby current No external trims needed for specified accuracy The AD737 output is negative going the AD736 is a positive output-going version of the same basic device GENERAL DESCRIPTION 12 The AD737 is a low power, precision, monolithic, true rms-to-dc The AD737 has both high (10 ) and low impedance input converter. It is laser trimmed to provide a maximum error of options. The high-Z FET input connects high source impedance 0.2 mV 0.3% of reading with sine wave inputs. Furthermore, input attenuators, and a low impedance (8 k) input accepts it maintains high accuracy while measuring a wide range of rms voltages of up to 0.9 V while operating from the minimum input waveforms, including variable duty cycle pulses and power supply voltage of 2.5 V. The two inputs can be used triac (phase) controlled sine waves. The low cost and small either single-ended or differentially. physical size of the AD737 make it suitable for upgrading the The AD737 achieves 1% of reading error bandwidth, exceeding performance of non-rms precision rectifiers in many applications. 10 kHz for input amplitudes from 20 mV rms to 200 mV rms, Compared to these circuits, the AD737 offers higher accuracy at while consuming only 0.72 mW. equal or lower cost. The AD737 is available in two performance grades. The AD737J The AD737 computes the rms value of both ac and dc input and AD737K grades operate over the commercial temperature voltages, and is ac-coupled by adding an input capacitor. In this range of 0C to 70C. The AD737JR-5 is tested with supply mode, the AD737 resolves input signal levels of 100 V rms or voltages of 2.5 V dc. The AD737A grade operates over the less, despite variations in temperature or supply voltage. High industrial temperature range of 40C to +85C. The AD737 is accuracy is maintained for input waveforms with crest factors of available in two low cost, 8lead packages: PDIP and SOIC N. 1 to 3 and crest factors at 2.5% or less with respect to full-scale PRODUCT HIGHLIGHTS input level. 1. Computes the average rectified, absolute, or true rms value The AD737 has no output buffer amplifier, thereby significantly of a signal regardless of waveform. reducing dc offset errors occurring at the output and making 2. Only one external component, an averaging capacitor, is the device highly compatible with high input impedance ADCs. required for the AD737 to perform true rms measurement. Requiring only 160 A of power supply current, the AD737 is 3. The standby power consumption of 125 W makes the optimized for use in portable multimeters and other battery- AD737 suitable for battery-powered applications. powered applications. In power-down mode, the standby supply current in is typically 25 A. Rev. J Document Feedback Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. rights of third parties that may result from its use. Specifications subject to change without notice. No Tel: 781.329.4700 19882015 Analog Devices, Inc. All rights reserved. license is granted by implication or otherwise under any patent or patent rights of Analog Devices. Technical Support www.analog.com Trademarks and registered trademarks are the property of their respective owners. 00828-001AD737 Data Sheet TABLE OF CONTENTS Features .............................................................................................. 1 DC Error, Output Ripple, and Averaging Error .................... 14 Functional Block Diagram .............................................................. 1 AC Measurement Accuracy and Crest Factor ........................ 14 General Description ......................................................................... 1 Calculating Settling Time .......................................................... 14 Product Highlights ........................................................................... 1 Applications Information .............................................................. 15 Revision History ............................................................................... 3 RMS MeasurementChoosing an Optimum Value for CAV 15 Specifications ..................................................................................... 4 Rapid Settling Times via the Average Responding Connection .................................................................................. 15 Absolute Maximum Ratings ............................................................ 7 Selecting Practical Values for Capacitors ................................ 15 Thermal Resistance ...................................................................... 7 Scaling Input and Output Voltages .......................................... 15 ESD Caution .................................................................................. 7 Additional Information ............................................................. 16 Pin Configurations and Function Descriptions ........................... 8 AD737 Evaluation Board ............................................................... 19 Typical Performance Characteristics ............................................. 9 Outline Dimensions ....................................................................... 21 Theory of Operation ...................................................................... 13 Ordering Guide .......................................................................... 22 Types of AC Measurement ........................................................ 13 Rev. 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