Low Cost, Low Power, True RMS-to-DC Converter Data Sheet AD736 FEATURES FUNCTIONAL BLOCK DIAGRAM CC +VS 8k Converts an ac voltage waveform to a dc voltage and then converts to the true rms, average rectified, or absolute value OUT 200 mV rms full-scale input range (larger inputs with input FULL WAVE RMS VIN CF RECTIFIER CORE 8k attenuator) (OPT) CF 12 High input impedance: 10 Low input bias current: 25 pA maximum CAV BIAS SECTION High accuracy: 0.3 mV 0.3% of reading COM CAV VS RMS conversion with signal crest factors up to 5 Wide power supply range: +2.8 V, 3.2 V to 16.5 V Figure 1. Low power: 200 A maximum supply current The AD736 allows the choice of two signal input terminals: a Buffered voltage output 12 high impedance FET input (10 ) that directly interfaces with No external trims needed for specified accuracy High-Z input attenuators and a low impedance input (8 k) that Related device: the AD737features a power-down control allows the measurement of 300 mV input levels while operating with standby current of only 25 A the dc output voltage from the minimum power supply voltage of +2.8 V, 3.2 V. The is negative and the output impedance is 8 k two inputs can be used either single ended or differentially. The AD736 has a 1% reading error bandwidth that exceeds GENERAL DESCRIPTION 10 kHz for the input amplitudes from 20 mV rms to 200 mV rms The AD736 is a low power, precision, monolithic true rms-to- while consuming only 1 mW. dc converter. It is laser trimmed to provide a maximum error of The AD736 is available in four performance grades. The 0.3 mV 0.3% of reading with sine wave inputs. Furthermore, AD736J and AD736K grades are rated over the 0C to +70C it maintains high accuracy while measuring a wide range of and 20C to +85C commercial temperature ranges. The input waveforms, including variable duty-cycle pulses and triac AD736A and AD736B grades are rated over the 40C to +85C (phase)-controlled sine waves. The low cost and small size of industrial temperature range. The AD736 is available in three this converter make it suitable for upgrading the performance low cost, 8-lead packages: PDIP, SOIC, and CERDIP. of non-rms precision rectifiers in many applications. Compared to these circuits, the AD736 offers higher accuracy at an equal PRODUCT HIGHLIGHTS or lower cost. 1. The AD736 is capable of computing the average rectified The AD736 can compute the rms value of both ac and dc input value, absolute value, or true rms value of various input signals. voltages. It can also be operated as an ac-coupled device by 2. Only one external component, an averaging capacitor, is adding one external capacitor. In this mode, the AD736 can required for the AD736 to perform true rms measurement. resolve input signal levels of 100 V rms or less, despite variations 3. The low power consumption of 1 mW makes the AD736 in temperature or supply voltage. High accuracy is also maintained suitable for many battery-powered applications. for input waveforms with crest factors of 1 to 3. In addition, 12 crest factors as high as 5 can be measured (introducing only 2.5% 4. A high input impedance of 10 eliminates the need for an additional error) at the 200 mV full-scale input level. external buffer when interfacing with input attenuators. The AD736 has its own output buffer amplifier, thereby pro- 5. A low impedance input is available for those applications that viding a great deal of design flexibility. Requiring only 200 A require an input signal up to 300 mV rms operating from low of power supply current, the AD736 is optimized for use in power supply voltages. portable multimeters and other battery-powered applications. Rev. I 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 www.analog.com license is granted by implication or otherwise under any patent or patent rights of Analog Devices. Trademarks and registered trademarks are the property of their respective owners. Fax: 781.461.3113 19882012 Analog Devices, Inc. All rights reserved. 00834-001AD736 Data Sheet TABLE OF CONTENTS Features .............................................................................................. 1 RMS MeasurementChoosing the Optimum Value for CAV .... 11 General Description ......................................................................... 1 Rapid Settling Times via the Average Responding Connection .................................................................................. 12 Functional Block Diagram .............................................................. 1 DC Error, Output Ripple, and Averaging Error ..................... 12 Product Highlights ........................................................................... 1 AC Measurement Accuracy and Crest Factor ............................ 12 Revision History ............................................................................... 2 Applications ..................................................................................... 13 Specifications ..................................................................................... 3 Connecting the Input ................................................................. 13 Absolute Maximum Ratings ............................................................ 5 Selecting Practical Values for Input Coupling (CC), Thermal Resistance ...................................................................... 5 Averaging (CAV), and Filtering (CF) Capacitors ...................... 14 ESD Caution .................................................................................. 5 Additional Application Concepts ............................................. 15 Pin Configuration and Function Descriptions ............................. 6 Evaluation Board ............................................................................ 17 Typical Performance Characteristics ............................................. 7 Outline Dimensions ....................................................................... 19 Theory of Operation ...................................................................... 10 Ordering Guide .......................................................................... 20 Types of AC Measurement ........................................................ 10 Calculating Settling Time Using Figure 16 ............................. 11 REVISION HISTORY 12/12Rev. H to Rev. I Changes to Features ......................................................................... 1 Changes to Features and Figure 1 .................................................. 1 Added Table 3 ...................................................................................6 Change to Error vs. Crest Factor Parameter, Table 1 .................. 3 Changes to Figure 21 and Figure 22 ........................................... 14 Changes to Operating Voltage Range Parameter, Table 1 .......... 4 Changes to Figure 23, Figure 24, and Figure 25 ........................ 15 Changes to Table 2 ........................................................................... 5 Updated Outline Dimensions ...................................................... 16 Added Table 3 Renumbered Sequentially ................................... 5 Changes to Ordering Guide ......................................................... 17 Changes to Figure 9 ......................................................................... 8 5/04Rev. E to Rev. F Changes to Figure 16 ....................................................................... 9 Changes to Specifications ................................................................ 2 Changes to Figure 18 ..................................................................... 10 Replaced Figure 18 ........................................................................ 10 Added Additional Application Concepts Section and Updated Outline Dimensions ...................................................... 16 Changes to Figure 25 ..................................................................... 15 Changes to Ordering Guide ......................................................... 16 Changes to Figure 29 ..................................................................... 17 4/03Rev. D to Rev. E Deleted Table 6 ............................................................................... 17 Changes to General Description ................................................. 1 Changes to Ordering Guide ......................................................... 20 Changes to Specifications ............................................................. 3 2/07Rev. G to Rev. H Changes to Absolute Maximum Ratings .................................... 4 Updated Layout ....................................................................... 9 to 12 Changes to Ordering Guide ......................................................... 4 Added Applications Section ......................................................... 13 11/02Rev. C to Rev. D Inserted Figure 21 to Figure 24 Renumbered Sequentially..... 13 Changes to Functional Block Diagram ....................................... 1 Deleted Figure 25 ........................................................................... 15 Changes to Pin Configuration ..................................................... 3 Added Evaluation Board Section................................................. 16 Figure 1 Replaced .......................................................................... 6 Inserted Figure 29 to Figure 34 Renumbered Sequentially..... 16 Changes to Figure 2 ....................................................................... 6 Inserted Figure 35 Renumbered Sequentially ........................... 17 Changes to Application Circuits Figures 4 to 8 ......................... 8 Added Table 6 ................................................................................. 17 Outline Dimensions Updated ...................................................... 8 2/06Rev. F to Rev. G Updated Format ................................................................. Universal Rev. I Page 2 of 20