Low Cost, Low Power, True RMS-to-DC Converter AD736 FEATURES GENERAL DESCRIPTION The AD736 is a low power, precision, monolithic true rms-todc converter. It is laser trimmed to provide a maximum error of 0.3 mV 0.3% of reading with sine wave inputs. Furthermore, it maintains high accuracy while measuring a wide range of input waveforms, including variable duty-cycle pulses and triac (phase)-controlled sine waves. The low cost and small size of this converter make it suitable for upgrading the performance of non-rms precision rectifiers in many applications. Compared to these circuits, the AD736 offers higher accuracy at an equal or lower cost. The AD736 can compute the rms value of both ac and dc input voltages. It can also be operated as an ac-coupled device by adding one external capacitor. In this mode, the AD736 can resolve input signal levels of 100 V rms or less, despite variations in temperature or supply voltage. High accuracy is also maintained for input waveforms with crest factors of 1 to 3. In addition, crest factors as high as 5 can be measured (introducing only 2.5% additional error) at the 200 mV full-scale input level. The AD736 has its own output buffer amplifier, thereby providing a great deal of design flexibility. Requiring only 200 A of power supply current, the AD736 is optimized for use in portable multimeters and other battery-powered applications. FUNCTIONAL BLOCK DIAGRAM CC 1 VIN 2 CF 3 8k FULL WAVE RECTIFIER AD736 8 COM 8k 7 +VS INPUT AMPLIFIER BIAS SECTION 6 OUTPUT rms CORE OUTPUT AMPLIFIER -VS 4 5 CAV 00834-001 Computes True rms value Average rectified value Absolute value Provides 200 mV full-scale input range (larger inputs with input attenuator) High input impedance: 1012 Low input bias current: 25 pA maximum High accuracy: 0.3 mV 0.3% of reading RMS conversion with signal crest factors up to 5 Wide power supply range: +2.8 V, -3.2 V to 16.5 V Low power: 200 mA maximum supply current Buffered voltage output No external trims needed for specified accuracy AD737--an unbuffered voltage output version with chip power-down also available Figure 1. The AD736 allows the choice of two signal input terminals: a high impedance FET input (1012 ) that directly interfaces with High-Z input attenuators and a low impedance input (8 k) that allows the measurement of 300 mV input levels while operating from the minimum power supply voltage of +2.8 V, -3.2 V. The two inputs can be used either single ended or differentially. The AD736 has a 1% reading error bandwidth that exceeds 10 kHz for the input amplitudes from 20 mV rms to 200 mV rms while consuming only 1 mW. The AD736 is available in four performance grades. The AD736J and AD736K grades are rated over the 0C to +70C and -20C to +85C commercial temperature ranges. The AD736A and AD736B grades are rated over the -40C to +85C industrial temperature range. The AD736 is available in three low cost, 8-lead packages: PDIP, SOIC, and CERDIP. PRODUCT HIGHLIGHTS 1. The AD736 is capable of computing the average rectified value, absolute value, or true rms value of various input signals. 2. Only one external component, an averaging capacitor, is required for the AD736 to perform true rms measurement. 3. The low power consumption of 1 mW makes the AD736 suitable for many battery-powered applications. 4. A high input impedance of 1012 eliminates the need for an external buffer when interfacing with input attenuators. 5. A low impedance input is available for those applications that require an input signal up to 300 mV rms operating from low power supply voltages. Rev. H 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 rights of third parties that may result from its use. Specifications subject to change without notice. No 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. One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 781.329.4700 www.analog.com Fax: 781.461.3113 (c)2007 Analog Devices, Inc. All rights reserved. AD736 TABLE OF CONTENTS Features .............................................................................................. 1 General Description ......................................................................... 1 Functional Block Diagram .............................................................. 1 Product Highlights ........................................................................... 1 Revision History ............................................................................... 2 Specifications..................................................................................... 3 Absolute Maximum Ratings............................................................ 5 ESD Caution.................................................................................. 5 RMS Measurement--Choosing the Optimum Value for CAV ....................................................................................................... 11 Rapid Settling Times via the Average Responding Connection.................................................................................. 12 DC Error, Output Ripple, and Averaging Error..................... 12 AC Measurement Accuracy and Crest Factor............................ 12 Applications..................................................................................... 13 Connecting the Input................................................................. 13 Pin Configuration and Function Descriptions............................. 6 Selecting Practical Values for Input Coupling (CC), Averaging (CAV), and Filtering (CF) Capacitors......................................... 14 Typical Performance Characteristics ............................................. 7 Evaluation Board ............................................................................ 16 Theory of Operation ...................................................................... 10 Outline Dimensions ....................................................................... 18 Types of AC Measurement ........................................................ 10 Ordering Guide .......................................................................... 19 Calculating Settling Time Using Figure 16 ............................. 11 REVISION HISTORY 2/07--Rev. G to Rev. H Updated Layout.......................................................................9 to 12 Added Applications Section......................................................... 13 Inserted Figure 21 to Figure 24; Renumbered Sequentially..... 13 Deleted Figure 25........................................................................... 15 Added Evaluation Board Section ................................................ 16 Inserted Figure 29 to Figure 34; Renumbered Sequentially..... 16 Inserted Figure 35; Renumbered Sequentially........................... 17 Added Table 6................................................................................. 17 2/06--Rev. F to Rev. G Updated Format.................................................................Universal Changes to Features......................................................................... 1 Added Table 3................................................................................... 6 Changes to Figure 21 and Figure 22............................................ 14 Changes to Figure 23, Figure 24, and Figure 25 ........................ 15 Updated Outline Dimensions ...................................................... 16 Changes to Ordering Guide ......................................................... 17 4/03--Rev. D to Rev. E Changes to General Description .................................................1 Changes to Specifications.............................................................3 Changes to Absolute Maximum Ratings....................................4 Changes to Ordering Guide .........................................................4 11/02--Rev. C to Rev. D Changes to Functional Block Diagram.......................................1 Changes to Pin Configuration .....................................................3 Figure 1 Replaced ..........................................................................6 Changes to Figure 2.......................................................................6 Changes to Application Circuits Figures 4 to 8 .........................8 Outline Dimensions Updated......................................................8 5/04--Rev. E to Rev. F Changes to Specifications ............................................................... 2 Replaced Figure 18 ........................................................................ 10 Updated Outline Dimensions ...................................................... 16 Changes to Ordering Guide ......................................................... 16 Rev. H | Page 2 of 20 AD736 SPECIFICATIONS At 25C 5 V supplies, ac-coupled with 1 kHz sine wave input applied, unless otherwise noted. Specifications in bold are tested on all production units at final electrical test. Results from those tests are used to calculate outgoing quality levels. Table 1. Parameter TRANSFER FUNCTION CONVERSION ACCURACY Total Error, Internal Trim 1 All Grades TMIN to TMAX A and B Grades J and K Grades vs. Supply Voltage @ 200 mV rms Input DC Reversal Error, DC-Coupled Nonlinearity 2 , 0 mV to 200 mV Total Error, External Trim ERROR VS. CREST FACTOR 3 Crest Factor = 1 to 3 Crest Factor = 5 INPUT CHARACTERISTICS High Impedance Input Signal Range (Pin 2) Continuous RMS Level Peak Transient Input Input Resistance Input Bias Current Low Impedance Input Signal Range (Pin 1) Continuous RMS Level Peak Transient Input Input Resistance Maximum Continuous Nondestructive Input Input Offset Voltage 4 J and K Grades A and B Grades vs. Temperature vs. Supply Conditions Min AD736J/AD736A AD736K/AD736B Typ Max Min Typ Max VOUT = Avg (VIN2) 1 kHz sine wave Using CC 0 mV rms to 200 mV rms 200 mV to 1 V rms 0.3/0.3 -1.2 @ 200 mV rms @ 200 mV rms 0.7/0.7 0.007 VS = 5 V to 16.5 V VS = 5 V to 3 V @ 600 mV dc @ 100 mV rms 0 mV rms to 200 mV rms 0 0 0 CAV, CF = 100 F CAV, CF = 100 F VS = +2.8 V, -3.2 V VS = 5 V to 16.5 V VS = +2.8 V, -3.2 V VS = 5 V VS = 16.5 V +0.06 -0.18 1.3 0.25 0.1/0.5 +0.1 -0.3 2.5 0.35 0 0 0 +0.06 -0.18 1.3 0.25 0.1/0.3 0.9 0.5/0.5 mV/% of reading % of reading/C +0.1 -0.3 2.5 0.35 %/V %/V % of reading % of reading mV/% of reading % additional error % additional error 200 1 0.9 2.7 2.7 4.0 4.0 1012 1 1012 1 25 300 1 6.4 mV/% of reading % of reading 0.7 2.5 200 1 VS = +2.8 V, -3.2 V VS = 5 V to 16.5 V VS = +2.8 V, -3.2 V VS = 5 V VS = 16.5 V 0.3/0.3 2.0 0.007 0.7 2.5 VS = 3 V to 16.5 V 1.7 3.8 11 8 All supply voltages VS = 5 V to 16.5 V VS = 5 V to 3 V 0.2/0.2 -1.2 0.5/0.5 2.0 8 50 80 Rev. H | Page 3 of 20 9.6 12 3 3 30 150 25 300 1 6.4 1.7 3.8 11 8 8 50 80 Unit 9.6 12 3 3 30 150 mV rms V rms V V V pA mV rms V rms V V V k V p-p mV mV V/C V/V V/V AD736 Parameter OUTPUT CHARACTERISTICS Output Offset Voltage J and K Grades A and B Grades vs. Temperature vs. Supply Output Voltage Swing 2 k Load Conditions AD736J/AD736A Typ Max 0.1 1 50 50 VS = 5 V to 16.5 V VS = 5 V to 3 V VS = +2.8 V, -3.2 V VS = 5 V No Load Output Current Short-Circuit Current Output Resistance FREQUENCY RESPONSE High Impedance Input (Pin 2) for 1% Additional Error VIN = 1 mV rms VIN = 10 mV rms VIN = 100 mV rms VIN = 200 mV rms 3 dB Bandwidth VIN = 1 mV rms VIN = 10 mV rms VIN = 100 mV rms VIN = 200 mV rms Low Impedance Input (Pin 1) for 1% Additional Error VIN = 1 mV rms VIN = 10 mV rms VIN = 100 mV rms VIN = 200 mV rms 3 dB Bandwidth VIN = 1 mV rms VIN = 10 mV rms VIN = 100 mV rms VIN = 200 mV rms POWER SUPPLY Operating Voltage Range Quiescent Current 200 mV rms, No Load TEMPERATURE RANGE Operating, Rated Performance Commercial Industrial Min VS = 16.5 V VS = 16.5 V @ dc 0 to 1.6 0 to 3.6 0 to 4 0 to 4 2 AD736K/AD736B Min Typ Max 0.1 0.5 0.5 20 130 1.7 1 50 50 0 to 1.6 0 to 3.6 0 to 4 0 to 4 2 3.8 5 12 0.3 0.3 20 130 Unit mV mV V/C V/V V/V 1.7 V 3.8 V 5 12 3 0.2 3 0.2 V V mA mA 1 6 37 33 1 6 37 33 kHz kHz kHz kHz 5 55 170 190 5 55 170 190 kHz kHz kHz kHz 1 6 90 90 1 6 90 90 kHz kHz kHz kHz 5 55 350 460 5 55 350 460 kHz kHz kHz kHz Sine wave input Sine wave input Sine wave input Sine wave input Zero signal Sine wave input 0C to 70C -40C to +85C +2.8, -3.2 5 160 230 16.5 200 270 AD736JN, AD736JR AD736AQ, AD736AR 1 +2.8, -3.2 5 160 230 16.5 200 270 V A A AD736KN, AD736KR AD736BQ, AD736BR Accuracy is specified with the AD736 connected as shown in Figure 18 with Capacitor CC. Nonlinearity is defined as the maximum deviation (in percent error) from a straight line connecting the readings at 0 mV rms and 200 mV rms. Output offset voltage is adjusted to zero. Error vs. crest factor is specified as additional error for a 200 mV rms signal. Crest factor = VPEAK/V rms. 4 DC offset does not limit ac resolution. 2 3 Rev. H | Page 4 of 20 AD736 ABSOLUTE MAXIMUM RATINGS Table 2. Parameter Supply Voltage Internal Power Dissipation 1 Input Voltage Output Short-Circuit Duration Differential Input Voltage Storage Temperature Range (Q) Storage Temperature Range (N, R) Lead Temperature (Soldering, 60 sec) ESD Rating 1 Rating 16.5 V 200 mW VS Indefinite +VS and -VS -65C to +150C -65C to +125C 300C 500 V Stresses above those listed under Absolute Maximum Ratings may cause permanent damage to the device. This is a stress rating only; functional operation of the device at these or any other conditions above those indicated in the operational section of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. ESD CAUTION 8-Lead PDIP: JA = 165C/W, 8-Lead CERDIP: JA = 110C/W, and 8-Lead SOIC: JA = 155C/W. Rev. H | Page 5 of 20 AD736 PIN CONFIGURATION AND FUNCTION DESCRIPTIONS AD736 8 COM +VS TOP VIEW CF 3 (Not to Scale) 6 OUTPUT -VS 4 7 5 CAV 00834-025 CC 1 VIN 2 Figure 2. Pin Configuration Table 3. Pin Function Descriptions Pin No. 1 Mnemonic CC 2 3 4 5 6 7 8 VIN CF -VS CAV OUTPUT +VS COM Description Coupling Capacitor. If dc coupling is desired at Pin 2, connect a coupling capacitor to this pin. If the coupling at Pin 2 is ac, connect this pin to ground. Note that this pin is also an input, with an input impedance of 8 k. Such an input is useful for applications with high input voltages and low supply voltages. High Input Impedance Pin. Connect an Auxiliary Low-Pass Filter Capacitor from the Output. Negative Supply Voltage if Dual Supplies Are Used, or Ground if Connected to a Single-Supply Source. Connect the Averaging Capacitor Here. DC Output Voltage. Positive Supply Voltage. Common. Rev. H | Page 6 of 20 AD736 TYPICAL PERFORMANCE CHARACTERISTICS 10V SINE WAVE INPUT, VS = 5V, CAV = 22F, CF = 4.7F, CC = 22F VIN = 200mV rms 1kHz SINE WAVE CAV = 100F CF = 22F 1V INPUT LEVEL (rms) 0.5 0.3 0.1 0 -0.1 100mV 1% ERROR 10mV -3dB 1mV -0.3 0 2 4 6 8 10 SUPPLY VOLTAGE (V) 12 14 16 100V 0.1 Figure 3. Additional Error vs. Supply Voltage 10 100 -3dB FREQUENCY (kHz) 10V SINE WAVE INPUT, VS = 5V, CAV = 22F, CF = 4.7F, CC = 22F DC-COUPLED 14 1V INPUT LEVEL (rms) 12 10 PIN 1 8 PIN 2 6 100mV 1% ERROR 10mV 10% ERROR 4 1mV 0 2 4 6 8 10 SUPPLY VOLTAGE (V) 12 14 16 100V 0.1 00834-003 Figure 4. Maximum Input Level vs. Supply Voltage 10 100 -3dB FREQUENCY (kHz) 1000 Figure 7. Frequency Response Driving Pin 2 16 6 1kHz SINE WAVE INPUT ADDITIONAL ERROR (% of Reading) 14 12 10 8 6 4 2 3ms BURST OF 1kHz = 3 CYCLES 200mV rms SIGNAL VS = 5V CC = 22F CF = 100F 5 4 CAV = 10F CAV = 33F 3 2 1 CAV = 100F CAV = 250F 0 2 4 6 8 10 SUPPLY VOLTAGE (V) 12 14 16 00834-004 0 1 Figure 5. Peak Buffer Output vs. Supply Voltage 0 1 2 3 4 CREST FACTOR (VPEAK /V rms) 5 00834-007 0 00834-006 -3dB 2 PEAK BUFFER OUTPUT (V) 1000 Figure 6. Frequency Response Driving Pin 1 16 PEAK INPUT BEFORE CLIPPING (V) 1 00834-005 -0.5 10% ERROR 00834-002 ADDITIONAL ERROR (% of Reading) 0.7 Figure 8. Additional Error vs. Crest Factor with Various Values of CAV Rev. H | Page 7 of 20 AD736 1.0 VIN = 200mV rms 1kHz SINE WAVE CAV = 100mF CF = 22mF VS = 5V 0.2 0 -0.2 -0.4 0 -0.5 -1.0 -1.5 VIN = SINE WAVE @ 1kHz CAV = 22F, CC = 47F, CF = 4.7F, VS = 5V -2.0 -0.6 -0.8 -60 -40 -20 0 20 40 60 80 TEMPERATURE (C) 100 120 140 -2.5 10mV Figure 9. Additional Error vs. Temperature VIN = 200mV rms CC = 47F CF = 47F VS = 5V CAV (F) 400 300 10 -0.5% 200 0.2 0.4 0.6 rms INPUT LEVEL (V) 0.8 1.0 1 10 00834-009 0 100 FREQUENCY (Hz) 1k 00834-012 -1% Figure 10. DC Supply Current vs. rms Input Level Figure 13. CAV vs. Frequency for Specified Averaging Error 1V VIN = 1kHz SINE WAVE INPUT AC-COUPLED VS = 5V INPUT LEVEL (rms) 1mV 1k 10k -3dB FREQUENCY (Hz) Figure 11. RMS Input Level (Pin 2) vs. -3 dB Frequency 100k 100mV 10mV 1mV 00834-010 100V 10V 100 -0.5% -1% VIN SINE WAVE AC-COUPLED CAV = 10F, CC = 47F, CF = 47F, VS = 5V 1 10 100 FREQUENCY (Hz) 1k 00834-013 10mV INPUT LEVEL (rms) 2V 100 VIN = 200mV rms 1kHz SINE WAVE CAV = 100F CF = 22F VS = 5V 500 100 1V Figure 12. Error vs. RMS Input Voltage (Pin 2), Output Buffer Offset Is Adjusted to Zero 600 DC SUPPLY CURRENT (A) 100mV INPUT LEVEL (rms) 00834-011 0.4 0.5 ERROR (% of Reading) 0.6 00834-008 ADDITIONAL ERROR (% of Reading) 0.8 Figure 14. RMS Input Level vs. Frequency for Specified Averaging Error Rev. H | Page 8 of 20 AD736 10nA 4.0 1nA INPUT BIAS CURRENT 3.0 2.5 2.0 2 4 6 8 10 SUPPLY VOLTAGE (V) 12 14 16 00834-014 0 Figure 15. Pin 2 Input Bias Current vs. Supply Voltage VS = 5V CC = 22F CF = 0F 100mV CAV = 10F 10mV CAV = 100F CAV = 33F 10ms 100ms 1s SETTLING TIME 10s 100s 00834-015 1mV 100V 1ms 100fA -55 -35 -15 5 25 45 65 TEMPERATURE (C) 85 105 Figure 17. Pin 2 Input Bias Current vs. Temperature 1V INPUT LEVEL (rms) 10pA 1pA 1.5 1.0 100pA Figure 16. RMS Input Level for Various Values of CAV vs. Settling Time Rev. H | Page 9 of 20 125 00834-016 INPUT BIAS CURRENT (pA) 3.5 AD736 THEORY OF OPERATION AC CC = 10F + DC OPTIONAL RETURN PATH FWR CURRENT MODE ABSOLUTE VALUE CC AD736 COM 1 8 8k VIN VIN 0.1F OUTPUT AMPLIFIER 2 INPUT AMPLIFIER IB<10pA +VS 6 rms OUTPUT 8k BIAS SECTION 3 7 rms TRANSLINEAR CORE CAV 4 5 0.1F TO COM PIN CA 33F + CF 10F (OPTIONAL) + 00834-017 -VS Figure 18. AD736 True RMS Circuit As shown by Figure 18, the AD736 has five functional subsections: the input amplifier, full-wave rectifier (FWR), rms core, output amplifier, and bias section. The FET input amplifier allows both a high impedance, buffered input (Pin 2) and a low impedance, wide dynamic range input (Pin 1). The high impedance input, with its low input bias current, is well suited for use with high impedance input attenuators. The output of the input amplifier drives a full-wave precision rectifier that, in turn, drives the rms core. The essential rms operations of squaring, averaging, and square rooting are performed in the core using an external averaging capacitor, CAV. Without CAV, the rectified input signal travels through the core unprocessed, as is done with the average responding connection (see Figure 19). A final subsection, an output amplifier, buffers the output from the core and allows optional low-pass filtering to be performed via the external capacitor, CF, which is connected across the feedback path of the amplifier. In the average responding connection, this is where all of the averaging is carried out. In the rms circuit, this additional filtering stage helps reduce any output ripple that was not removed by the averaging capacitor, CAV. TYPES OF AC MEASUREMENT The AD736 is capable of measuring ac signals by operating as either an average responding converter or a true rms-to-dc converter. As its name implies, an average responding converter computes the average absolute value of an ac (or ac and dc) voltage or current by full-wave rectifying and low-pass filtering the input signal; this approximates the average. The resulting output, a dc average level, is scaled by adding (or reducing) gain; this scale factor converts the dc average reading to an rms equivalent value for the waveform being measured. For example, the average absolute value of a sine wave voltage is 0.636 times VPEAK; the corresponding rms value is 0.707 x VPEAK. Therefore, for sine wave voltages, the required scale factor is 1.11 (0.707/0.636). In contrast to measuring the average value, true rms measurement is a universal language among waveforms, allowing the magnitudes of all types of voltage (or current) waveforms to be compared to one another and to dc. RMS is a direct measure of the power or heating value of an ac voltage compared to that of a dc voltage; an ac signal of 1 V rms produces the same amount of heat in a resistor as a 1 V dc signal. Rev. H | Page 10 of 20 AD736 Mathematically, the rms value of a voltage is defined (using a simplified equation) as ( ) V rms = Avg V 2 This involves squaring the signal, taking the average, and then obtaining the square root. True rms converters are smart rectifiers; they provide an accurate rms reading regardless of the type of waveform being measured. However, average responding converters can exhibit very high errors when their input signals deviate from their precalibrated waveform; the magnitude of the error depends on the type of waveform being measured. For example, if an average responding converter is calibrated to measure the rms value of sine wave voltages and then is used to measure either symmetrical square waves or dc voltages, the converter has a computational error 11% (of reading) higher than the true rms value (see Table 4). CALCULATING SETTLING TIME USING FIGURE 16 Figure 16 can be used to closely approximate the time required for the AD736 to settle when its input level is reduced in amplitude. The net time required for the rms converter to settle is the difference between two times extracted from the graph (the initial time minus the final settling time). As an example, consider the following conditions: a 33 F averaging capacitor, a 100 mV initial rms input level, and a final (reduced) 1 mV input level. From Figure 16, the initial settling time (where the 100 mV line intersects the 33 F line) is approximately 80 ms. The settling time corresponding to the new or final input level of 1 mV is approximately 8 seconds. Therefore, the net time for the circuit to settle to its new value is 8 seconds minus 80 ms, which is 7.92 seconds. Note that because of the smooth decay characteristic inherent with a capacitor/diode combination, this is the total settling time to the final value (that is, not the settling time to 1%, 0.1%, and so on, of the final value). In addition, this graph provides the worst-case settling time because the AD736 settles very quickly with increasing input levels. RMS MEASUREMENT--CHOOSING THE OPTIMUM VALUE FOR CAV Because the external averaging capacitor, CAV, holds the rectified input signal during rms computation, its value directly affects the accuracy of the rms measurement, especially at low frequencies. Furthermore, because the averaging capacitor appears across a diode in the rms core, the averaging time constant increases exponentially as the input signal is reduced. This means that as the input level decreases, errors due to nonideal averaging decrease, and the time required for the circuit to settle to the new rms level increases. Therefore, lower input levels allow the circuit to perform better (due to increased averaging) but increase the waiting time between measurements. Obviously, when selecting CAV, a trade-off between computational accuracy and settling time is required. Table 4. Error Introduced by an Average Responding Circuit when Measuring Common Waveforms Waveform Type 1 V Peak Amplitude Undistorted Sine Wave Symmetrical Square Wave Undistorted Triangle Wave Gaussian Noise (98% of Peaks <1 V) Rectangular Pulse Train SCR Waveforms 50% Duty Cycle 25% Duty Cycle Crest Factor (VPEAK/V rms) 1.414 1.00 1.73 3 2 10 True RMS Value (V) 0.707 1.00 0.577 0.333 0.5 0.1 Average Responding Circuit Calibrated to Read RMS Value of Sine Waves (V) 0.707 1.11 0.555 0.295 0.278 0.011 % of Reading Error Using Average Responding Circuit 0 +11.0 -3.8 -11.4 -44 -89 2 4.7 0.495 0.212 0.354 0.150 -28 -30 Rev. H | Page 11 of 20 AD736 RAPID SETTLING TIMES VIA THE AVERAGE RESPONDING CONNECTION Because the average responding connection shown in Figure 19 does not use the CAV averaging capacitor, its settling time does not vary with the input signal level. It is determined solely by the RC time constant of CF and the internal 8 k resistor in the output amplifier's feedback path. In most cases, the combined magnitudes of both the dc and ac error components need to be considered when selecting appropriate values for Capacitor CAV and Capacitor CF. This combined error, representing the maximum uncertainty of the measurement, is termed the averaging error and is equal to the peak value of the output ripple plus the dc error. EO IDEAL EO CC 10F + DC ERROR = EO - EO (IDEAL) (OPTIONAL) AD736 1 FULL WAVE RECTIFIER VIN VIN 2 CF 3 4 AVERAGE EO = EO DOUBLE-FREQUENCY RIPPLE +VS 8k +VS 7 INPUT AMPLIFIER TIME Figure 20. Output Waveform for Sine Wave Input Voltage OUTPUT As the input frequency increases, both error components decrease rapidly; if the input frequency doubles, the dc error and ripple reduce to one quarter and one half of their original values, respectively, and rapidly become insignificant. VOUT 6 BIAS SECTION OUTPUT AMPLIFIER -VS -VS COM 8 00834-019 8k CC rms CORE 5 CAV AC MEASUREMENT ACCURACY AND CREST FACTOR + CF 33F POSITIVE SUPPLY 0.1F +VS 0.1F NEGATIVE SUPPLY -VS 00834-018 COMMON Figure 19. AD736 Average Responding Circuit DC ERROR, OUTPUT RIPPLE, AND AVERAGING ERROR Figure 20 shows the typical output waveform of the AD736 with a sine wave input applied. As with all real-world devices, the ideal output of VOUT = VIN is never achieved exactly. Instead, the output contains both a dc and an ac error component. The crest factor of the input waveform is often overlooked when determining the accuracy of an ac measurement. Crest factor is defined as the ratio of the peak signal amplitude to the rms amplitude (crest factor = VPEAK/V rms). Many common waveforms, such as sine and triangle waves, have relatively low crest factors (2). Other waveforms, such as low duty-cycle pulse trains and SCR waveforms, have high crest factors. These types of waveforms require a long averaging time constant (to average out the long periods between pulses). Figure 8 shows the additional error vs. the crest factor of the AD736 for various values of CAV. As shown in Figure 20, the dc error is the difference between the average of the output signal (when all the ripple in the output is removed by external filtering) and the ideal dc output. The dc error component is therefore set solely by the value of the averaging capacitor used. No amount of post filtering (that is, using a very large CF) allows the output voltage to equal its ideal value. The ac error component, an output ripple, can be easily removed by using a large enough post filtering capacitor, CF. Rev. H | Page 12 of 20 AD736 APPLICATIONS CONNECTING THE INPUT 1 This input structure provides four input configurations as shown in Figure 21, Figure 22, Figure 23, and Figure 24. Figure 21 and Figure 22 show the high impedance configurations, and Figure 23 and Figure 24 show the low impedance connections used to extend the input voltage range. 2 3 4 CC VIN CF COM 8 AD736 +VS 7 OUTPUT 6 -VS CAV +VS VOUTDC 5 CAV 00834-028 The inputs of the AD736 resemble an op amp, with noninverting and inverting inputs. The input stages are JFETs accessible at Pin 1 and Pin 2. Designated as the high impedance input, Pin 2 is connected directly to a JFET gate. Pin 1 is the low impedance input because of the scaling resistor connected to the gate of the second JFET. This gate-resistor junction is not externally accessible and is servo-ed to the voltage level of the gate of the first JFET, as in a classic feedback circuit. This action results in the typical 8 k input impedance referred to ground or reference level. -VS Figure 23. Low-Z AC-Coupled Input Connection 1 CC 2 VIN 3 CF 4 -VS COM 8 AD736 +VS 7 OUTPUT 6 +VS VOUTDC CAV 5 1M CC 2 VIN 3 CF 4 -VS AD736 +VS 7 OUTPUT 6 +VS -VS Figure 24. Low-Z DC-Coupled Input Connection VOUTDC CAV 5 00834-026 CAV -VS Figure 21. High-Z AC-Coupled Input Connection (Default) 1 CC 2 VIN 3 CF 4 -VS COM 8 AD736 +VS 7 OUTPUT 6 +VS VOUTDC CAV 5 00834-027 CAV -VS 00834-029 CAV COM 8 1 Figure 22. High-Z DC-Coupled Input Connection Rev. H | Page 13 of 20 AD736 SELECTING PRACTICAL VALUES FOR INPUT COUPLING (CC), AVERAGING (CAV), AND FILTERING (CF) CAPACITORS Table 5 provides practical values of CAV and CF for several common applications. In addition, if the input voltage has more than 100 mV of dc offset, then the ac-coupling network shown in Figure 27 should be used in addition to CC. The input coupling capacitor, CC, in conjunction with the 8 k internal input scaling resistor, determine the -3 dB low frequency roll-off. This frequency, FL, is equal to FL = Note that at FL, the amplitude error is approximately -30% (-3 dB) of the reading. To reduce this error to 0.5% of the reading, choose a value of CC that sets FL at one-tenth of the lowest frequency to be measured. 1 2 (8000)(Value of CC in Farads) Table 5. Capacitor Selection Chart Application General-Purpose RMS Computation RMS Input Level 0 V to 1 V 0 mV to 200 mV General Purpose Average Responding 0 V to 1 V 0 mV to 200 mV SCR Waveform Measurement 0 mV to 200 mV 0 mV to 100 mV Audio Applications Speech Music 300 Hz 20 Hz Max Crest Factor 5 5 5 5 5 5 5 5 CAV (F) 150 15 33 3.3 None None None None 100 82 50 47 CF (F) 10 1 10 1 33 3.3 33 3.3 33 27 33 27 Settling Time 1 to 1% 360 ms 36 ms 360 ms 36 ms 1.2 sec 120 ms 1.2 sec 120 ms 1.2 sec 1.0 sec 1.2 sec 1.0 sec 3 10 1.5 100 0.5 68 18 ms 2.4 sec Settling time is specified over the stated rms input level with the input signal increasing from zero. Settling times are greater for decreasing amplitude input signals. OPTIONAL AC COUPLING CAPACITOR VIN CC 10F + 0.01F (OPTIONAL) 1kV 200mV +VS 9M CC 1N4148 2V 8k 20V 47k 1W 90k 200V -VS 1N4148 CF 3 10k -VS -VS FULL WAVE RECTIFIER VIN 2 900k AD736 1 4 8 COM +VS 8k INPUT AMPLIFIER 7 +VS 1F OUTPUT 6 BIAS SECTION OUTPUT AMPLIFIER rms CORE OUTPUT CAV 5 + 1F CAV 33F + CF 10F (OPTIONAL) Figure 25. AD736 with a High Impedance Input Attenuator Rev. H | Page 14 of 20 00834-020 1 0 mV to 200 mV 0 mV to 100 mV Low Frequency Cutoff (-3 dB) 20 Hz 200 Hz 20 Hz 200 Hz 20 Hz 200 Hz 20 Hz 200 Hz 50 Hz 60 Hz 50 Hz 60 Hz AD736 3 -IN AD711 CC 10F CC 6 + 2 8k AD736 1 FULL WAVE RECTIFIER VIN +IN 2 +VS 8k CF OUTPUT BIAS SECTION 3 -VS -VS +VS 1F 7 INPUT AMPLIFIER INPUT IMPEDANCE: 1012||10pF COM 8 OUTPUT AMPLIFIER rms CORE 4 OUTPUT 6 CAV 5 + 1F 00834-021 CAV 33F + CF 10F (OPTIONAL) Figure 26. Differential Input Connection CC 10F + (OPTIONAL) 8k CC AD736 1 FULL WAVE RECTIFIER VIN DC-COUPLED VIN 2 0.1F -VS +VS +VS 1F 7 OUTPUT BIAS SECTION 3 1M +VS 8k INPUT AMPLIFIER CF AC-COUPLED COM 8 OUTPUT AMPLIFIER rms CORE 4 OUTPUT 6 CAV 5 39M + OUTPUT VOS ADJUST CAV 33F + 1F 00834-022 1M CF 10F (OPTIONAL) -VS Figure 27. External Output VOS Adjustment CC 10F + COM 8k AD736 1 FULL WAVE RECTIFIER VIN 0.1F VIN 2 VS 2 +VS 8k 7 INPUT AMPLIFIER 1M VS 2 8 100k OUTPUT CF 3 -VS 4 BIAS SECTION OUTPUT AMPLIFIER rms CORE 6 4.7F CAV 4.7F 9V 5 + 33F 100k + CF 10F (OPTIONAL) Figure 28. Battery-Powered Option Rev. H | Page 15 of 20 00834-023 CC AD736 EVALUATION BOARD 00834-033 An evaluation board, AD736-EVALZ, is available for experimentation or becoming familiar with rms-to-dc converters. Figure 29 is a photograph of the board, and Figure 30 is the top silkscreen showing the component locations. Figure 31, Figure 32, Figure 33, and Figure 34 show the layers of copper, and Figure 35 shows the schematic of the board configured as shipped. The board is designed for multipurpose applications and can be used for the AD737 as well. 00834-030 Figure 31. Evaluation Board--Component-Side Copper 00834-032 00834-034 Figure 29. AD736 Evaluation Board Figure 32. Evaluation Board--Secondary-Side Copper Figure 30. Evaluation Board--Component-Side Silkscreen Figure 35 shows the board schematic with all movable jumpers. The jumper positions in black are default connections; the dottedoutline jumpers are optional connections. The board is tested prior to shipment and only requires a power supply connection and a precision meter to perform measurements. 00834-035 As shipped, the board is configured for dual supplies and high impedance input. Optional jumper locations enable low impedance and dc input connections. Using the low impedance input (Pin 1) often enables higher input signals than otherwise possible. A dc connection enables an ac plus dc measurement, but care must be taken so that the opposite polarity input is not dc-coupled to ground. Figure 33. Evaluation Board--Internal Power Plane 00834-036 Table 6 is the bill of materials for the AD736 evaluation board. Figure 34. Evaluation Board--Internal Ground Plane Rev. H | Page 16 of 20 AD736 -VS +VS GND2 GND3 GND4 C1 10F 25V W1 DC COUP VIN J1 LO-Z W4 LO-Z IN -VS +VS W3 AC COUP R3 0 + CC P2 HI-Z SEL CIN 0.1F + C2 10F 25V + GND1 HI-Z 1 IN 2 GND COM CC VIN AD736 +VS 8 R4 0 C6 7 0.1F +VS VOUT W2 3 R1 1M C4 0.1F 4 SEL J3 CF OUT -VS CAV 6 J2 CF1 5 CAV CAV 33F 16V+ NORM PD -VS CF2 00834-032 +VS FILT Figure 35. Evaluation Board Schematic Table 6. Evaluation Board Bill of Materials Qty 1 1 2 3 1 5 1 4 2 1 1 1 2 4 Name Test loop Test loop Capacitors Capacitors Capacitor Test loops Integrated circuit Test loops Connectors Header Header Resistor Resistors Headers Description Red Green Tantalum 10 F, 25 V 0.1 F, 16 V, 0603, X7R Tantalum 33 F, 16V, 20%, 6032 Purple RMS-to-dc converter Black BNC, right angle 6-pin, 2 x 3 3-pin 1 M, 1/10 W, 1%, 0603 0 , 5%, 0603 2-pin, 0.1" center Reference Designator +VS -VS C1, C2 C4, C6, CIN CAV CAV, HI Z, LO Z, VIN, VOUT DUT GND1, GND2, GND3, GND4 J1, J2 J3 P2 R1 R3, R4 W1, W2, W3, W4 Rev. H | Page 17 of 20 Manufacturer Components Corp. Components Corp. Nichicon Corp. KEMET Corp. Nichicon Corp. Components Corp. Analog Devices, Inc. Components Corp. AMP 3M Molex, Inc. Panasonic Corp. Panasonic Corp. Molex, Inc. Mfg. Part Number TP-104-01-02 TP-104-01-05 F931E106MCC C0603C104K4RACTU F931C336MCC TP-104-01-07 AD736JRZ TP-104-01-00 227161-1 929836-09-03 22-10-2031 ERJ3EKF1004V ERJ3GEY0R00V 22-10-2021 AD736 OUTLINE DIMENSIONS 0.400 (10.16) 0.365 (9.27) 0.355 (9.02) 8 5 1 4 0.280 (7.11) 0.250 (6.35) 0.240 (6.10) 0.100 (2.54) BSC 0.210 (5.33) MAX 0.060 (1.52) MAX 0.015 (0.38) MIN 0.150 (3.81) 0.130 (3.30) 0.115 (2.92) SEATING PLANE 0.022 (0.56) 0.018 (0.46) 0.014 (0.36) 0.325 (8.26) 0.310 (7.87) 0.300 (7.62) 0.195 (4.95) 0.130 (3.30) 0.115 (2.92) 0.015 (0.38) GAUGE PLANE 0.014 (0.36) 0.010 (0.25) 0.008 (0.20) 0.430 (10.92) MAX 0.005 (0.13) MIN 0.070 (1.78) 0.060 (1.52) 0.045 (1.14) 070606-A COMPLIANT TO JEDEC STANDARDS MS-001 CONTROLLING DIMENSIONS ARE IN INCHES; MILLIMETER DIMENSIONS (IN PARENTHESES) ARE ROUNDED-OFF INCH EQUIVALENTS FOR REFERENCE ONLY AND ARE NOT APPROPRIATE FOR USE IN DESIGN. CORNER LEADS MAY BE CONFIGURED AS WHOLE OR HALF LEADS. Figure 36. 8-Lead Plastic Dual In-Line Package [PDIP] Narrow Body (N-8) Dimensions shown in inches and (millimeters) 8 0.055 (1.40) MAX 5.00 (0.1968) 4.80 (0.1890) 5 0.310 (7.87) 0.220 (5.59) 1 4 4.00 (0.1574) 3.80 (0.1497) 0.100 (2.54) BSC 0.320 (8.13) 0.290 (7.37) 0.405 (10.29) MAX 0.023 (0.58) 0.014 (0.36) 0.25 (0.0098) 0.10 (0.0040) 0.150 (3.81) MIN 0.200 (5.08) 0.125 (3.18) 0.070 (1.78) 0.030 (0.76) SEATING PLANE 15 0 0.015 (0.38) 0.008 (0.20) CONTROLLING DIMENSIONS ARE IN INCHES; MILLIMETER DIMENSIONS (IN PARENTHESES) ARE ROUNDED-OFF INCH EQUIVALENTS FOR REFERENCE ONLY AND ARE NOT APPROPRIATE FOR USE IN DESIGN. 5 4 1.27 (0.0500) BSC 0.060 (1.52) 0.015 (0.38) 0.200 (5.08) MAX 8 1 COPLANARITY 0.10 SEATING PLANE 6.20 (0.2441) 5.80 (0.2284) 1.75 (0.0688) 1.35 (0.0532) 0.51 (0.0201) 0.31 (0.0122) 0.50 (0.0196) 45 0.25 (0.0099) 8 0 0.25 (0.0098) 0.17 (0.0067) 1.27 (0.0500) 0.40 (0.0157) COMPLIANT TO JEDEC STANDARDS MS-012-A A CONTROLLING DIMENSIONS ARE IN MILLIMETERS; INCH DIMENSIONS (IN PARENTHESES) ARE ROUNDED-OFF MILLIMETER EQUIVALENTS FOR REFERENCE ONLY AND ARE NOT APPROPRIATE FOR USE IN DESIGN. Figure 37. 8-Lead Ceramic Dual In-Line Package [CERDIP] (Q-8) Dimensions shown in inches and (millimeters) Figure 38. 8-Lead Standard Small Outline Package [SOIC_N] Narrow Body (R-8) Dimensions shown in millimeters and (inches) Rev. H | Page 18 of 20 012407-A 0.005 (0.13) MIN AD736 ORDERING GUIDE Model AD736AQ AD736BQ AD736AR AD736AR-REEL AD736AR-REEL7 AD736ARZ 1 AD736ARZ-R71 AD736ARZ-RL1 AD736BR AD736BR-REEL AD736BR-REEL7 AD736BRZ1 AD736BRZ-R71 AD736BRZ-RL1 AD736JN AD736JNZ1 AD736KN AD736KNZ1 AD736JR AD736JR-REEL AD736JR-REEL7 AD736JRZ1 AD736JRZ-RL1 AD736JRZ-R71 AD736KR AD736KR-REEL AD736KR-REEL7 AD736KRZ1 AD736KRZ-RL1 AD736KRZ-R71 AD736-EVALZ1 1 Temperature Range -40C to +85C -40C to +85C -40C to +85C -40C to +85C -40C to +85C -40C to +85C -40C to +85C -40C to +85C -40C to +85C -40C to +85C -40C to +85C -40C to +85C -40C to +85C -40C to +85C 0C to +70C 0C to +70C 0C to +70C 0C to +70C 0C to +70C 0C to +70C 0C to +70C 0C to +70C 0C to +70C 0C to +70C 0C to +70C 0C to +70C 0C to +70C 0C to +70C 0C to +70C 0C to +70C Package Description 8-Lead CERDIP 8-Lead CERDIP 8-Lead SOIC_N 8-Lead SOIC_N 8-Lead SOIC_N 8-Lead SOIC_N 8-Lead SOIC_N 8-Lead SOIC_N 8-Lead SOIC_N 8-Lead SOIC_N 8-Lead SOIC_N 8-Lead SOIC_N 8-Lead SOIC_N 8-Lead SOIC_N 8-Lead PDIP 8-Lead PDIP 8-Lead PDIP 8-Lead PDIP 8-Lead SOIC_N 8-Lead SOIC_N 8-Lead SOIC_N 8-Lead SOIC_N 8-Lead SOIC_N 8-Lead SOIC_N 8-Lead SOIC_N 8-Lead SOIC_N 8-Lead SOIC_N 8-Lead SOIC_N 8-Lead SOIC_N 8-Lead SOIC_N Evaluation Board Z = RoHS compliant part. Rev. H | Page 19 of 20 Package Option Q-8 Q-8 R-8 R-8 R-8 R-8 R-8 R-8 R-8 R-8 R-8 R-8 R-8 R-8 N-8 N-8 N-8 N-8 R-8 R-8 R-8 R-8 R-8 R-8 R-8 R-8 R-8 R-8 R-8 R-8 AD736 NOTES (c)2007 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. C00834-0-2/07(H) Rev. H | Page 20 of 20