Low Cost, Low Power, True RMS-to-DC Converter AD736 FEATURES GENERAL DESCRIPTION The AD736 is a low power, precision, monolithic true rms-to-dc 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 ac-coupled 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 (while 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 8k CC 1 VIN 2 CF 3 FULL WAVE RECTIFIER AD736 8 COM 8k 7 +VS INPUT AMPLIFIER BIAS SECTION 6 OUTPUT RMS CORE OUTPUT AMPLIFIER -VS 4 5 CAV 00834-F-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 of 1012 V Low input bias current: 25 pA max 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 max 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 may 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 The AD736 is capable of computing the average rectified value, absolute value, or true rms value of various input signals. Only one external component, an averaging capacitor, is required for the AD736 to perform true rms measurement. The low power consumption of 1 mW makes the AD736 suitable for many battery-powered applications. A high input impedance of 1012 eliminates the need for an external buffer when interfacing with input attenuators. 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. F 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: 781.326.8703 (c) 2004 Analog Devices, Inc. All rights reserved. AD736 TABLE OF CONTENTS Rapid Settling Times via the Average Responding Connection ............................................................................. 11 Specifications..................................................................................... 3 Absolute Maximum Ratings............................................................ 5 Pin Configuration ........................................................................ 5 ESD Caution.................................................................................. 5 DC Error, Output Ripple, and Averaging Error ..................... 11 AC Measurement Accuracy and Crest Factor ........................ 11 Typical Performance Characteristics ............................................. 6 Selecting Practical Values for Input Coupling (CC), Averaging (CAV), and Filtering (CF) Capacitors..................................... 11 Calculating Settling Time Using Figure 16 ............................... 8 Application Circuits ....................................................................... 13 Types of AC Measurement.......................................................... 9 Outline Dimensions ....................................................................... 15 Theory of Operation ...................................................................... 10 Ordering Guide .......................................................................... 16 RMS Measurement--Choosing the Optimum Value for CAV ........................................................................... 10 REVISION HISTORY 5/04--Data Sheet Changed from Rev. E to Rev. F. Changes to Specifications ............................................................ 2 Replaced Figure 18 ..................................................................... 10 Updated Outline Dimensions ................................................... 16 Changes to Ordering Guide ...................................................... 16 4/03--Data Sheet Changed from 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--Data Sheet Changed from 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 Rev. F | Page 2 of 16 AD736 SPECIFICATIONS Table 1. @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. Conditions CONVERSION ACCURACY Total Error, Internal Trim1 All Grades 1 kHz sine wave Using CC 0 mV rms-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 TMIN to TMAX A and B Grades J and K Grades vs. Supply Voltage @ 200 mV rms Input DC Reversal Error, DC-Coupled Nonlinearity2, 0 mV-200 mV Total Error, External Trim ERROR VS. CREST FACTOR3 Crest Factor = 1 to 3 Crest Factor = 5 INPUT CHARACTERISTICS High Impedance Input (Pin 2) Signal Range Continuous rms Level Peak Transient Input Input Resistance Input Bias Current Low Impedance Input (Pin 1) Signal Range Continuous rms Level Peak Transient Input Input Resistance Maximum Continuous Nondestructive Input Input Offset Voltage4 J and K Grades A and B Grades vs. Temperature vs. Supply VS = 5 V to 16.5 V VS = 5 V to 3 V @ 600 mV dc @ 100 mV rms 0 mV rms-200 mV rms Min AD736J/AD736A AD736K/AD736B Typ Max Min Typ Max VOUT = Avg(VIN2) Parameter TRANSFER FUNCTION 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 1 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 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. 3 Error versus 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 Rev. F | Page 3 of 16 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 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 Conditions Min 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 VS = 16.5 V VS = 16.5 V @ dc 0-1.6 0-3.6 0-4 0-4 2 Min AD736K/AD736B Typ Max 0.1 0.5 0.5 20 130 1.7 3.8 5 12 1 50 50 0-1.6 0-3.6 0-4 0-4 2 0.3 0.3 20 130 1.7 3.8 5 12 Unit mV mV V/C V/V V/V 3 0.2 3 0.2 V V 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 Rev. F | Page 4 of 16 +2.8, -3.2 5 160 230 16.5 200 270 AD736KN, AD736KR AD736BQ, AD736BR V A A AD736 ABSOLUTE MAXIMUM RATINGS Table 2. PIN CONFIGURATION 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. 5 8k CC 1 VIN 2 CF 3 FULL WAVE RECTIFIER 8 COM 8k 7 +VS INPUT AMPLIFIER BIAS SECTION 6 OUTPUT RMS CORE -VS 4 OUTPUT AMPLIFIER 5 CAV Figure 2. Pin Configuration for 8-Lead PDIP (N-8), 8-Lead SOIC (RN-8), and 8-Lead CERDIP (Q-8) Packages 8-Lead PDIP Package: JA = 165C/W 8-Lead CERDIP Package: JA = 110C/W 8-Lead SOIC Package: JA = 155C/W ESD CAUTION ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily accumulate on the human body and test equipment and can discharge without detection. Although this product features proprietary ESD protection circuitry, permanent damage may occur on devices subjected to high energy electrostatic discharges. Therefore, proper ESD precautions are recommended to avoid performance degradation or loss of functionality. Rev. F | Page 5 of 16 AD736 00834-F-001 Parameter Supply Voltage Internal Power Dissipation5 Input Voltage Output Short-Circuit Duration Differential Input Voltage Storage Temperature Range (Q) Storage Temperature Range (N, R) Lead Temperature Range (Soldering 60 sec) ESD Rating AD736 TYPICAL PERFORMANCE CHARACTERISTICS 10V VIN = 200mV rms 1kHz SINE WAVE CAV = 100F CF = 22F 1V INPUT LEVEL (rms) 0.5 SINE WAVE INPUT, VS = 5V, CAV = 22F, CF = 4.7F, CC = 22F 0.3 0.1 0 -0.1 100mV 1% ERROR 10mV -3dB -0.5 0 2 4 6 8 10 SUPPLY VOLTAGE (V) 12 14 10% ERROR 100V 0.1 16 Figure 3. Additional Error vs. Supply Voltage 10 100 -3dB FREQUENCY (kHz) 1000 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 2 0 2 4 6 8 10 SUPPLY VOLTAGE (V) 12 14 -3dB 100V 0.1 16 10 100 -3dB FREQUENCY (kHz) 1000 Figure 7. Frequency Response Driving Pin 2 Figure 4. Maximum Input Level vs. Supply Voltage 16 6 1kHz SINE WAVE INPUT ADDITIONAL ERROR (% OF READING) 14 12 10 8 6 00834-F-004 4 2 0 1 0 2 4 6 8 10 SUPPLY VOLTAGE (V) 12 14 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 00834-F-007 0 00834-F-006 1mV 00834-F-003 PEAK INPUT BEFORE CLIPPING (V) 1 Figure 6. Frequency Response Driving Pin 1 16 PEAK BUFFER OUTPUT (V) 00834-F-005 1mV -0.3 00834-F-002 ADDITIONAL ERROR (% OF READING) 0.7 CAV = 250F 0 16 Figure 5. Peak Buffer Output vs. Supply Voltage 1 2 3 4 CREST FACTOR (VPEAK/V rms) Figure 8. Additional Error vs. Crest Factor vs. CAV Rev. F | Page 6 of 16 5 AD736 1.0 VIN = 200mV rms 1kHz SINE WAVE CAV = 100F CF = 22F VS = 5V 0.2 0 -0.2 -0.4 -0.6 -0.8 -60 -40 -20 0 20 40 60 80 TEMPERATURE (C) 100 120 0 -0.5 -1.0 -1.5 VIN = SINE WAVE @ 1kHz CAV = 22F, CC = 47F, CF = 4.7F, VS = 5V -2.0 -2.5 10mV 140 Figure 9. Additional Error vs. Temperature 100mV INPUT LEVEL (rms) 1V 2V Figure 12. Error vs. RMS Input Voltage (Pin 2), Output Buffer Offset Is Adjusted to Zero 600 100 VIN = 200mV rms 1kHz SINE WAVE CAV = 100F CF = 22F VS = 5V 500 VIN = 200mV rms CC = 47F CF = 47F VS = 5V 400 CAV (F) DC SUPPLY CURRENT (A) 00834-F-011 0.4 0.5 ERROR (% OF READING) 0.6 00834-F-008 ADDITIONAL ERROR (% OF READING) 0.8 300 10 -0.5% 200 0 0.2 0.4 0.6 rms INPUT LEVEL (V) 0.8 1 10 1.0 100 FREQUENCY (Hz) 1k Figure 13. CAV vs. Frequency for Specified Averaging Error Figure 10. DC Supply Current vs. RMS Input Level 1V 10mV VIN = 1kHz SINE WAVE INPUT AC-COUPLED VS = 5V -1% INPUT LEVEL (rms) -0.5% 100mV 1mV 10V 100 1k 10k -3dB FREQUENCY (Hz) Figure 11. -3 dB Frequency vs. RMS Input Level (Pin 2) 10mV VIN SINE WAVE AC-COUPLED CAV = 10F, CC = 47F, CF = 47F, VS = 5V 00834-F-010 100V 1mV 100k 1 10 100 FREQUENCY (Hz) 00834-F-013 INPUT LEVEL (rms) 00834-F-012 00834-F-009 100 -1% 1k Figure 14. RMS Input Level vs. Frequency for Specified Averaging Error Rev. F | Page 7 of 16 AD736 4.0 CALCULATING SETTLING TIME USING FIGURE 16 Figure 16 may 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. INPUT BIAS CURRENT (pA) 3.5 3.0 2.5 2.0 1.0 00834-F-014 1.5 0 2 4 6 8 10 SUPPLY VOLTAGE (V) 12 14 16 Figure 15. Pin 2 Input Bias Current vs. Supply Voltage 1V VS = 5V CC = 22F CF = 0F 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 (i.e., not the settling time to 1%, 0.1%, and so on, of the final value). Also, this graph provides the worst-case settling time since the AD736 settles very quickly with increasing input levels. INPUT LEVEL (rms) 100mV CAV = 10F 10mV CAV = 100F CAV = 33F 00834-F-015 1mV 100V 1ms 10ms 100ms 1s SETTLING TIME 10s 100s Figure 16. Settling Time vs. RMS Input Level for Various Values of CAV 10nA INPUT BIAS CURRENT 1nA 100pA 10pA 100fA -55 00834-F-016 1pA -35 -15 5 25 45 65 TEMPERATURE (C) 85 105 125 Figure 17. Pin 2 Input Bias Current vs. Temperature Rev. F | Page 8 of 16 AD736 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 then 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 that of VPEAK; the corresponding rms value is 0.707 times VPEAK. Therefore, for sine wave voltages, the required scale factor is 1.11 (0.707 divided by 0.636). 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 is then used to measure either symmetrical square waves or dc voltages, the converter will have a computational error 11% (of reading) higher than the true rms value (see Table 3). 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. Table 3. 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 True rms Value (V) 0.707 1.00 0.577 Average Responding Circuit Calibrated to Read RMS Value of Sine Waves Will Read (V) 0.707 1.11 0.555 % of Reading Error Using Average Responding Circuit 0 11.0 -3.8 3 2 10 0.333 0.5 0.1 0.295 0.278 0.011 -11.4 -44 -89 2 4.7 0.495 0.212 0.354 0.150 -28 -30 Rev. F | Page 9 of 16 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-F-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, which 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 (Figure 19). RMS MEASUREMENT--CHOOSING THE OPTIMUM VALUE FOR CAV Since 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 while 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. 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. Rev. F | Page 10 of 16 AD736 EO RAPID SETTLING TIMES VIA THE AVERAGE RESPONDING CONNECTION IDEAL EO DC ERROR = EO - EO (IDEAL) Because the average responding connection shown in Figure 19 does not use the CAV averaging capacitor, its settling time does not vary with 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. TIME CC 10F + Figure 20. Output Waveform for Sine Wave Input Voltage (OPTIONAL) 1 8 AD736 FULL WAVE RECTIFIER VIN VIN 2 COM +VS 8k 7 INPUT AMPLIFIER CF 3 4 +VS OUTPUT BIAS SECTION 6 VOUT OUTPUT AMPLIFIER -VS -VS 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. 8k CC rms CORE 5 CAV + CF 33F +VS POSITIVE SUPPLY COMMON 0.1F -VS 00834-F-018 0.1F NEGATIVE SUPPLY 00834-F-019 AVERAGE EO = EO DOUBLE-FREQUENCY RIPPLE AC MEASUREMENT ACCURACY AND CREST FACTOR 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 time periods between pulses). Figure 8 shows the additional error versus the crest factor of the AD736 for various values of CAV. Figure 19. AD736 Average Responding Circuit SELECTING PRACTICAL VALUES FOR INPUT COUPLING (CC), AVERAGING (CAV), AND FILTERING (CF) CAPACITORS 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. As shown, the dc error is the difference between the average of the output signal (when all the ripple in the output has been 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 (i.e., using a very large CF) will allow the output voltage to equal its ideal value. The ac error component, an output ripple, may be easily removed by using a large enough post-filtering capacitor, CF. In most cases, the combined magnitudes of both the dc and ac error components need to be considered when selecting appropriate values for capacitors CAV and 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. Table 4 provides practical values of CAV and CF for several common applications. The input coupling capacitor, CC, in conjunction with the 8 k internal input scaling resistor, determines the -3 dB low frequency roll-off. This frequency, FL, is equal to FL = 1 2 (8,000)(The Value of C C in Farads ) Note that at FL, the amplitude error is approximately -30% (-3 dB) of reading. To reduce this error to 0.5% of reading, choose a value of CC that sets FL at one-tenth of the lowest frequency to be measured. In addition, if the input voltage has more than 100 mV of dc offset, then the ac-coupling network shown in Figure 23 should be used in addition to capacitor CC. Rev. F | Page 11 of 16 AD736 Table 4. 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 SCR Waveform Measurement 0 mV to 200 mV 0 mV to 200 mV 0 mV to 100 mV Audio Applications Speech Music 6 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 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 F CF (F) 10 1 10 1 33 3.3 33 3.3 33 27 33 27 F Settling Time6 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 F 100 F 0.5 F 68 F 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. Rev. F | Page 12 of 16 AD736 APPLICATION CIRCUITS OPTIONAL AC COUPLING CAPACITOR VIN CC 10F + 0.01F (OPTIONAL) 1kV +VS 200mV 9M 1N4148 2V 900k 8k CC 1 FULL WAVE RECTIFIER VIN 2 20V 47k 1W 1N4148 90k 200V +VS 8k +VS 1F 7 INPUT AMPLIFIER CF OUTPUT 3 -VS -VS 10k COM 8 AD736 BIAS SECTION -VS OUTPUT AMPLIFIER rms CORE 4 OUTPUT 6 CAV 5 + 00834-F-020 CAV 33F + 1F CF 10F (OPTIONAL) Figure 21. AD736 with a High Impedance Input Attenuator 3 AD711 6 CC 10F CC + 2 8k 1 AD736 FULL WAVE RECTIFIER VIN +IN 2 INPUT IMPEDANCE: 1012 INPUT IMPEDANCE: 10pF CF 3 -VS -VS 4 8 COM +VS 8k 7 INPUT AMPLIFIER +VS 1F OUTPUT BIAS SECTION 6 OUTPUT AMPLIFIER rms CORE OUTPUT CAV 5 + 1F CAV 33F + CF 10F (OPTIONAL) Figure 22. Differential Input Connection Rev. F | Page 13 of 16 00834-F-021 -IN AD736 CC 10F + (OPTIONAL) 8k CC 1 FULL WAVE RECTIFIER VIN DC-COUPLED VIN 2 +VS 8k CF AC-COUPLED OUTPUT BIAS SECTION 3 1M -VS -VS OUTPUT 6 OUTPUT AMPLIFIER rms CORE 4 +VS CAV 5 39M + OUTPUT VOS ADJUST CAV 33F + 1F 00834-F-022 -VS +VS 1F 7 INPUT AMPLIFIER 0.1F 1M COM 8 AD736 CF 10F (OPTIONAL) Figure 23. External Output VOS Adjustment CC 10F + CC 8k 1 AD736 FULL WAVE RECTIFIER VIN 0.1F VIN 2 8k COM VS +VS 2 8 7 INPUT AMPLIFIER 1M 100k OUTPUT CF BIAS SECTION 3 -VS OUTPUT AMPLIFIER rms CORE 4 6 4.7F CAV 4.7F 9V 5 + 33F 100k 00834-F-023 + CF 10F (OPTIONAL) Figure 24. Battery-Powered Option CC CC + 8k AD736 1 VIN FULL WAVE RECTIFIER 2 INPUT AMPLIFIER 8 7 +VS 6 3 COM OUTPUT Figure 25. Low Z, AC-Coupled Input Connection Rev. F | Page 14 of 16 00834-F-024 VIN AD736 OUTLINE DIMENSIONS 0.375 (9.53) 0.365 (9.27) 0.355 (9.02) 8 5 1 4 0.295 (7.49) 0.285 (7.24) 0.275 (6.98) 0.325 (8.26) 0.310 (7.87) 0.300 (7.62) 0.100 (2.54) BSC 0.150 (3.81) 0.135 (3.43) 0.120 (3.05) 0.015 (0.38) MIN 0.180 (4.57) MAX 0.150 (3.81) 0.130 (3.30) 0.110 (2.79) 0.022 (0.56) 0.018 (0.46) 0.014 (0.36) 0.015 (0.38) 0.010 (0.25) 0.008 (0.20) SEATING PLANE 0.060 (1.52) 0.050 (1.27) 0.045 (1.14) COMPLIANT TO JEDEC STANDARDS MO-095AA 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 Figure 26. 8-Lead Plastic Dual In-Line Package [PDIP] (N-8) Dimensions shown in inches and (millimeters) 0.005 (0.13) MIN 0.055 (1.40) MAX 8 5 0.310 (7.87) 0.220 (5.59) PIN 1 1 4 0.100 (2.54) BSC 0.320 (8.13) 0.290 (7.37) 0.405 (10.29) MAX 0.200 (5.08) MAX 0.200 (5.08) 0.125 (3.18) 0.023 (0.58) 0.014 (0.36) 0.060 (1.52) 0.015 (0.38) 0.150 (3.81) MIN SEATING 0.070 (1.78) PLANE 0.030 (0.76) 15 0 0.015 (0.38) 0.008 (0.20) CONTROLLING DIMENSIONS ARE IN INCHES; MILLIMETERS DIMENSIONS (IN PARENTHESES) ARE ROUNDED-OFF INCH EQUIVALENTS FOR REFERENCE ONLY AND ARE NOT APPROPRIATE FOR USE IN DESIGN Figure 27. 8-Lead Ceramic Dual In-Line Package [CERDIP] (Q-8) Dimensions shown in inches and (millimeters) Rev. F | Page 15 of 16 AD736 5.00 (0.1968) 4.80 (0.1890) 8 5 4.00 (0.1574) 3.80 (0.1497) 1 4 1.27 (0.0500) BSC 0.25 (0.0098) 0.10 (0.0040) 6.20 (0.2440) 5.80 (0.2284) 1.75 (0.0688) 1.35 (0.0532) 0.51 (0.0201) COPLANARITY SEATING 0.31 (0.0122) 0.10 PLANE 0.50 (0.0196) x 45 0.25 (0.0099) 8 0.25 (0.0098) 0 1.27 (0.0500) 0.40 (0.0157) 0.17 (0.0067) COMPLIANT TO JEDEC STANDARDS MS-012AA 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 28. 8-Lead Standard Small Outline Package [SOIC] Narrow Body (R-8) Dimensions shown in millimeters and (inches) ORDERING GUIDE Model AD736JN AD736KN AD736AQ AD736BQ AD736AR AD736AR-Reel AD736AR-Reel-7 AD736BR AD736BR-Reel AD736BR-Reel-7 AD736JR AD736JR-Reel AD736JR-Reel-7 AD736JRZ1 AD736JRZ-RL1 AD736JRZ-R71 AD736KR AD736KR-Reel AD736KR-Reel-7 AD736KRZ1 AD736KRZ-RL1 AD736KRZ-R71 1 Temperature Range 0C to +70C 0C to +70C -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 Package Description PDIP PDIP CERDIP CERDIP SOIC SOIC SOIC SOIC SOIC SOIC SOIC SOIC SOIC SOIC SOIC SOIC SOIC SOIC SOIC SOIC SOIC SOIC Z = Pb-Free Part. (c) 2004 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. C00834-0-5/04(F) Rev. F | Page 16 of 16 Package Option N-8 N-8 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 R-8 R-8 R-8 R-8 R-8 R-8