a FEATURES 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 10 12 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 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 physical 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 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. The AD736 allows the choice of two signal input terminals: a high impedance (1012 ) FET input that directly interfaces with high Z input attenuators and a low impedance (8 k) input Low Cost, Low Power, True RMS-to-DC Converter AD736 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 -VS 4 OUTPUT AMPLIFIER 5 CAV that allows the measurement of 300 mV input level while operating from the minimum power supply voltage of +2.8 V, -3.2 V. The two inputs may be used either singly or differentially. The AD736 achieves a 1% of reading error bandwidth exceeding 10 kHz for 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 commercial temperature range of 0C to +70C. The AD736A and AD736B grades are rated over the industrial temperature range of -40C to +85C. The AD736 is available in three low cost, 8-lead packages: plastic miniDIP, plastic SOIC, and hermetic 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 requiring up to 300 mV rms input signal operating from low power supply voltages. REV. D 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. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices. 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) Analog Devices, Inc., 2002 AD736-SPECIFICATIONS Parameter (@ 25C 5 V supplies, ac-coupled with 1 kHz sine wave input applied, unless otherwise noted.) Conditions VOUT = TRANSFER FUNCTION CONVERSION ACCURACY Total Error, Internal Trim1 All Grades TMIN-TMAX A and B Grades J and K Grades vs. Supply Voltage @ 200 mV rms Input @ 200 mV rms Input DC Reversal Error, DC-Coupled Nonlinearity2, 0-200 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 Continuous rms Level Peak Transient Input Peak Transient Input Peak Transient Input Input Resistance Input Bias Current Low Impedance Input (Pin 1) Signal Range Continuous rms Level Continuous rms Level Peak Transient Input Peak Transient Input 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. Supply AD736J/AD736A Min Typ Max ( ) Avg VIN 2 1 kHz Sine Wave AC-Coupled Using CC 0-200 mV rms 200 mV-1 V rms 0.3/0.3 -1.2 @ 200 mV rms @ 200 mV rms 0.007 AD736K/AD736B Min Typ Max VOUT = Unit ( ) Avg VIN 2 0.2/0.2 0.3/0.3 mV/ % of Reading -1.2 2.0 % of Reading 0.5/0.5 2.0 0.007 0.5/0.5 mV/ % of Reading % of Reading/C +0.1 -0.3 0.7/0.7 VS = 5 V to 16.5 V VS = 5 V to 3 V 0 0 +0.06 -0.18 +0.1 -0.3 0 0 +0.06 -0.18 @ 600 mV dc @ 100 mV rms 0-200 mV rms 0 1.3 +0.25 0.1/0.5 2.5 +0.35 0 1.3 2.5 +0.25 +0.35 0.1/0.3 % of Reading % of Reading mV/ % of Reading 0.7 2.5 % Additional Error % Additional Error 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 0.7 2.5 200 1 0.9 2.7 4.0 1012 1 VS = 3 V to 16.5 V 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 1.7 3.8 11 8 6.4 All Supply Voltages AC-Coupled 8 50 80 VS = 5 V to 16.5 V VS = 5 V to 3 V -2- 0.9 4.0 200 1 2.7 1012 1 25 300 l 25 mV rms V rms V V V pA 9.6 mV rms V rms V V V k 12 12 V p-p 3 3 30 150 3 3 30 150 mV mV V/C V/V V/V 9.6 6.4 1.7 3.8 11 8 8 50 80 300 l %/V %/V REV. D AD736 Parameter Conditions OUTPUT CHARACTERISTICS Output Offset Voltage 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 Output Voltage Swing 2 k Load VS = +2.8 V, -3.2 V 2 k Load VS = 5 V 2 k Load VS = 16.5 V No Load VS = 16.5 V Output Current Short-Circuit Current Output Resistance @ dc 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 FREQUENCY RESPONSE 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 = l 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 AD736J/AD736A Min Typ Max 0.1 1 50 50 0 to +1.6 0 to +3.6 0 to +4 0 to +4 2 AD736K/AD736B Min Typ Max 0.5 0.5 20 130 +1.7 +3.8 +5 +12 0.1 1 50 50 0 to +1.6 0 to +3.6 0 to +4 0 to +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 +2.8, -3.2 Zero Signal Sine Wave Input TEMPERATURE RANGE Operating, Rated Performance Commercial (0C to +70C) Industrial (-40C to +85C) 5 160 230 AD736J AD736A 16.5 200 270 +2.8, -3.2 5 160 230 16.5 200 270 V A A AD736K AD736B NOTES l Accuracy is specified with the AD736 connected as shown in Figure 1 with capacitor C C. 2 Nonlinearity is defined as the maximum deviation (in percent error) from a straight line connecting the readings at 0 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 = V PEAK/V rms. 4 DC offset does not limit ac resolution. Specifications are subject to change without notice. Specifications shown in boldface are tested on all production units at final electrical test. Results from those tests are used to calculate outgoing quality levels. REV. D -3- AD736 ABSOLUTE MAXIMUM RATINGS 1 PIN CONFIGURATION Supply Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16.5 V Internal Power Dissipation2 . . . . . . . . . . . . . . . . . . . . 200 mW Input Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VS Output Short-Circuit Duration . . . . . . . . . . . . . . . . Indefinite Differential Input Voltage . . . . . . . . . . . . . . . . . . +VS and -VS Storage Temperature Range (Q) . . . . . . . . . -65C to +150C Storage Temperature Range (N, R) . . . . . . . -65C to +125C Operating Temperature Range AD736J/AD736K . . . . . . . . . . . . . . . . . . . . . . . 0C to +70C AD736A/AD736B . . . . . . . . . . . . . . . . . . . . -40C to +85C Lead Temperature Range (Soldering 60 sec) . . . . . . . . . 300C ESD Rating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 500 V 8-Lead MiniDIP (N-8), 8-Lead SOIC (RN-8), 8-Lead CERDIP (Q-8) 8k CC 1 FULL WAVE RECTIFIER VIN 2 CF 3 NOTES 1 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. 2 8-Lead Plastic Package: JA = 165C/W 8-Lead CERDIP Package: JA = 110C/W 8-Lead Small Outline Package: JA = 155C/W AD736 8 COM 8k 7 +VS INPUT AMPLIFIER BIAS SECTION 6 OUTPUT rms CORE OUTPUT AMPLIFIER -VS 4 5 CAV ORDERING GUIDE Model Temperature Range Package Description Package Option AD736JN AD736KN AD736JR AD736KR AD736AQ AD736BQ AD736JR-Reel AD736JR-Reel-7 AD736KR-Reel AD736KR-Reel-7 0C to +70C 0C to +70C 0C to +70C 0C to +70C -40C to +85C -40C to +85C 0C to +70C 0C to +70C 0C to +70C 0C to +70C Plastic Mini-DIP Plastic Mini-DIP Plastic SOIC Plastic SOIC CERDIP CERDIP Plastic SOIC Plastic SOIC Plastic SOIC Plastic SOIC N-8 N-8 RN-8 RN-8 Q-8 Q-8 RN-8 RN-8 RN-8 RN-8 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 the AD736 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. -4- REV. D Typical Performance Characteristics-AD736 16 +0.3 +0.1 0 -0.1 -0.3 -0.5 12 10 PIN 1 8 PIN 2 6 2 4 6 8 10 12 SUPPLY VOLTAGE - V 14 100V 2 16 10V SINE WAVE INPUT, VS = 5V, CAV = 22F, CF = 4.7F, CC = 22F 4 6 8 10 12 SUPPLY VOLTAGE - V 2 INPUT LEVEL - rms 100mV 1% ERROR 10mV 1% ERROR 10% ERROR -3dB 10% ERROR 1 10 100 FREQUENCY - kHz 1000 100V 0.1 1 10 100 FREQUENCY - kHz 0.4 0.2 0 -0.2 -0.4 3 2 1 CAV = 100F CAV = 250F 1 2 3 4 CREST FACTOR (VPEAK/V rms) 5 10mV VIN = 1kHz SINE WAVE INPUT AC-COUPLED VS = 5V VIN = 1kHz SINE WAVE INPUT VS = 5V CAV = 22F CC = 10F 500 INPUT LEVEL - rms 0.6 DC SUPPLY CURRENT - A VIN = 200mV rms 1kHz SINE WAVE CAV = 100F CF = 22F 4 TPC 6. Additional Error vs. Crest Factor vs. CAV 600 0.8 3ms BURST OF 1kHz = 3 CYCLES CAV = 10F 200mV rms SIGNAL VS = 5V CC = 22F CAV = 33F CF = 100F 8 0 1000 TPC 5. Frequency Response Driving Pin 2 TPC 4. Frequency Response Driving Pin 1 100k 6 SINE WAVE INPUT, VS = 5V, CAV = 22F, CF = 4.7F, CC = 22F 1mV 1k 10k -3dB FREQUENCY - Hz TPC 3. Peak Buffer Output vs. Supply Voltage 10mV 1mV 100V 0.1 10V 100 16 100mV -3dB ADDITIONAL ERROR - % OF READING 14 1V 1V INPUT LEVEL - rms 0 TPC 2. Maximum Input Level vs. Supply Voltage TPC 1. Additional Error vs. Supply Voltage 10V 1mV 4 0 0 VIN = 1kHz SINE WAVE INPUT AC-COUPLED VS = 5V 14 ADDITIONAL ERROR - % OF READING +0.5 10mV DC-COUPLED INPUT LEVEL - rms VIN = 200mV rms 1kHz SINE WAVE CAV = 100F CF = 22F PEAK INPUT BEFORE CLIPPING - V ADDITIONAL ERROR - % OF READING +0.7 400 300 1mV 100V 200 -0.6 -0.8 -60 -40 -20 100 0 20 40 60 80 100 120 140 TEMPERATURE - C TPC 7. Additional Error vs. Temperature REV. D 0 0.2 0.4 0.6 0.8 rms INPUT LEVEL - V TPC 8. DC Supply Current vs. rms lnput Level -5- 1.0 10V 100 1k 10k -3dB FREQUENCY - Hz TPC 9. -3 dB Frequency vs. rms Input Level (Pin 2) 100k AD736 1V 100 1.0 VIN = 200mV rms CC = 47F CF = 47F VS = 5V -0.5% 100mV -0.5 -1.0 10 -0.5% -1.5 VIN = SINE WAVE @ 1kHz CAV = 22F, CC = 47F, CF = 4.7F, VS = 5V -2.0 -2.5 10mV 100mV INPUT LEVEL - rms 1V 1 10 2V VIN = SINE WAVE AC-COUPLED CAV = 10F, CC = 47F, CF = 47F, VS = 5V 1mV 1k 100 FREQUENCY - Hz TPC 11. CAV vs. Frequency for Specified Averaging Error 1V 4.0 2.5 2.0 CAV = 100F CAV = 10F 10mV 10 100 FREQUENCY - Hz 1k 10nA 1nA INPUT BIAS CURRENT INPUT LEVEL rms 100mV 3.0 1 TPC 12. rms Input Level vs. Frequency for Specified Averaging Error VS = 5V CC = 22F CF = 0F 3.5 CAV = 33F 1mV 100pA 10pA 1pA 1.5 1.0 10mV -1% TPC 10. Error vs. rms Input Voltage (Pin 2), Output Buffer Offset is Adjusted to Zero INPUT BIAS CURRENT - pA -1% INPUT LEVEL - rms 0 CAV - F ERROR - % OF READING 0.5 0 2 4 6 8 10 12 SUPPLY VOLTAGE - V 14 16 TPC 13. Pin 2 Input Bias Current vs. Supply Voltage 100V 1ms 10ms 100ms 1s SETTLING TIME 10s 100s TPC 14. Settling Time vs. rms Input Level for Various Values of CAV -6- 100fA -55 -35 -15 +5 +25 +45 +65 +85 +105 +125 TEMPERATURE - C TPC 15. Pin 2 Input Bias Current vs. Temperature REV. D AD736 value for the waveform being measured. For example, the average absolute value of a sine wave voltage is 0.636 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). CALCULATING SETTLING TIME USING TPC 14 TPC 14 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, an initial rms input level of 100 mV, and a final (reduced) input level of 1 mV. From TPC 14, the initial settling time (where the 100 mV line intersects the 33 F line) is approximately 80 ms. 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. 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. 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. As an 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 will have a computational error 11% (of reading) higher than the true rms value (see Table I). TYPES OF AC MEASUREMENT The AD736 is capable of measuring ac signals by operating as either an average responding 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 Table I. 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 REV. D Crest Factor (VPEAK/V rms) True rms Value Average Responding Circuit Calibrated to Read rms Value of Sine Waves Will Read 1.414 1.00 1.73 0.707 V 1.00 V 0.577 V 0.707 V 1.11 V 0.555 V 0% 11.0% -3.8% 3 2 10 0.333 V 0.5 V 0.1 V 0.295 V 0.278 V 0.011 V -11.4% -44% -89% 2 4.7 0.495 V 0.212 V 0.354 V 0.150 V -28% -30% -7- % of Reading Error Using Average Responding Circuit AD736 AD736 THEORY OF OPERATION RMS MEASUREMENT--CHOOSING THE OPTIMUM VALUE FOR CAV As shown by Figure 1, the AD736 has five functional subsections: input amplifier, full-wave rectifier, rms core, output amplifier, and bias section. The FET input amplifier allows both a high impedance, buffered input (Pin 2) or a low impedance, widedynamic-range input (Pin 1). The high impedance input, with its low input bias current, is well suited for use with high impedance input attenuators. 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 it takes 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. The output of the input amplifier drives a full-wave precision rectifier, which in turn, drives the rms core. It is in the core that the essential rms operations of squaring, averaging, and square rooting are performed, 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 2). A final subsection, an output amplifier, buffers the output from the core and also allows optional low-pass filtering to be performed via the external capacitor, CF, 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. CC 10F + (OPTIONAL) 8k CC 1 CC 10F (OPTIONAL) + 2 8 8k 7 +VS CF -VS CURRENT MODE ABSOLUTE VALUE -VS 8 1 COM +VS INPUT AMPLIFIER 3 CC FULL WAVE RECTIFIER VIN VIN AD736 4 OUTPUT BIAS SECTION VOUT 6 OUTPUT AMPLIFIER rms CORE COM 5 CAV + 8k CF 33F VIN +VS POSITIVE SUPPLY 0.1F VIN 2 7 -VS 3 COMMON 0.1F FET OP AMP IB<10pA CF +VS NEGATIVE SUPPLY 8k -VS Figure 2. AD736 Average Responding Circuit rms TRANSLINEAR CORE 6 4 5 RMS OUTPUT RAPID SETTLING TIMES VIA THE AVERAGE RESPONDING CONNECTION Because the average responding connection shown in Figure 2 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. CAV CAV 33F CF 10F (OPTIONAL) +VS POSITIVE SUPPLY 0.1F COMMON 0.1F NEGATIVE SUPPLY -VS Figure 1. AD736 True rms Circuit -8- REV. D AD736 DC ERROR, OUTPUT RIPPLE, AND AVERAGING ERROR 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). TPC 6 shows the additional error versus the crest factor of the AD736 for various values of CAV. Figure 3 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 exactly achieved; instead, the output contains both a dc and an ac error component. EO IDEAL EO DC ERROR = EO - EO (IDEAL) AVERAGE EO = EO SELECTING PRACTICAL VALUES FOR INPUT COUPLING (CC), AVERAGING (C AV), AND FILTERING (CF) CAPACITORS DOUBLE-FREQUENCY RIPPLE TIME Table II provides practical values of CAV and CF for several common applications. Figure 3. Output Waveform for Sine Wave Input Voltage 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 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. The input coupling capacitor, CC, in conjunction with the 8 k internal input scaling resistor, determines the -3 dB low frequency rolloff. This frequency, FL, is equal to: FL = 1 2(8 , 000)(TheValue of CC 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 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. In addition, if the input voltage has more than 100 mV of dc offset, then the ac-coupling network shown in Figure 6 should be used in addition to capacitor CC. 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. Table II. AD737 Capacitor Selection Chart Application RMS Input Level General-Purpose rms Computation 0-1 V 0-200 mV General-Purpose Average Responding 0-1 V 0-200 mV SCR Waveform Measurement Audio Applications Speech Music Low Frequency Cutoff (-3 dB) Max Crest Factor 20 Hz 200 Hz 20 Hz 200 Hz 20 Hz 200 Hz 5 5 5 5 CAV CF Settling Time* to 1% 150 F 15 F 33 F 3.3 F None None 10 F 1 F 10 F 1 F 33 F 3.3 F 360 ms 36 ms 360 ms 36 ms 1.2 sec 120 ms 33 F 3.3 F 33 F 27 F 1.2 sec 120 ms 1.2 sec 1.0 sec 20 Hz 200 Hz 50 Hz 60 Hz 5 5 None None 100 F 82 F 0-100 mV 50 Hz 60 Hz 5 5 50 F 47 F 33 F 27 F 1.2 sec 1.0 sec 0-200 mV 0-100 mV 300 Hz 20 Hz 3 10 1.5 F 100 F 0.5 F 68 F 18 ms 2.4 sec 0-200 mV *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. D -9- AD736 Applications Circuits OPTIONAL AC-COUPLING CAPACITOR VIN CC 10F + 0.01F (OPTIONAL) 1kV +VS 200mV 9M 8k CC 1 1N4148 FULL WAVE RECTIFIER VIN 2V 2 900k 20V 47k 1W 1N4148 -VS 10k 7 1F CF 3 -VS +VS 8k INPUT AMPLIFIER 90k 200V COM 8 AD736 +VS OUTPUT BIAS SECTION OUTPUT 6 OUTPUT AMPLIFIER -VS rms CORE 4 CAV 5 + CAV 33F + 1F CF 10F (OPTIONAL) Figure 4. AD736 with a High Impedance Input Attenuator -IN 3 CC 10F 6 + AD711 CC 8k 1 2 AD736 FULL WAVE RECTIFIER VIN +IN 2 INPUT IMPEDANCE: 1012 CF INPUT IMPEDANCE: 10pF 3 -VS COM +VS 8k 7 1F INPUT AMPLIFIER +VS OUTPUT BIAS SECTION 6 OUTPUT AMPLIFIER -VS 4 8 rms CORE OUTPUT CAV 5 + 1F CAV 33F + CF (OPTIONAL) 10F Figure 5. Differential Input Connection -10- REV. D AD736 DC-COUPLED CC 10F + AC-COUPLED CC COM 1 FULL WAVE RECTIFIER 2 +VS 8k 7 1F INPUT AMPLIFIER 0.1F CF AC-COUPLED OUTPUT BIAS SECTION 3 1M 6 OUTPUT AMPLIFIER -VS CAV rms CORE 4 +VS 1M 8 AD736 VIN DC-COUPLED VIN (OPTIONAL) 8k 5 39M + OUTPUT VOS -VS ADJUST CAV 33F + 1F CF 10F (OPTIONAL) Figure 6. External Output VOS Adjustment CF 10F + CC 8k 1 0.1F VIN 8 AD736 FULL WAVE RECTIFIER VIN 2 COM VS +VS 2 8k 7 INPUT AMPLIFIER 1M 100k CF OUTPUT BIAS SECTION 3 OUTPUT AMPLIFIER -VS 6 4.7F CAV 4.7F 9V rms CORE 4 5 + 33F 100k + CF (OPTIONAL) 10F Figure 7. Battery-Powered Option VIN CC CC 8k + AD736 1 VIN FULL WAVE RECTIFIER 2 8 7 INPUT AMPLIFIER 6 3 COM +VS VOUT Figure 8. Low Z, AC-Coupled Input Connection REV. D -11- AD736 OUTLINE DIMENSIONS 8-Lead Standard Small Outline Package [SOIC] Narrow Body (RN-8) 8-Lead Ceramic DIP-Glass Hermetic Seal [CERDIP] (Q-8) Dimensions shown in inches and (millimeters) Dimensions shown in millimeters and (inches) 1 5 4 8 5 0.310 (7.87) 0.220 (5.59) PIN 1 6.20 (0.2440) 5.80 (0.2284) C00834-0-11/02(D) 8 4.00 (0.1574) 3.80 (0.1497) 0.055 (1.40) MAX 0.005 (0.13) MIN 5.00 (0.1968) 4.80 (0.1890) 1 4 0.100 (2.54) BSC 1.27 (0.0500) BSC 0.25 (0.0098) 0.10 (0.0040) COPLANARITY SEATING 0.10 PLANE 0.51 (0.0201) 0.33 (0.0130) 0.320 (8.13) 0.290 (7.37) 0.405 (10.29) MAX 0.50 (0.0196) 45 0.25 (0.0099) 1.75 (0.0688) 1.35 (0.0532) 0.060 (1.52) 0.015 (0.38) 0.200 (5.08) MAX 0.150 (3.81) MIN 0.200 (5.08) 0.125 (3.18) 8 0.25 (0.0098) 0 1.27 (0.0500) 0.41 (0.0160) 0.19 (0.0075) 0.023 (0.58) 0.014 (0.36) 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 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 8-Lead Plastic Dual-in-Line Package [PDIP] (N-8) Dimensions shown in inches and (millimeters) 0.375 (9.53) 0.365 (9.27) 0.355 (9.02) 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 0.180 (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) MIN SEATING PLANE 0.060 (1.52) 0.050 (1.27) 0.045 (1.14) 0.150 (3.81) 0.135 (3.43) 0.120 (3.05) 0.015 (0.38) 0.010 (0.25) 0.008 (0.20) 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 Revision History Location Page 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 -12- REV. D PRINTED IN U.S.A. 8