ANALOG DEVICES AD736: Low Cost Low Power True RMS-to-DC Converter Data Sheet (Rev H 2007-02-01-)
Contents
1. 3 dB Bandwidth Sine wave input Vin 1 mV rms 5 5 kHz Vi 10 mV rms 55 55 kHz Vin 100 mV rms 170 170 kHz Vin 200 mV rms 190 190 kHz Low Impedance Input Pin 1 Sine wave input for 1 Additional Error Vin 1 mV rms 1 1 kHz Vi 10 mV rms 6 6 kHz Vin 100 mV rms 90 90 kHz Vin 200 mV rms 90 90 kHz 3 dB Bandwidth Sine wave input Vin 1 mV rms 5 5 kHz Vi 10 mV rms 55 55 kHz Vin 100 mV rms 350 350 kHz Vin 200 mV rms 460 460 kHz POWER SUPPLY Operating Voltage Range 2 8 3 2 5 16 5 2 8 3 2 5 16 5 V Quiescent Current Zero signal 160 200 160 200 uA 200 mV rms No Load Sine wave input 230 270 230 270 Operating Rated Performance Commercial Industrial 0 C to 70 C 40 C to 85 C AD736JN AD736JR AD736AQ AD736AR AD736KN AD736KR AD736BQ AD736BR 1 Accuracy is specified with the AD736 connected as shown in Figure 18 with Capacitor Cc 2 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 vs crest factor is specified as additional error for 200 mV rms signal Crest factor Vpeax V rms DC offset does not limit ac resolution Rev H Page 4 of 20 ABSOLUTE MAXIMUM RATINGS Table 2 Parameter Rating Supply Voltage 16 5V Internal Power Dissipation 200 mW Input Voltage Vs2. 0 9 0 9 V Vs 5 V 2 7 2 7 V Vs 16 5V 4 0 4 0 V Input Resistance 1072 1072 Q Input Bias Current Vs 3 V to 16 5 V 1 25 1 25 pA Low Impedance Input Signal Range Pin 1 Continuous RMS Level Vs 2 8 V 3 2 V 300 300 mV rms Vs 5 V to 16 5 V 1 1 V rms Peak Transient Input Vs 2 8 V 32V 1 7 1 7 V Vs 5V 3 8 3 8 V Vs 16 5V 11 11 V Input Resistance 6 4 8 9 6 6 4 8 9 6 kQ Maximum Continuous All supply voltages 12 12 V p p Nondestructive Input Input Offset Voltage J and K Grades 3 3 mV A and B Grades 3 3 mV vs Temperature 8 30 8 30 yv C vs Supply Vs 5V to 16 5 V 50 150 50 150 uV V Vs 5 V to 3 V 80 80 uV V Rev H Page 3 of 20 AD736 AD736J AD736A AD736K AD736B Parameter Conditions Min Typ Max Min Typ Max Unit OUTPUT CHARACTERISTICS Output Offset Voltage Jand K Grades 0 1 0 5 0 1 0 3 mV A and B Grades 0 5 0 3 mV vs Temperature 1 20 1 20 uV C vs Supply Vs 5Vto 16 5 V 50 130 50 130 uV V Vs 5 V to 3 V 50 50 uV V Output Voltage Swing 2 kO Load Vs 2 8 V 3 2 Oto 1 7 Oto 1 7 V 1 6 1 6 Vs 5V Oto 3 8 Oto 3 8 V 3 6 3 6 Vs 16 5V 0to4 5 0to4 5 V No Load Vs 16 5V Oto4 12 Oto4 12 V Output Current 2 2 mA Short Circuit Current 3 3 mA Output Resistance Q dc 0 2 0 2 Q FREQUENCY RESPONSE High Impedance Input Pin 2 Sine wave input for 1 Additional Error Vin 1 mV rms 1 1 kHz Vn 10 mV rms 6 6 kHz Vin 100 mV rms 37 37 kHz Vin 200 mV rms 33 33 kHz
3. 100pA 10pA 1pA ur Te 55 35 15 5 25 45 65 85 105 TEMPERATURE C Figure 17 Pin 2 Input Bias Current vs Temperature 00834 016 AD736 THEORY OF OPERATION FWR CURRENT MODE ABSOLUTE VALUE AMPLIFIER Ips10pA 40pF OPTIONAL OUTPUT AMPLIFIER 00834 017 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 anda 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 Cay 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 Cr which is connected across the feedback path of the amplif
4. 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 Cr 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 Cr 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 Cr 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 DC ERROR Eg Eo IDEAL appt s AVERAGE Eg DOUBLE FREQUENCY RIPPLE 00834 019 TIME Figure 20 Output Waveform for Sine Wave Input Voltage 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 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
5. 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 Cay 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 Mea
6. 0 3 30 bs 0 115 2 92 y 0 38 0 015 kl 0 130 3 30 ri MIN GAUGE 0 115 2 92 SEATING PLANE 0 014 0 36 0 022 0 58 0 005 0 13 vaso 70 9371 0 018 0 46 gt lt gt FS MIN MAX 0 014 0 36 0 070 1 78 0 060 1 52 0 045 1 14 COMPLIANT 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 070606 A 0 005 0 13 0 055 1 40 MIN M 5 00 0 1968 0 310 7 87 4 80 0 1890 0 220 5 59 4 8 4 00 0 1574 6 20 0 2441 3 80 0 1497 3 4 5 80 0 2284 0 405 29 0 320 8 13 S 290 7 1 27 0 0500 0 50 0 0196 45o BSC 79510 00901 0 200 G3 08 0 060 1 52 s 20088 0 25 0 0099 0 015 0 38 0 25 0 0098 35 0 0532 0 10 0 0040 Epy 0 200 5 08 0 150 3 81 COPLANARITY 0 51 0 0201 gt i e MIN m F lt 1 27 0 0500 0 125 3 18 0 31 0 0122 0 25 0 0098 475 5487 0 015 0 38 SEATING 25 0 0 40 0 0157 0 023 0 58 58 Wie EP 0 008 0 20 PLANE 0 17 0 0067 0 014 0 36 7 es 070 1 78 PLANE T 0 030 0 76 CONTROLLING DIMENSIONS ARE IN INCHES
7. 0 C to 70 C 8 Lead PDIP N 8 AD736KNZ 0 C to 70 C 8 Lead PDIP N 8 AD736JR 0 C to 70 C 8 Lead SOIC_N R 8 AD736JR REEL 0 C to 70 C 8 Lead SOIC_N R 8 AD736JR REEL7 0 C to 70 C 8 Lead SOIC_N R 8 AD736JRZ 0 C to 70 C 8 Lead SOIC_N R 8 AD736JRZ RL 0 C to 70 C 8 Lead SOIC R 8 AD736JRZ R7 0 C to 70 C 8 Lead SOIC R 8 AD736KR 0 C to 70 C 8 Lead SOIC_N R 8 AD736KR REEL 0 C to 70 C 8 Lead SOIC_N R 8 AD736KR REEL7 0 C to 70 C 8 Lead SOIC R 8 AD736KRZ 0 C to 70 C 8 Lead SOIC R 8 AD736KRZ RL 0 C to 70 C 8 Lead SOIC R 8 AD736KRZ R7 0 C to 70 C 8 Lead SOIC_N R 8 AD736 EVALZ Evaluation Board 17 RoHS compliant part Rev H Page 19 of 20 AD736 NOTES 2007 Analog Devices Inc All rights reserved Trademarks and ANALOG registered trademarks are the property of their respective owners C00834 0 2 07 H DEVICES www analo g com Rev H Page 20 of 20
8. 11 RMS Input Level Pin 2 vs dB Frequency ERROR of Reading 00834 008 Cay HF 00834 009 INPUT LEVEL rms 00834 010 Vin SINE WAVE 1kHz Cay 22uF Cc 47pF 4 7pF Vs 5 5 10mV 100mV 2V INPUT LEVEL rms Figure 12 Error vs RMS Input Voltage Pin 2 Output Buffer Offset Is Adjusted to Zero Vin 200mV rms Cc 47uF Cp 47uF Vs 5V FREQUENCY Hz Figure 13 Cav vs Frequency for Specified Averaging Error Vin SINE WAVE AC COUPLED Cay 10pF Cc 47pF 47pF Vg 5 1 10 100 1k FREQUENCY Hz 00834 011 00834 012 00834 013 Figure 14 RMS Input Level vs Frequency for Specified Averaging Error Rev H Page 8 of 20 lt 2 E z ul x x 2 o o lt m E 0 2 4 6 8 10 12 14 16 SUPPLY VOLTAGE V Figure 15 Pin 2 Input Bias Current vs Supply Voltage Vs 5V Cc 22yF Cr OpF 100mV Cay 10 Cay 100 E Cay 33uF E 2 1mV 100 1ms 10ms 100ms 1s 10s 100s SETTLING TIME Figure 16 RMS Input Level for Various Values of Cav vs Settling Time INPUT BIAS CURRENT 00834 014 00834 015 Rev Page 9 of 20 1nA
9. 120 ms Responding 0 mV to 200 mV 20 Hz None 33 1 2 sec 200 Hz None 3 3 120 ms SCR Waveform Measurement 0 mV to 200 mV 50 Hz 5 100 33 1 2 sec 60 Hz 5 82 27 1 0 sec 0 mV to 100 mV 50Hz 5 50 33 1 2 sec 60 Hz 5 47 27 1 0 sec Audio Applications Speech 0 mV to 200 mV 300 Hz 3 1 5 0 5 18 ms Music 0 mV to 100 mV 20Hz 10 100 68 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 1kV 0 01 40pF OPTIONAL Figure 25 AD736 with a High Impedance Input Attenuator Rev H Page 14 of 20 00834 020 AD736 2 AD711 Cc 10uF IN 6 2 FULL WAVE INO RECTIFIER INPUT AMPLIFIER 00834 021 10uF OPTIONAL Figure 26 Differential Input Connection wi FULL WAVE RECTIFIER DC COUPLED VINO INPUT 1uF 0 1 AMPLIFIER AC COUPLED 1MQ O OUTPUT 1MO OUTPUT Vos ADJUST Cr 10 OPTIONAL 00834 022 Figure 27 External Output Vos Adjustment s FULL 0 1 WAVE Vin O RECTIFIER INPUT AMPLIFIER OUTPUT AMPLIFIER 10 OPTIONAL 00834 023 Figure 28 Battery Powered Option Rev H Page 15 of 20 AD736 EVALUATION BOARD An evaluation board AD736 EVALZ is available for experimentation or becoming familiar with rms to dc convert
10. 20 GND1 GND2 GND3 GND4 Vs Vs VIN Figure 35 Evaluation Board Schematic Table 6 Evaluation Board Bill of Materials 00834 032 AD736 Qty Name Description Reference Designator Manufacturer Mfg Part Number 1 Test loop Red TVs Components Corp TP 104 01 02 1 Test loop Green Vs Components Corp TP 104 01 05 2 Capacitors Tantalum 10 pF 25 V C1 C2 Nichicon Corp F931E106MCC 3 Capacitors 0 1 uF 16 V 0603 X7R C4 C6 CIN KEMET Corp C0603C104KARACTU 1 Capacitor Tantalum 33 uF 16V 2096 6032 CAV Nichicon Corp F931C336MCC 5 Test loops Purple CAV HI Z LO Z VIN VOUT Components Corp TP 104 01 07 1 Integrated circuit RMS to dc converter DUT Analog Devices Inc AD736JRZ 4 Test loops Black GND1 GND2 GND3 GND4 Components Corp TP 104 01 00 2 Connectors BNC right angle 21 72 227161 1 1 6 pin 2 x J3 3M 929836 09 03 1 Header 3 pin P2 Molex Inc 22 10 2031 1 Resistor 1 1 10 W 1 0603 R1 Panasonic Corp ERJ3EKF1004V 2 Resistors 0 O 596 0603 Panasonic Corp ERJ3GEYOROOV 4 Headers 2 pin 0 1 center W1 W2 W3 W4 Molex Inc 22 10 2021 Rev H Page 17 of 20 AD736 OUTLINE DIMENSIONS 0 400 10 16 0 365 9 27 0 355 9 02 8 5 0 280 7 11 0 250 6 35 H 4 0 240 6 10 0 325 8 26 4 0 310 7 87 0 100 2 54 0 300 7 62 BSC 0 060 1 52 0 195 4 95 0 210 533 i 0 13
11. 20 Mathematically the rms value of a voltage is defined using a simplified equation as rms Jag This involves squaring the signal taking the average 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 1196 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 uF averaging capacitor 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 uF line is approximately 80 ms AD736 The settling
12. ANALOG DEVICES Low Cost Low Power True RMS to DC Converter AD736 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 10 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 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 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 as an ac coupled device by adding one external capacitor In this mode the AD736 can resolve input signal levels of 100 uV rms or less des
13. Block Diagram sse 1 Product Highlights oo ee 1 REVISION HistoEy i 2 Sp cifications acie etn rrt e 3 Absolute Maximum Ratings seen 5 ESD Caution 5 Pin Configuration and Function Descriptions 6 Typical Performance Characteristics sss 7 Theory of Operations l RH E Io RR 10 Types of AC Measurement aaa 10 Calculating Settling Time Using Figure 16 11 REVISION HISTORY 2 07 Rev G to Rev H Updated Layout 9 to 12 Added Applications Section sse 13 Inserted Figure 21 to Figure 24 Renumbered Sequentially 13 Deleted Figure 25 etre tte ede erede 15 Added Evaluation Board Section sss 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 Updated Format conr is Universal Changes to Featuresuu eee re erint 1 Added Table EO e i Rein 6 Changes to Figure 21 and Figure 22 sse 14 Changes to Figure 23 Figure 24 and Figure 25 15 Updated Outline Dimensions see 16 Changes to Ordering 17 5 04 Rev E to Rev F Changes to Specifications seen 2 Replaced Fi
14. MILLIMETER DIMENSIONS IN PARENTHESES ARE ROUNDED OFF INCH 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 COMPLIANT TO JEDEC STANDARDS MS 012 AA 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 012407 A 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 AD736 ORDERING GUIDE Model Temperature Range Package Description Package Option AD736AQ 40 C to 85 C 8 Lead CERDIP Q 8 AD736BQ 40 C to 85 C 8 Lead CERDIP Q 8 AD736AR 40 C to 85 C 8 Lead SOIC R 8 AD736AR REEL 40 C 85 C 8 Lead SOIC_N R 8 AD736AR REEL7 40 C 85 C 8 Lead SOIC_N R 8 AD736ARZ 40 C to 85 C 8 Lead SOIC R 8 AD736ARZ R7 40 C to 85 C 8 Lead SOIC R 8 AD736ARZ RL 40 C to 85 C 8 Lead SOIC R 8 AD736BR 40 C to 85 C 8 Lead SOIC R 8 AD736BR REEL 40 C 85 C 8 Lead SOIC R 8 AD736BR REEL7 40 C 85 C 8 Lead SOIC_N R 8 AD736BRZ 40 C to 85 C 8 Lead SOIC R 8 AD736BRZ R7 40 C to 85 C 8 Lead SOIC R 8 AD736BRZ RL 40 C to 85 C 8 Lead SOIC R 8 AD736JN 0 C to 70 C 8 Lead PDIP N 8 AD736JNZ 0 C to 70 C 8 Lead PDIP N 8 AD736KN
15. Output Short Circuit Duration Indefinite Differential Input Voltage Vs and Vs Storage Temperature Range O 65 C to 150 C Storage Temperature Range N R 65 C to 125 C Lead Temperature Soldering 60 sec 300 C ESD Rating 500 V AD736 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 Oja 165 C W 8 Lead CERDIP 110 C W and 8 Lead SOIC 155 C W ESD electrostatic discharge sensitive device Charged devices and circuit boards can discharge without detection Although this product features patented or proprietary protection circuitry damage may occur on devices subjected to high energy ESD Aviad Therefore proper ESD precautions should be taken to avoid performance degradation or loss of functionality Rev H Page 5 of 20 AD736 PIN CONFIGURATION AND FUNCTION DESCRIPTIONS AD736 L2 sop view Ls lt o BH z 00834 025 Figure 2 Pin Configuration Table 3 Pin Function Descriptions Pin No Mnemonic Description 1 Cc Coupling Capacitor If dc coupling is desired at Pin 2 connect a coupling capa
16. TIONS AD736 At 25 C 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 AD736J AD736A AD736K AD736B Parameter Conditions Min Typ Max Min Typ Max Unit TRANSFER FUNCTION Vour VAvg Vin CONVERSION ACCURACY 1 kHz sine wave Total Error Internal Trim Using Cc All Grades mV rms to 200 mV rms 0 3 0 3 0 5 0 5 0 2 0 2 0 3 0 3 mV of reading 200 mV to 1 V rms 12 2 0 12 2 0 of reading Tmn to Tmax A and B Grades 200 mV rms 0 7 0 7 0 5 0 5 mV of reading J and K Grades 200 mV rms 0 007 0 007 of reading C vs Supply Voltage 200 mV rms Input Vs 5Vto 16 5V 0 0 06 0 1 0 0 06 0 1 96 V Vs 5Vto 3V 0 0 18 0 3 0 0 18 0 3 96 V DC Reversal Error DC Coupled 600 mV dc 1 3 2 5 1 3 2 5 96 of reading Nonlinearity O mV to 200 mV amp 100 mV rms 0 0 25 0 35 0 0 25 0 35 96 of reading Total Error External Trim 0 mV rms to 200 mV rms 0 1 0 5 0 1 0 3 mV of reading ERROR VS CREST FACTOR Crest Factor 1 to 3 Cav Cr 100 UF 0 7 0 7 additional error Crest Factor 5 Cav Cr 100 UF 2 5 2 5 additional error INPUT CHARACTERISTICS High Impedance Input Signal Range Pin 2 Continuous RMS Level Vs 2 8 V 3 2 V 200 200 mV rms Vs 5Vto 16 5V 1 1 Vrms Peak Transient Input Vs 2 8 V 3 2 V
17. citor 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 Such an input is useful for applications with high input voltages and low supply voltages 2 Vin High Input Impedance Pin 3 Cr Connect an Auxiliary Low Pass Filter Capacitor from the Output 4 Vs Negative Supply Voltage if Dual Supplies Are Used or Ground if Connected to a Single Supply Source 5 Cav Connect the Averaging Capacitor Here 6 OUTPUT DC Output Voltage 7 Vs Positive Supply Voltage 8 COM Common Rev H Page 6 of 20 AD736 TYPICAL PERFORMANCE CHARACTERISTICS 10V Vin 200mV rms SINE WAVE INPUT Vs 5 1kHz SINE WAVE Cay 22yF 4 7gF Cc 22uF Bg 05 Cav 5 1004F P dU 5 22uF 2 T 5 03 2 100mV al x 1 ERROR 04 gt A t c E ERE E z 5 10mV 2 0 1 y z 7 o T Sus 1mV lt s 10 ERROR 0 5 100 2 0 2 4 6 8 10 12 14 16 5 0 1 1 10 100 1000 5 SUPPLY VOLTAGE V 3dB FREQUENCY kHz Figure 3 Additional Error vs Supply Voltage Figure 6 Frequency Respon
18. e input voltage range Vs 00834 029 VOUTpc Figure 24 Low Z DC Coupled Input Connection V s 00834 026 Figure 21 High Z AC Coupled Input Connection Default 00834 027 Vs Figure 22 High Z DC Coupled Input Connection Rev H Page 13 of 20 AD736 SELECTING PRACTICAL VALUES FOR INPUT COUPLING AVERAGING AND FILTERING Cr CAPACITORS Table 5 provides practical values of Cav and C for several common applications The input coupling capacitor Cc in conjunction with the 8 internal input scaling resistor determine the 3 dB low frequency roll off This frequency Fr is equal to 1 F Table 5 Capacitor Selection Chart 8000 Value of in Farads Note that at Fi the amplitude error is approximately 3096 3 dB of the reading To reduce this error to 0 5 of the reading choose a value of Cc that sets 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 27 should be used in addition to Cc Low Frequency Crest Cav Cr Application RMS Input Level Cutoff 3 dB Factor pF uF Settling Time to 1 General Purpose RMS Computation OVto1V 20Hz 5 150 10 360 ms 200 Hz 5 15 1 36 ms 0 mV to 200 mV 20Hz 5 33 10 360 ms 200 Hz 5 3 3 1 36 ms General Purpose OVto1V 20 Hz None 33 1 2 sec Average 200 Hz None 3 3
19. either single ended or differentially The AD736 has a 196 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 0 C to 70 C and 20 C to 85 C commercial temperature ranges The AD736A and AD736B grades are rated over the 40 C to 85 C 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 10 O 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 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 2007 Analog Devices Inc All rights reserved AD736 TABLE OF CONTENTS Features ay ayasa aaa qawa 1 General Description ineo do tedio eit tie e RENE 1 Functional
20. ers 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 2 8 d 5 S Figure 29 AD736 Evaluation Board GND ANALOG GND4 D nae ta z vI 22 Jug si E p F Otw P J3 HL wo Figure 30 Evaluation Board Component Side Silkscreen 00834 034 00834 032 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 35 shows the board schematic with all movable jumpers The jumper positions in black are default connections the dotted outline 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 Table 6 is the bill of materials for the AD736 evaluation board 00834 035 00834 036 Figure 34 Evaluation Board Internal Ground Plane Rev H Page 16 of
21. gured8y e edat 10 Updated Outline Dimensions see 16 Changes to Ordering Guide RMS Measurement Choosing the Optimum Value for Cav 11 Rapid Settling Times via the Average Responding Connection E 12 DC Error Output Ripple and Averaging Error 12 AC Measurement Accuracy and Crest 12 Applications 13 Connecting the 13 Selecting Practical Values for Input Coupling Cc Averaging Cay and Filtering Capacitors 14 Evaltiation Board 16 Outline Dimensions oett 18 Ordering Gideon eim 19 4 03 Rev D to Rev E Changes to General Description sss 1 Changes to 3 Changes to Absolute Maximum Ratings ss 4 Changes to Ordering Guide sse 4 11 02 Rev C to Rev D Changes to Functional Block Diagram ss 1 Changes to Pin Configuration 3 Figure 1 Replaced 6 Changes to Figure 22 66 6 Changes to Application Circuits Figures 4 to 8 8 Outline Dimensions Updated sss 8 Rev H Page 2 of 20 SPECIFICA
22. ier 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 the corresponding rms value is 0 707 x 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
23. pite 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 pro viding a great deal of design flexibility Requiring only 200 uA of power supply current the AD736 is optimized for use in portable multimeters and other battery powered applications Rev H Information fumished 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 FUNCTIONAL BLOCK DIAGRAM f FULL WAVE ViN 2 RECTIFIER INPUT AMPLIFIER 00834 001 Figure 1 The AD736 allows the choice of two signal input terminals a high impedance FET input 10 that directly interfaces with High Z input attenuators and a low impedance input 8 kQ that allows the measurement of 300 mV input levels while operating from the minimum power supply voltage of 42 8 V 3 2 V The two inputs can be used
24. se Driving Pin 1 10V DC COUPLED SINE WAVE INPUT Vs 5V Cay 22uF 4 7uF Cc 22uF 5 n a a I 2100 ul m x gt u ui 1 ERROR 5 2 s E z x 10 ERROR lt 100pV o 3 0 1 1 10 100 1000 5 SUPPLY VOLTAGE V 8 3dB FREQUENCY kHz 8 Figure 4 Maximum Input Level vs Supply Voltage Figure 7 Frequency Response Driving Pin 2 6 3ms BURST OF 1kHz 54 3 CYCLES 5 200mVrms SIGNAL Cay 10pF c 5 Vs 5V E Cc 22uF E amp Cr 100 5 s 4 H Cay 5 x in g 3 TE 72 5 ul m EI E a 1 Cay 100 Cay 250 5 0 0 2 4 6 8 10 12 14 16 1 2 3 4 5 SUPPLY VOLTAGE V 8 CREST FACTOR rms 8 Figure 5 Peak Buffer Output vs Supply Voltage Figure 8 Additional Error vs Crest Factor with Various Values of Cav Rev H Page 7 of 20 AD736 ADDITIONAL ERROR of Reading DC SUPPLY CURRENT uA INPUT LEVEL rms Vin 200mV rms 1kHz SINE WAVE Cay 100mF Cr 22mF TEMPERATURE C Figure 9 Additional Error vs Temperature 600 Vin 200mV rms 1kHz SINE WAVE 500 400 300 200 0 0 2 0 4 0 6 0 8 1 0 rms INPUT LEVEL V Figure 10 DC Supply Current vs rms Input Level 100 1k 10k 100k 3dB FREQUENCY Hz Figure
25. signal amplitude to the rms amplitude crest factor rms Many common waveforms such as sine and triangle waves have relatively low crest factors lt 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 Rev H Page 12 of 20 AD736 APPLICATIONS CONNECTING THE INPUT 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 Vs 8 input impedance referred to ground or reference level 00834 028 Figure 23 Low Z AC Coupled Input Connection 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 th
26. suring Common Waveforms Average Responding Circuit Crest Factor TrueRMS Calibrated to Read RMS Value of of Reading Error Using Waveform Type 1 V Peak Amplitude Vpeax Vrms Value Sine Waves V Average Responding Circuit Undistorted Sine Wave 1 414 0 707 0 707 0 Symmetrical Square Wave 1 00 1 00 1 11 11 0 Undistorted Triangle Wave 1 73 0 577 0 555 3 8 Gaussian Noise 9896 of Peaks 1 V 3 0 333 0 295 114 Rectangular 2 0 5 0 278 44 Pulse Train 10 0 1 0 011 89 SCR Waveforms 5096 Duty Cycle 2 0 495 0 354 28 25 Duty Cycle 47 0 212 0 150 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 Cay averaging capacitor its settling time does not vary with the input signal level It is determined solely by the RC time constant of Cr and the internal 8 resistor in the output amplifier s feedback path POSITIVE SUPPLY ws 0 1pF COMMON 0 1pF NEGATIVE suPPLY 4 gt Vs Figure 19 AD736 Average Responding Circuit 00834 018 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 Vour Vi is never achieved exactly Instead the output contains both a dc and an ac error component As shown in Figure 20
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