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FEATURES COMPUTES True RMS Value Average Rectified Value Absolute Value PROVIDES 200 mV Full-Scale Input Range (Larger Inputs with Input Attenuator) Direct Interfacing with 3 1/2 Digit CMOS A/D Converters High Input Impedance of 1012 Low Input Bias Current: 25 pA max High Accuracy: 0.2 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: 160 A max Supply Current No External Trims Needed for Specified Accuracy AD736--A General Purpose, Buffered Voltage Output Version Also Available PRODUCT DESCRIPTION
Low Cost, Low Power, True RMS-to-DC Converter AD737*
FUNCTIONAL BLOCK DIAGRAM
8k CC 1 FULL WAVE RECTIFIER INPUT AMPLIFIER BIAS SECTION RMS CORE
AD737
8 COM
VIN
2
8k
7 +VS
POWER DOWN
3
6 OUTPUT
-VS 4
5 CAV
The AD737 is a low power, precision, monolithic true rms-to-dc converter. It is laser trimmed to provide a maximum error of 0.2 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 AD737 offers higher accuracy at equal or lower cost. The AD737 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 AD737 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 AD737 has no output buffer amplifier, thereby significantly reducing dc offset errors occuring at the output. This allows the device to be highly compatible with high input impedance A/D converters. Requiring only 160 A of power supply current, the AD737 is optimized for use in portable multimeters and other battery powered applications. This converter also provides a "power down" feature which reduces the power supply standby current to less than 30 A.
*Protected under U.S. Patent Number 5,495,245.
The AD737 allows the choice of two signal input terminals: a high impedance (1012 ) FET input which will directly interface with high Z input attenuators and a low impedance (8 k) input which 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 singly or differentially. The AD737 achieves a 1% of reading error bandwidth exceeding 10 kHz for input amplitudes from 20 mV rms to 200 mV rms while consuming only 0.72 mW. The AD737 is available in four performance grades. The AD737J and AD737K grades are rated over the commercial temperature range of 0C to +70C. The AD737A and AD737B grades are rated over the industrial temperature range of -40C to +85C. The AD737 is available in three low-cost, 8-lead packages: plastic DIP, plastic SO and hermetic cerdip.
PRODUCT HIGHLIGHTS
1. The AD737 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 AD737 to perform true rms measurement. 3. The low power consumption of 0.72 mW makes the AD737 suitable for many battery powered applications.
REV. C
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 which 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 World Wide Web Site: http://www.analog.com Fax: 781/326-8703 (c) Analog Devices, Inc., 1999
AD737-SPECIFICATIONS otherwise noted.)
Model Conditions Min
(@ +25 C,
5 V supplies, ac coupled with 1 kHz sine-wave input applied unless
AD737K/B Typ
V OUT =
2
AD737J/A Typ
Max
2
Min
Max
Units
TRANSFER FUNCTION CONVERSION ACCURACY Total Error, Internal Trim1 All Grades 1 kHz Sine Wave ac Coupled Using C C 0-200 mV rms 200 mV-1 V rms
V OUT =
Avg.(V IN )
Avg.(V IN )
0.2/0.3 -1.2 0.007 0 0 0 +0.06 -0.18 1.3 +0.25 0.1/0.2 0.7 2.5
0.4/0.5 2.0 0.5/0.7
0.2/0.2 -1.2 0.007
0.2/0.3 2.0 0.3/0.5
mV/ % of Reading % of Reading mV/ % of Reading % of Reading/C %/V %/V % of Reading % of Reading mV/ % of Reading % Additional Error % Additional Error
TMIN-TMAX A&B Grades @ 200 mV rms J&K Grades @ 200 mV rms vs. Supply Voltage @ 200 mV rms Input VS = 5 V to 16.5 V @ 200 mV rms Input VS = 5 V to 3 V dc Reversal Error, dc Coupled @ 600 mV dc Nonlinearity2, 0-200 mV @ 100 mV rms Total Error, External Trim 0-200 mV rms ERROR vs. CREST FACTOR3 Crest Factor 1 to 3 CAV , CF = 100 F Crest Factor = 5 CAV , CF = 100 F INPUT CHARACTERISTICS High Impedance Input (Pin 2) Signal Range Continuous rms Level VS = +2.8 V, -3.2 V Continuous rms Level VS = 5 V to 16.5 V Peak Transient Input VS = +2.8 V, -3.2 V Peak Transient Input VS = 5 V Peak Transient Input VS = 16.5 V Input Resistance Input Bias Current VS = 5 V Low Impedance Input (Pin 1) Signal Range Continuous rms Level VS = +2.8 V, -3.2 V Continuous rms Level VS = 5 V to 16.5 V Peak Transient Input VS = +2.8 V, -3.2 V Peak Transient Input VS = 5 V Peak Transient Input VS = 16.5 V Input Resistance Maximum Continuous Nondestructive Input All Supply Voltages Input Offset Voltage4 ac Coupled J&K Grades A&B Grades vs. Temperature vs. Supply VS = 5 V to 16.5 V vs. Supply VS = 5 V to 3 V OUTPUT CHARACTERISTICS Output Voltage Swing No Load VS = +2.8 V, -3.2 V No Load VS = 5 V No Load VS = 16.5 V Output Resistance @ dc FREQUENCY RESPONSE High Impedance Input (Pin 2) For 1% Additional Error Sine-Wave Input VIN = 1 mV rms VIN = 10 mV rms VIN = 100 mV rms VIN = 200 mV rms 3 dB Bandwidth Sine-Wave Input VIN = 1 mV rms VIN = 10 mV rms VIN = 100 mV rms VIN = 200 mV rms
+0.1 -0.3 2.5 +0.35
0 0 0
+0.06 -0.18 1.3 +0.25 0.1/0.2 0.7 2.5
+0.1 -0.3 2.5 +0.35
200 1 0.9 4.0 1012 1 25 2.7 0.9 4.0 1012 1 2.7
200 1
25
mV rms V rms V V V pA
6.4
1.7 3.8 11 8
300 l
9.6 12 3 3 30 150
6.4
1.7 3.8 11 8
300 l
9.6 12 3 3 30 150
mV rms V rms V V V k V p-p mV mV V/C V/V V/V
8 50 80
8 50 80
0 to -1.6 0 to -3.3 0 to -4 6.4
-1.7 -3.4 -5 8
9.6
0 to -1.6 0 to -3.3 0 to -4 6.4
-1.7 -3.4 -5 8
9.6
V V V k
1 6 37 33 5 55 170 190
1 6 37 33 5 55 170 190
kHz kHz kHz kHz kHz kHz kHz kHz
-2-
REV. C
AD737
Model Conditions Min AD737J/A Typ Max Min AD737K/B Typ Max Units
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 = 1 mV rms VIN = 10 mV rms VIN = 100 mV rms VIN = 200 mV rms POWER SUPPLY Operating Voltage Range Quiescent Current VIN = 200 mV rms, No Load Power Down Mode Current TEMPERATURE RANGE Operating, Rated Performance Commercial (0C to +70C) Industrial (-40C to +85C)
Sine-Wave Input 1 6 90 90 Sine-Wave Input 5 55 350 460 +2.8, -3.2 5 120 170 25 16.5 160 210 40 +2.8, -3.2 5 55 350 460 5 120 170 25 16.5 160 210 40 kHz kHz kHz kHz V A A A 1 6 90 90 kHz kHz kHz kHz
Zero Signal Sine-Wave Input Pin 3 Tied to +VS
AD737J AD737A
AD737K AD737B
NOTES l Accuracy is specified with the AD737 connected as shown in Figure 16 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. 3 Error vs. Crest Factor is specified as additional error for a 200 mV rms signal. C.F. = 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.
Supply Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16.5 V Internal Power Dissipation2 . . . . . . . . . . . . . . . . . . . . . 200 mW Input Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 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 AD737J/K . . . . . . . . . . . . . . . . . . . . . . . . . . . 0C to +70C AD737A/B . . . . . . . . . . . . . . . . . . . . . . . . . . -40C to +85C Lead Temperature Range (Soldering 60 sec) . . . . . . . . +300C ESD Rating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 500 V
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 DIP Package: JA = 165C/W 8-Lead Cerdip Package: JA = 110C/W 8-Lead Small Outline Package: JA = 155C/W
ABSOLUTE MAXIMUM RATINGS 1
ORDERING GUIDE Temperature Range -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 Package Description Cerdip Cerdip Plastic DIP SOIC 13" Tape and Reel 7" Tape and Reel Plastic DIP SOIC 13" Tape and Reel 7" Tape and Reel Package Option Q-8 Q-8 N-8 SO-8 SO-8 SO-8 N-8 SO-8 SO-8 SO-8
Model AD737AQ AD737BQ AD737JN AD737JR AD737JR-REEL AD737JR-REEL7 AD737KN AD737KR AD737KR-REEL AD737KR-REEL7
PIN CONFIGURATIONS Plastic DIP (N-8), Cerdip (Q-8), SOIC (SO-8)
AD737
8 FULL WAVE RECTIFIER INPUT AMPLIFIER BIAS SECTION RMS CORE COM
8k CC 1
VIN
2
8k
7 +VS
POWER DOWN
3
6 OUTPUT
-VS 4
5 CAV
REV. C
-3-
AD737 -Typical Characteristics
Figure 1. Additional Error vs. Supply Voltage
Figure 2. Maximum Input Level vs. Supply Voltage
Figure 3. Power Down Current vs. Supply Voltage
Figure 4. Frequency Response Driving Pin 1
Figure 5. Frequency Response Driving Pin 2
Figure 6. Additional Error vs. Crest Factor vs. CAV
Figure 7. Additional Error vs. Temperature
Figure 8. DC Supply Current vs. RMS lnput Level
Figure 9. 23 dB Frequency vs. RMS Input Level (Pin 2)
-4-
REV. C
Applying the AD737
Figure 10. Error vs. RMS Input Voltage (Pin 2) Using Circuit of Figure 21
Figure 11. C AV vs. Frequency for Specified Averaging Error
Figure 12. RMS Input Level vs. Frequency for Specified Averaging Error
Figure 13. Pin 2 Input Bias Current vs. Supply Voltage
Figure 14. Settling Time vs. RMS Input Level for Various Values of CAV
Figure 15. Pin 2 Input Bias Current vs. Temperature
CALCULATING SETTLING TIME USING FIGURE 14
TYPES OF AC MEASUREMENT
The graph of Figure 14 may be used to closely approximate the time required for the AD737 to settle when its input level is reduced in amplitude. The net time required for the rms converter to settle will be 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 Figure 14, the initial settling time (where the 100 mV line intersects the 33 F line) is around 80 ms. The settling time corresponding to the new or final input level of 1 mV is approximately 8 seconds. Therefore, the net time for the circuit to settle to its new value will be 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%, etc., of final value). Also, this graph provides the worst case settling time, since the AD737 will settle very quickly with increasing input levels.
The AD737 is capable of measuring ac signals by operating as either an average responding or a true rms-to-de 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 will approximate 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 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). 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 dc: an ac signal of 1 volt rms will produce the same amount of heat in a resistor as a 1 volt dc signal.
REV. C
-5-
AD737
Table I. Error Introduced by an Average Responding Circuit When Measuring Common Waveforms
Waveform Type 1 Volt Peak Amplitude Crest Factor (VPEAK /V rms) True rms Value Average Responding Circuit Calibrated to Read rms Value of Sine Waves Will Read % of Reading Error* Using Average Responding Circuit
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
1.414
0.707 V
0.707 V
0%
1.00 1.73
1.00 V 0.577 V
1.11 V 0.555 V
+11.0% -3.8%
3 2 10 2 4.7
0.333 V 0.5 V 0.1 V 0.495 V 0.212 V
0.295 V 0.278 V 0.011 V 0.354 V 0.150 V
-11.4% -44% -89% -28% -30%
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 will depend upon 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 de voltages, the converter will have a computational error 11% (of reading) higher than the true rms value (see Table I).
AD737 THEORY OF OPERATION
As shown by Figure 16, the AD737 has four functional subsections: input amplifier, full-wave rectifier, rms core and bias sections. The FET input amplifier allows both a high impedance, buffered input (Pin 2) or a low impedance, wide-dynamic-range
CC 10 F
(OPTIONAL
input (Pin 1). The high impedance input, with its low input bias current, is well suited for use with high impedance input attenuators. The input signal may be either dc or ac coupled to the input amplifier. Unlike other rms converters, the AD737 permits both direct and indirect ac coupling of the inputs. AC coupling is provided by placing a series capacitor between the input signal and Pin 2 (or Pin 1) for direct coupling and between Pin 1 and ground (while driving Pin 2) for indirect coupling. 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 17). A final subsection, the bias section, permits a "power down" function. This reduces the idle current of the AD737 from 160 A down to a mere 30 A. This feature is selected by tying Pin 3 to the +VS terminal. In the average responding connection, all of the averaging is carried out by an RC post filter consisting of an 8 k internal scale-factor resistor connected between Pins 6 and 8 and an external averaging capacitor, CF. In the rms circuit, this additional filtering stage helps reduce any output ripple which was not removed by the averaging capacitor, CAV.
RMS MEASUREMENT - CHOOSING THE OPTIMUM VALUE FOR C AV
8k CC 1
AD737
FULL WAVE RECTIFIER 8k RMS CORE
8
COM
VIN
2
INPUT AMPLIFIER BIAS SECTION
7 +VS OUTPUT
CF 10 F
(OPTIONAL)
POWER 3 DOWN -VS 4
6
VOUT
5
CAV
CAV 33 F POSITIVE SUPPLY 0.1 F COMMON NEGATIVE SUPPLY 0.1 F -VS +VS
Figure 16. AD737 True RMS Circuit
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 will increase exponentially as the input signal is reduced. This means that as the input level decreases, errors due to nonideal averaging will reduce while the time it takes for the circuit to settle to the new rms level will increase. 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. -6- REV. C
AD737
RAPID SETTLING TIMES VIA THE AVERAGE RESPONDING CONNECTION (FIGURE 17) AC MEASUREMENT ACCURACY AND CREST FACTOR
Because the average responding connection does not use an 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 output scaling resistor.
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 (C.F. = 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 6 shows the additional error vs. crest factor of the AD737 for various values of CAV.
SELECTING PRACTICAL VALUES FOR INPUT COUPLING (C C), AVERAGING (CAV) AND FILTERING (CF) CAPACITORS
Table II provides practical values of CAV and C F for several common applications.
Table II. AD737 Capacitor Selection Chart
Application rms Input Level Low Max CAV Frequency Crest Cutoff Factor (-3 dB) CF Settling Time* to 1%
Figure 17. AD737 Average Responding Circuit
DC ERROR, OUTPUT RIPPLE, AND AVERAGING ERROR
General Purpose 0-1 V rms Computation
20 Hz 200 Hz
5 5 5 5
150 F 10 F 360 ms 15 F 1 F 36 ms 33 F 10 F 360 ms 3.3 F 1 F 36 ms None None None None 33 F 1.2 sec 3.3 F 120 ms 33 F 1.2 sec 3.3 F 120 ms
Figure 18 shows the typical output waveform of the AD737 with a sine-wave input voltage 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.
0-200 mV 20 Hz 200 Hz General Purpose Average Responding 0-1 V 20 Hz 200 Hz
0-200 mV 20 Hz 200 Hz SCR Waveform Measurement 0-200 mV 50 Hz 60 Hz 0-100 mV 50 Hz 60 Hz Audio Applications 5 5 5 5
100 F 33 F 1.2 sec 82 F 27 F 1.0 sec 50 F 47 F 33 F 1.2 sec 27 F 1.0 sec
Figure 18. Output Waveform for Sine-Wave Input Voltage
Speech Music
0-200 mV 300 Hz 0-100 mV 20 Hz
3 10
1.5 F 0.5 F 18 ms 100 F 68 F 2.4 sec
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. 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. As the input frequency increases, both error components decrease rapidly: if the input frequency doubles, the dc error and ripple reduce to 1/4 and 1/2 their original values, respectively, and rapidly become insignificant.
* Settling time is specified over the stated rms input level with the input signal increasing from zero. Settling times will be greater for decreasing amplitude input signals.
The input coupling capacitor, CC, in conjunction with the 8 k internal input scaling resistor, determine 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 will be 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 the lowest frequency to be measured. In addition, if the input voltage has more than 100 mV of dc offset, than the ac coupling network at Pin 2 should be used in addition to capacitor CC .
REV. C
-7-
AD737-Applications Circuits
Figure 19. 3 1/2 Digit DVM Circuit
Figure 20. Battery Powered Operation for 200 mV max RMS Full-Scale Input Figure 21. External Scale Factor Trim
Figure 22. dB Output Connection
Figure 23. DC Coupled VOS and Scale Factor Trims
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
8-Lead Small Outline Package (SO-8)
0.1968 (5.00) 0.1890 (4.80)
8 5 4
8-Lead Plastic DIP Package (N-8)
8-Lead Cerdip Package (Q-8)
PRINTED IN U.S.A.
0.1574 (4.00) 0.1497 (3.80) PIN 1
1
0.2440 (6.20) 0.2284 (5.80)
0.0500 (1.27) BSC 0.0098 (0.25) 0.0040 (0.10) SEATING PLANE 0.0688 (1.75) 0.0532 (1.35) 0.0192 (0.49) 0.0138 (0.35) 8 0.0098 (0.25) 0 0.0075 (0.19)
0.0196 (0.50) 0.0099 (0.25)
45
0.0500 (1.27) 0.0160 (0.41)
-8-
REV. C
C1175b-0-12/99 (rev. C)

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