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a
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
Low Cost, Low Power, True RMS-to-DC Converter AD737*
FUNCTIONAL BLOCK DIAGRAM
8k CC 1
AD737
FULL WAVE RECTIFIER INPUT AMPLIFIER
8 COM
VIN 2
8k
7 +VS
POWER 3 DOWN
BIAS SECTION
RMS CORE
6 OUTPUT
-VS 4
5 CAV
Requiring only 160 A of power supply current, the AD737 is optimized for use in portable multimeters and other batterypowered applications. This converter also provides a power-down feature that reduces the power supply standby current to less than 30 A. The AD737 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 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 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 SOIC, and hermetic CERDIP.
PRODUCT HIGHLIGHTS
GENERAL DESCRIPTION
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 occurring at the output. This allows the device to be highly compatible with high input impedance A/D converters.
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.
*Protected under U.S. Patent Number 5,495,245.
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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
5 V supplies, otherwise AD737-SPECIFICATIONS (@ 25 C, noted.) ac-coupled with 1 kHz sine wave input applied, unless
AD737J/AD737A Typ Max AD737K/AD737B Min Typ Max
Parameter TRANSFER FUNCTION CONVERSION ACCURACY Total Error, Internal Trim1 All Grades
Conditions
Min
Unit
VOUT =
1 kHz Sine Wave AC-Coupled Using CC 0-200 mV rms 200 mV-1 V rms
Avg VIN
()
2
VOUT =
Avg VIN
()
2
0.2/0.3 -1.2
0.4/0.5 2.0 0.5/0.7
0.2/0.2 -1.2
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 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 Peak Transient Input
@ 200 mV rms @ 200 mV rms 0.007
0.007
VS = 5 V to 16.5 V VS = 5 V to 3 V @ 600 mV dc @ 100 mV rms 0-200 mV rms
0 0
+0.06 -0.18 1.3 +0.25 0.1/0.2
+0.1 -0.3 2.5 +0.35
0 0
+0.06 -0.18 1.3 +0.25 0.1/0.2
+0.1 -0.3 2.5 +0.35
0
0
CAV, CF = 100 F CAV, CF = 100 F
0.7 2.5
0.7 2.5
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 VS = 5 V
200 1 0.9 4.0 2.7 1012 1 0.9 4.0 25 2.7 1012 1
200 1
Input Resistance Input Bias Current Low Impedance Input (Pin 1) Signal Range Continuous rms Level Peak Transient Input
25
mV rms V rms V V V pA
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 6.4 All Supply Voltages AC-Coupled
300 l 1.7 3.8 11 8 1.7 3.8 11 8
300 l
Input Resistance Maximum Continuous Nondestructive Input Input Offset Voltage4 J and K Grades A and B Grades vs. Temperature vs. Supply vs. Supply
9.6 12 3 3 30 150
6.4
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
VS = 5 V to 16.5 V VS = 5 V to 3 V
8 50 80
8 50 80
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AD737
Parameter OUTPUT CHARACTERISTICS Output Voltage Swing No Load Conditions AD737J/AD737A Min Typ Max Min AD737K/AD737B Typ Max Unit
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 VIN = 200 mV rms, No Load Power-Down Mode Current TEMPERATURE RANGE Operating, Rated Performance Commercial (0C to +70C) Industrial (-40C to +85C)
VS = +2.8 V, -3.2 V VS = 5 V VS = 16.5 V @ dc
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
Sine Wave Input 1 6 37 33 Sine Wave Input 5 55 170 190 Sine Wave Input 1 6 90 90 Sine Wave Input 5 55 350 460 +2.8, -3.2 Zero Signal Sine Wave Input Pin 3 Tied to +VS 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 5 55 170 190 kHz kHz kHz kHz 1 6 37 33 kHz kHz kHz kHz
AD737J AD737A
AD737K AD737B
NOTES l Accuracy is specified with the AD737 connected as shown in Figure 1 with capacitor CC. 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 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.
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AD737
ABSOLUTE MAXIMUM RATINGS 1 PIN CONFIGURATIONS Plastic DIP (N-8), CERDIP (Q-8), SOIC (SOIC-8)
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 AD737J/AD737K . . . . . . . . . . . . . . . . . . . . . . 0C to +70C AD737A/AD737B . . . . . . . . . . . . . . . . . . . -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
8k CC 1
AD737
FULL WAVE RECTIFIER INPUT AMPLIFIER
8 COM
VIN 2
8k
7 +VS
POWER 3 DOWN
BIAS SECTION
RMS CORE
6 OUTPUT
-VS 4
5 CAV
ORDERING GUIDE
Model AD737AQ AD737BQ AD737JN AD737JR AD737JR-REEL AD737JR-REEL7 AD737KN AD737KR AD737KR-REEL AD737KR-REEL7
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 R-8 R-8 R-8 N-8 R-8 R-8 R-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 AD737 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.
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Typical Performance Characteristics-AD737
0.7
ADDITIONAL ERROR - % of Reading
PEAK INPUT BEFORE CLIPPING - V
16 VIN = 200mV rms SINE WAVE @ 1kHz CAV = 100 F CF = 22 F DC-COUPLED 14
SUPPLY CURRENT - A
25
0.5
12 10 PIN 1 8 PIN 2 6 4 2 0 0 2 4 6 8 10 12 SUPPLY VOLTAGE - V 14 16
20
0.3
0.1 0 -0.1
15
10
-0.3
-0.5
0
2
4 6 8 10 12 SUPPLY VOLTAGE - V
14
16
5
0
2
4 6 8 10 12 14 16 DUAL SUPPLY VOLTAGE - V
18
TPC 1. Additional Error vs. Supply Voltage
TPC 2. Maximum Input Level vs. Supply Voltage
TPC 3. Power-Down Current vs. Supply Voltage
10V SINE WAVE INPUT, VS = 5V, CAV = 22 F, CF = 4.7 F, CC = 22 F 1V
INPUT LEVEL - rms INPUT LEVEL - rms
10V
ADDITIONAL ERROR - % of Reading
6
SINE WAVE INPUT, VS = 5V, CAV = 22 F, CF = 4.7 F, CC = 22 F 1V
8
4
100mV 1% ERROR 10mV -3dB 1mV 10% ERROR 100 V 0.1
100mV 1% ERROR 10mV
3ms BURST OF 1kHz = 3 CYCLES CAV = 10 F 200mV rms SIGNAL VS = 5V CC = 22 F CAV = 33 F CF = 100 F
3
2
1mV
10% ERROR -3dB
1
CAV = 100 F CAV = 250 F
1
10 100 FREQUENCY - kHz
1000
100 V 0.1
0
1
10 100 FREQUENCY - kHz
1000
1
2 3 4 CREST FACTOR (VPEAK/V rms)
5
TPC 4. Frequency Response Driving Pin 1
TPC 5. Frequency Response Driving Pin 2
TPC 6. Additional Error vs. Crest Factor vs. CAV
0.8
ADDITIONAL ERROR - % of Reading
500 VIN = 200mV rms SINE WAVE @ 1kHz CC = 22 F CAV = 100 F CF = 22 F VS = 5V
10mV VIN = SINE WAVE AC-COUPLED VS = 5V INPUT LEVEL - rms 1mV
0.6 0.4 0.2 0 -0.2 -0.4 -0.6
400
DC SUPPLY CURRENT -
A
300
200
100 V
100
-0.8 -60 -40 -20
0 20 40 60 80 100 120 140 TEMPERATURE - C
0
0
0.2
0.4 0.6 0.8 RMS INPUT LEVEL - V
1.0
10 V 100
1k 10k -3dB FREQUENCY - Hz
100k
TPC 7. Additional Error vs. Temperature
TPC 8. DC Supply Current vs. RMS Input Level
TPC 9. -3 dB Frequency vs. RMS Input Level (Pin 2)
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AD737
1.0 0.5
100 VIN = 200mV rms CC = 47 F CF = 47 F VS = 5V
F
1V
ERROR - % of Reading
-1%
-0.5%
-0.5 -1.0 -1.5 -2.0 VIN = SINE WAVE @ 1kHz CAV = 22 F, CC = 47 F, CF = 4.7 F, VS = 5V 100mV INPUT LEVEL - rms 1V 2V
10 -0.5%
INPUT LEVEL - rms
0
100mV
CAV -
10mV VIN = SINE WAVE AC-COUPLED CAV = 10 F, CC = 47 F, CF = 47 F, VS = 5V
-1% 1 10
1mV
-2.5 10mV
100 FREQUENCY - Hz
1k
1
10 100 FREQUENCY - Hz
1k
TPC 10. Error vs. RMS Input Voltage (Pin 2) Using Circuit of Figure 6
TPC 11. CAV vs. Frequency for Specified Averaging Error
TPC 12. RMS Input Level vs. Frequency for Specified Averaging Error
4.0
1V VS = 5V CC = 22 F CF = 0 F
10nA
INPUT BIAS CURRENT - pA
3.5
1nA INPUT BIAS CURRENT
3.0
INPUT LEVEL - rms
100mV CAV = 10 F 10mV CAV = 33 F CAV = 100 F
100pA
2.5
10pA
2.0
1mV
1.5
1pA
1.0
0
2
4 6 8 10 12 SUPPLY VOLTAGE - V
14
16
100 V 1ms
10ms
100ms 1s SETTLING TIME
10s
100s
100fA -55 -35 -15
5 25 45 65 85 105 125 TEMPERATURE - C
TPC 13. Pin 2 Input Bias Current vs. Supply Voltage
TPC 14. Settling Time vs. RMS Input Level for Various Values of CAV
TPC 15. Pin 2 Input Bias Current vs. Temperature
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AD737
CALCULATING SETTLING TIME USING TPC 14
TPC 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 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 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 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 AD737 settles very quickly with increasing input levels.
TYPES OF AC MEASUREMENT
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 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. Mathematically, the rms value of a voltage is defined (using a simplified equation) as:
V rms = Avg V 2
()
The AD737 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 value for the waveform being measured. For example, the average
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).
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
Crest Factor (VPEAK/V rms) 1.414 1.00 1.73 3 2 10 2 4.7
True RMS Value 0.707 V 1.00 V 0.577 V 0.333 V 0.5 V 0.1 V 0.495 V 0.212 V
Average Responding Circuit Calibrated to Read RMS Value of Sine Waves Will Read 0.707 V 1.11 V 0.555 V 0.295 V 0.278 V 0.011 V 0.354 V 0.150 V
% of Reading Error Using Average Responding Circuit 0% 11.0% -3.8% -11.4% -44% -89% -28% -30%
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AD737
AD737 THEORY OF OPERATION
As shown by Figure 1, 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 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.
CC 10 F (OPTIONAL) +
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 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.
RAPID SETTLING TIMES VIA THE AVERAGE RESPONDING CONNECTION
CURRENT MODE ABSOLUTE VALUE CC
1 8
8k VIN
COM
Because the average responding connection shown in Figure 2 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.
+ CF 10 F (OPTIONAL)
CC 10 F + (OPTIONAL) CC 8k
1
VIN
2
8k FET OP AMP IB<10pA
7 +VS
AD737
FULL WAVE RECTIFIER INPUT AMPLIFIER BIAS SECTION RMS CORE 8k
8
COM +VS + CF 33 F
POWER DOWN
3
BIAS SECTION
6
VIN
2
7
OUTPUT
TRANSLINEAR CORE -VS
4 5
POWER 3 DOWN
6
VOUT
OUTPUT
CAV
-VS 4
5
CAV
CAV 33 F + POSITIVE SUPPLY 0.1 F COMMON 0.1 F NEGATIVE SUPPLY -VS +VS
POSITIVE SUPPLY 0.1 F COMMON 0.1 F NEGATIVE SUPPLY
+VS
-VS
Figure 2. AD737 Average Responding Circuit
DC ERROR, OUTPUT RIPPLE, AND AVERAGING ERROR
Figure 1. AD737 True RMS Circuit
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, 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 that was not removed by the averaging capacitor, CAV. -8-
Figure 3 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.
EO IDEAL EO DC ERROR = EO - EO (IDEAL)
AVERAGE EO = EO DOUBLE-FREQUENCY RIPPLE TIME
Figure 3. Output Waveform for Sine Wave Input Voltage
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AD737
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. 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
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 AD737 for various values of CAV.
SELECTING PRACTICAL VALUES FOR INPUT COUPLING (CC), AVERAGING (C AV), AND FILTERING (CF) CAPACITORS
Table II 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 rolloff. This frequency, FL, is equal to: FL = 1 2(8 , 000)(TheValue of CC in Farads)
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
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 at Pin 2 should be used in addition to capacitor CC.
Table II. AD737 Capacitor Selection Chart
Application General-Purpose RMS Computation
RMS Input Level 0-1 V 0-200 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
Max Crest Factor 5 5 5 5
CAV 150 F 15 F 33 F 3.3 F None None None None 100 F 82 F 50 F 47 F 1.5 F 100 F
CF 10 F 1 F 10 F 1 F 33 F 3.3 F 33 F 3.3 F 33 F 27 F 33 F 27 F 0.5 F 68 F
Settling Time* 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
General-Purpose Average Responding
0-1 V
0-200 mV SCR Waveform Measurement 0-200 mV 0-100 mV Audio Applications Speech Music
5 5 5 5
0-200 mV 0-100 mV
300 Hz 20 Hz
3 10
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.
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AD737 Applications Circuits
SWITCH CLOSED ACTIVATES POWER DOWN MODE: AD737 DRAWS JUST 40 A IN THIS MODE 1PRV 0.01 F 200mV 1N4148 9M 2V 900k 20V 90k 200V 10k 47k 1W 1N4148
3
CC 10 F + CC
1
+
1F
20k +VS AD589 1.23V 200k 3 1/2 DIGIT AD7136 TYPE CONVERTER REF HIGH REF LOW +V COMMON OUTPUT 1M LOW 0.1 F CAV ANALOG HIGH + 33 F -VS + 9V
VIN
8k
AD737
FULL WAVE RECTIFIER INPUT AMPLIFIER BIAS SECTION RMS CORE
COM
8
50k
VIN
2
+VS 8k
7
6
POWER DOWN -VS
4
5
1F
+
Figure 4. 3 1/2 Digit DVM Circuit
INPUT SCALE FACTOR ADJ R2 R1 C1 5k 69.8k 0.47 F 1%
INPUT
CC
COM
5V
CF 0.47 F
R3 78.7k
R4 5k
R5 80.6k
5V
VIN
+VS
C2 0.01 F
AD737JR
POWER DOWN
OUTPUT
OUTPUT ZERO ADJUST
0.01 F
AD8541AR
-VS
OUTPUT
CAV
C3 0.01 F
+
5V
C4 2.2 F
CAV 33 F
R7 100k
2.5V
C5 1F
+
R8 100k
Figure 5. Battery-Powered Operation for 200 mV Max RMS Full-Scale Input
-10-
REV. D
AD737
CC 10 F + CC 8k
1
100 SCALE FACTOR ADJUST
AD737
FULL WAVE RECTIFIER INPUT AMPLIFIER 8k
COM
8
200 +VS
7
VIN
2
CF 10 F
+
OUTPUT POWER DOWN
3
BIAS SECTION RMS CORE
6
VOUT
-VS
4
CAV
5
+ CAV 33 F
Figure 6. External Scale Factor Trim
13 Q1 14
CC 10 F +
CC 8k
1
AD737
FULL WAVE RECTIFIER
INPUT AMPLIFIER
8k
COM 8 NOT CONNECTED
12 *
1k +3500 PPM/ C
60.4
SCALE FACTOR TRIM
PRECISION RESISTOR CORP TYPE PT/ST
VIN
2
7
+VS
2k
POWER DOWN
OUTPUT
3
31.6k
BIAS SECTION
RMS CORE
6
AD711
-VS
4
CAV
5
dB OUTPUT 100mV/dB
+
CAV
**RCAL **R1 IREF
* 10 Q2 11 9 *Q1, Q2 PART OF RCA CA3046 OR SIMILAR NPN TRANSITOR ARRAY
**R1 + RCAL IN OHMS = 10,000
4.3V 0dB INPUT LEVEL IN VOLTS
Figure 7. dB Output Connection
OFFSET ADJUST 500k +VS 1M 1k -VS
CC
1
8k
COM
AD737
FULL WAVE RECTIFIER INPUT AMPLIFIER
499 1k SCALE FACTOR ADJUST
8
VIN
2
7
+VS
3
6
VOUT
Figure 8. DC-Coupled VOS and Scale Factor Trims
REV. D
-11-
AD737
OUTLINE DIMENSIONS 8-Lead Standard Small Outline Package [SOIC] Narrow Body (R-8)
Dimensions shown in millimeters and (inches)
5.00 (0.1968) 4.80 (0.1890)
8 5 4
8-Lead Ceramic Dip-Glass Hermetic Seal [CERDIP] (Q-8)
Dimensions shown in inches and (millimeters)
8
5
4.00 (0.1574) 3.80 (0.1497)
1
6.20 (0.2440) 5.80 (0.2284)
PIN 1
1 4
0.310 (7.87) 0.220 (5.59)
0.100 (2.54) BSC 1.27 (0.0500) BSC 0.25 (0.0098) 0.10 (0.0040) COPLANARITY SEATING 0.10 PLANE 1.75 (0.0688) 1.35 (0.0532) 8 0.25 (0.0098) 0 0.19 (0.0075) 0.50 (0.0196) 0.25 (0.0099) 45 0.200 (5.08) MAX 1.27 (0.0500) 0.41 (0.0160) 0.200 (5.08) 0.125 (3.18) 0.023 (0.58) 0.014 (0.36) 0.405 (10.29) MAX 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) 0.320 (8.13) 0.290 (7.37)
0.51 (0.0201) 0.33 (0.0130)
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
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)
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) 0.015 (0.38) MIN SEATING PLANE 0.060 (1.52) 0.050 (1.27) 0.045 (1.14)
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)
0.150 (3.81) 0.135 (3.43) 0.120 (3.05)
0.015 (0.38) 0.010 (0.25) 0.008 (0.20)
Revision History
Location 12/02--Data Sheet changed from REV. C to REV. D. Page
Changes to FUNCTIONAL BLOCK DIAGRAM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Changes to PIN CONFIGURATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Figure 1 replaced . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Changes to Figure 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Figure 5 replaced . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Changes to Application Circuits Figures 4, 6-8 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 OUTLINE DIMENSIONS updated . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 -12- REV. D
PRINTED IN U.S.A.
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
C00828-0-12/02(D)
0.005 (0.13) MIN
0.055 (1.40) MAX

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