REV. E
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Tel: 781/329-4700 www.analog.com
Fax: 781/326-8703 © 2003 Analog Devices, Inc. All rights reserved.
AD737
*
Low Cost, Low Power,
True RMS-to-DC Converter
*Protected under U.S. Patent Number 5,495,245.
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: 2.5 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
FUNCTIONAL BLOCK DIAGRAM
COM
OUTPUT
C
C
V
IN
AD737
FULL
WAVE
RECTIFIER
BIAS
SECTION RMS CORE
8k
8k
–V
S
+V
S
C
AV
1
2
3
4
8
7
6
5
INPUT
AMPLIFIER
POWER
DOWN
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 cir-
cuits, 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 exter-
nal capacitor. In this mode, the AD737 can resolve input signal
levels of 100 mV rms or less, despite variations in temperature or
supply voltage. High accuracy is also maintained for input wave-
forms 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.
Requiring only 160 mA of power supply current, the AD737 is opti-
mized for use in portable multimeters and other battery-powered
applications. This converter also provides a power-down feature
that reduces the power supply standby current to less than 30 mA.
The AD737 allows the choice of two signal input terminals: a
high impedance (10
12
W) FET input that directly interfaces with
high Z input attenuators and a low impedance (8 kW) input that
allows the measurement of 0.9 V input levels while operating
from the minimum power supply voltage of ±2.5 V. The two inputs
may be used either single-ended or differentially.
The AD737 achieves 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 –20∞C to +85∞C. The AD737JR-5 is tested with supply
voltages of ±2.5 V dc. The AD737A and AD737B grades are
rated over the industrial temperature range of –40∞C to +85∞C.
The AD737 is available in three low cost, 8-lead packages:
PDIP, SOIC, and 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 battery-powered applications.
REV. E–2–
AD737–SPECIFICATIONS
AD737J/AD737A AD737K/AD737B AD737J-5
Parameter Conditions Min Typ Max Min Typ Max Min Typ Max Unit
TRANSFER FUNCTION
VAvgV
OUT IN
=
()
2
CONVERSION ACCURACY
Total Error E
IN
= 0 to 200 mV rms 0.2/0.3 0.4/0.5 0.2/0.2 0.2/0.3 ±mV/±POR
1
±V
S
= ±2.5 V 0.2/0.3 0.4/0.5 POR
±V
S
= ±2.5 V, Input 0.2/0.3 0.4/0.5 POR
to Pin 1
E
IN
= 200 mV – 1 V rms –1.2 ±2.0 –1.2 ±2.0 POR
T
MIN
to T
MAX
A and B Grades E
IN
= 200 mV rms 0.5/0.7 0.3/0.5 ±mV/±POR
J and K Grades E
IN
= 200 mV rms,
±V
S
= ±2.5 V 0.007 0.007 0.02 ±POR/∞C
vs. Supply Voltage E
IN
= 200 mV rms
±V
S
= ±2.5 V to ±5 V 0–0.18 –0.3 0 –0.18 –0.3 0 –0.18 –0.3 %/V
±V
S
= ±5 V to ±16.5 V 0+0.06 +0.1 0 +0.06 +0.1 0 +0.06 +0.1 %/V
DC Reversal Error, V
IN
= 600 mV dc 1.3 2.5 1.3 2.5 POR
DC-Coupled
V
IN
= 200 mV dc, 1.7 2.5 POR
±V
S
= ±2.5 V
Nonlinearity
2
E
IN
= 0 to 200 mV rms
E
IN
= 100 mV rms 0+0.25 +0.35 0+0.25 +0.35 POR
Input to Pin 1, E
IN
= 100 mV rms,
AC-Coupled
3
after correction
±V
S
= ±2.5 V 0.02 0.1 POR
Total Error, External Trim E
IN
= 0 to 200 mV rms 0.1/0.2 0.1/0.2 0.1/0.2 ±mV/±POR
ADDITIONAL CREST
FACTOR ERROR
4
Crest Factor = 1 to 3 C
AV
= C
F
= 100 mF0.7 0.7 %
C
AV
= 22 mF,
C
F
= 100 mF, ±V
S
=
±2.5 V, Input to Pin 1 1.7 %
Crest Factor = 5 C
AV
= C
F
= 100 mF2.5 2.5 %
INPUT CHARACTERISTICS
High Z Input (Pin 2)
Signal Range
Continuous RMS Level ±V
S
= +2.5 V 200 mV rms
±V
S
= +2.8 V, –3.2 V 200 200 mV rms
±V
S
= ±5 V to ±16.5 V 11 V rms
Peak Transient Input ±V
S
= +2.5 V,
Input to Pin 1 ±0.6 V
±V
S
= +2.8 V, –3.2 V ±0.9 ±0.9 V
±V
S
= ±5 V ±2.7 ±2.7 V
±V
S
= ±16.5 V ±4.0 ±4.0 V
Input Resistance 10
12
10
12
10
12
W
Input Bias Current ±V
S
= ±5 V 1 25 1 25 1 25 pA
Low Z Input (Pin 1)
Signal Range
Continuous RMS Level ±V
S
= +2.5 V 300 mV rms
±V
S
= +2.8 V, –3.2 V 300 300 mV rms
±V
S
= ±5 V to ±16.5 V l l V rms
Peak Transient Input ±V
S
= +2.5 V ±1.7 V
±V
S
= +2.8 V, –3.2 V ±1.7 ±1.7 V
±V
S
= ±5 V ±3.8 ±3.8 V
±V
S
= ±16.5 V ±11 ±11 V
Input Resistance 6.4 8 9.6 6.4 8 9.6 6.4 8 9.6 kW
Maximum Continuous
Nondestructive Input,
All Supply Voltages ±12 ±12 ±12 V p-p
Input Offset Voltage
5
AC-Coupled ±3±3±3mV
vs. Temperature 8 30 8 30 8 30 mV/∞C
vs. Supply V
S
= ±2.5 V to ±5 V 80 80 80 mV/V
V
S
= ±5 V to ±16.5 V 50 150 50 150 mV/V
(@ 25ⴗC, ⴞ5 V supplies, except AD737J-5, ±2.5 V, CAV = 33 F, CC = 10 F, f = 1 kHz,
sine wave input applied to Pin 2, unless otherwise specified.)
REV. E
AD737
–3–
AD737J/AD737A AD737K/AD737B AD737J-5
Parameter Conditions Min Typ Max Min Typ Max Min Typ Max Unit
OUTPUT CHARACTERISTICS No Load
Output Voltage Swing ±V
S
= +2.8 V, –3.2 V 1.6 –1.7 –1.6 –1.7 V
±V
S
= ±5 V –3.3 –3.4 –3.3 –3.4 V
±V
S
= ±16.5 V –4 –5 –4 –5 V
±V
S
= ±2.5 V, (Pin 1) –1.1 –0.9 V
Output Resistance dc 6.4 8 9.6 6.4 8 9.6 6.4 8 9.6 kΩ
FREQUENCY RESPONSE
High Impedance Input (Pin 2)
For 1% Additional Error V
IN
= 1 mV rms 1 1 1 kHz
V
IN
= 10 mV rms 6 6 6 kHz
V
IN
= 100 mV rms 37 37 37 kHz
V
IN
= 200 mV rms 33 33 33 kHz
For 3 dB Bandwidth V
IN
= 1 mV rms 5 5 5 kHz
V
IN
= 10 mV rms 55 55 55 kHz
V
IN
= 100 mV rms 170 170 170 kHz
V
IN
= 200 mV rms 190 190 190 kHz
Low Z Input (Pin 1)
For 1% Additional Error V
IN
= 1 mV rms 1 1 1 kHz
V
IN
= 10 mV rms 6 6 6 kHz
V
IN
= 40 mV rms 25 kHz
V
IN
= 100 mV rms 90 90 90 kHz
V
IN
= 200 mV rms 90 90 90 kHz
For 3 dB Bandwidth V
IN
= 1 mV rms 5 5 5 kHz
V
IN
= 10 mV rms 55 55 55 kHz
V
IN
= 100 mV rms 350 350 350 kHz
V
IN
= 200 mV rms 460 460 460 kHz
POWER DOWN
Disable Voltage 0 0 V
Input Current, PD Enabled +V
PD
= +V
S
11 11 µA
POWER SUPPLY
Operating Voltage Range +2.8/–3.2 ±5±16.5 +2.8/–3.2 ±5±16.5 ±2.5 ±5±16.5 V
Current No Input 120 160 120 160 120 160 µA
Rated Input 170 210 170 210 170 210 µA
Power Down 25 40 25 40 25 40 µA
TEMPERATURE RANGE See the Ordering Guide
NOTES
l
POR = % of Reading.
2
Nonlinearity is defined as the maximum deviation (in percent error) from a straight line connecting the readings at 0 V and 200 mV rms.
3
After fourth order error correction using the equation y = – 0.31009x
4
– 0.21692x
3
– 0.06939x
2
+ 0.99756x + 11.1 ⫻ 10
–6
, where y is the corrected result and x is the
device output between 0.01 V and 0.3 V.
4
Crest factor error is specified as the additional error resulting from the specific crest factor, using a 200 mV rms signal as a reference. The crest factor is defined
as V
PEAK
/V rms.
5
DC offset does not limit ac resolution.
Specifications subject to change without notice.
Specifications shown in boldface are tested on all production units at final electrical test. Results from these tests are used to calculate outgoing quality levels.
REV. E–4–
AD737
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.
ABSOLUTE MAXIMUM RATINGS
1
Supply Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ±16.5 V
Internal Power Dissipation
2
. . . . . . . . . . . . . . . . . . . . 200 mW
Input Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ±V
S
Output Short-Circuit Duration . . . . . . . . . . . . . . . . Indefinite
Differential Input Voltage . . . . . . . . . . . . . . . . . . +V
S
and –V
S
Storage Temperature Range (Q) . . . . . . . . . –65°C to +150°C
Storage Temperature Range (N, R) . . . . . . . –65°C to +125°C
Lead Temperature Range (Soldering 60 sec) . . . . . . . . . 300°C
ESD Rating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 500 V
NOTES
1
Stresses above those listed under Absolute Maximum Ratings may cause perma-
nent 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 PDIP Package:
JA
= 165°C/W.
8-Lead CERDIP Package:
JA
= 110°C/W.
8-Lead SOIC:
JA
= 155°C/W.
PIN CONFIGURATION
PDIP (N-8), CERDIP (Q-8), SOIC (R-8)
COM
OUTPUT
C
C
V
IN
AD737
FULL
WAVE
RECTIFIER
BIAS
SECTION RMS CORE
8k⍀
8k⍀
–V
S
+V
S
C
AV
1
2
3
4
8
7
6
5
INPUT
AMPLIFIER
POWER
DOWN
ORDERING GUIDE
Temperature Package Package
Model Range Description Option
AD737AQ –40°C to +85°CCERDIP Q-8
AD737BQ –40°C to +85°CCERDIP Q-8
AD737JN 0°C to 70°CPDIP N-8
AD737JR 0°C to 70°CSOIC R-8
AD737JR-REEL 0°C to 70°C13" Tape and Reel R-8
AD737JR-REEL7 0°C to 70°C7" Tape and Reel R-8
AD737JR-5 0°C to 70°CSOIC R-8
AD737JR-5-REEL 0°C to 70°C13" Tape and Reel R-8
AD737JR-5-REEL7 0°C to 70°C7" Tape and Reel R-8
AD737KN 0°C to 70°CPDIP N-8
AD737KR 0°C to 70°CSOIC R-8
AD737KR-REEL 0°C to 70°C13" Tape and Reel R-8
AD737KR-REEL7 0°C to 70°C7" Tape and Reel R-8
REV. E
Typical Performance Characteristics–AD737
–5–
SUPPLY VOLTAGE (V)
ADDITIONAL ERROR (% of Reading)
–0.5 02 16
4681012 14
–0.3
–0.1
0.1
0.3
0.5
0.7
0
V
IN
= 200mV rms
SINE WAVE @ 1kHz
C
AV
= 100F
C
F
= 22F
TPC 1. Additional Error vs.
Supply Voltage
FREQUENCY (kHz)
INPUT LEVEL (V rms)
100
0.1 1 1000
10 100
1m
10m
100m
1
10
SINE WAVE INPUT, V
S
= 5V,
C
AV
= 22F, C
F
= 4.7F, C
C
= 22F
–3dB
10% ERROR
1% ERROR
TPC 4. Frequency Response
Driving Pin 1
TEMPERATURE (C)
ADDITIONAL ERROR (% of Reading)
–0.8
–60
–0.6
–0.4
–0.2
0
0.2
0.4
V
IN
= 200mV rms
SINE WAVE @ 1kHz
C
C
= 22F
C
AV
= 100F
C
F
= 22F
V
S
= 5V
–40 –20 0 20 40 60 80 100 120 140
0.6
0.8
TPC 7. Additional Error vs.
Temperature
SUPPLY VOLTAGE (V)
PEAK INPUT BEFORE CLIPPING ( V)
002 16
4681012 14
2
4
6
8
12
16
DC-COUPLED
14
10
PIN 1
PIN 2
TPC 2. Maximum Input Level vs.
Supply Voltage
FREQUENCY (kHz)
INPUT LEVEL (V rms)
100
0.1 1 1000
10 100
1m
10m
100m
1
10 SINE WAVE INPUT, VS = 5V,
CAV = 22F, C F = 4.7F, C C = 22F
–3dB
10% ERROR
1% ERROR
TPC 5. Frequency Response
Driving Pin 2
RMS INPUT LEVEL (V)
DC SUPPLY CURRENT (A)
00
100
200
300
400
0.2 0.4 0.6 0.8 1.0
500
TPC 8. DC Supply Current vs.
RMS Input Level
DUAL SUPPLY VOLTAGE (V)
SUPPLY CURRENT (A)
502 16
4681012 14
10
20
25
15
18
TPC 3. Power-Down Current vs.
Supply Voltage
CREST FACTOR (VPEAK/V rms)
ADDITIONAL ERROR (% of Reading)
015
234
1
2
3
4
6
3ms BURST OF 1kHz =
3 CYCLES
200mV rms SIGNAL
VS = 5V
CC = 22F
CF = 100F
8
CAV = 33F
CAV = 100F
CAV = 250F
CAV = 10F
TPC 6. Additional Error vs.
Crest Factor vs. C
AV
–3dB FREQUENCY (Hz)
INPUT LEVEL (V rms)
10
100 1k 100k10k
100
1m
10m
VIN = SINE WAVE
AC-COUPLED
VS = 5V
TPC 9. RMS Input Level (Pin 2)
vs. –3 dB Frequency
REV. E–6–
AD737
INPUT LEVEL (V rms)
ERROR (% of Reading)
–2.5
10m 100m 1 2
–2.0
–1.5
–1.0
–0.5
0
0.5
1.0
VIN = SINE WAVE @ 1kHz
CAV = 22F, C C = 47F,
CF = 4.7F, V S = 5V
TPC 10. Error vs. RMS Input
Voltage (Pin 2) Using Circuit
of Figure 6
SUPPLY VOLTAGE (V)
1.0 0
1.5
2.0
2.5
3.0
246810
3.5
12 14 16
4.0
INPUT BIAS CURRENT (pA)
TPC 13. Pin 2 Input Bias Current
vs. Supply Voltage
FREQUENCY (kHz)
INPUT LEVEL (V rms)
100
0.1 1 1000
10 100
1m
10m
100m
1
10 SINE WAVE INPUT, VS = 2.5V,
CAV = 22F, C F = 4.7F, C C = 22F
TPC 16. Frequency Response
Driving Pin 1
100
10
FREQUENCY (Hz)
CAV (F)
10
1
100 1k
VIN = 200mV rms
CC = 47F
CF = 47F
VS = 5V
–1%
–0.5%
TPC 11. C
AV
vs. Frequency for
Specified Averaging Error
100
1m
SETTLING TIME (Seconds)
INPUT LEVEL (V rms)
1m
10m
100m
1
10m 100m 1 10 100
CAV = 100F
CAV = 33F
CAV = 10F
VS = 5V
CC = 22F
CF = 0F
TPC 14. Settling Time vs. RMS Input
Level for Various Values of C
AV
FREQUENCY (kHz)
INPUT LEVEL (V rms)
100
0.1 1 1000
10 100
1m
10m
100m
1
10 SINE WAVE INPUT, V
S
= 2.5V,
C
AV
= 22F, C
F
= 4.7F, C
C
= 22F
3dB
0.5%
10%
1%
TPC 17. Error Contours Driving Pin 1
FREQUENCY (Hz)
INPUT LEVEL (V rms)
1m 110 1k
100
10m
100m
1
V
IN
= SINE WAVE
AC-COUPLED
C
AV
= 10F, C
C
= 47F,
C
F
= 47F, V
S
= 5V
–1% –0.5%
TPC 12. RMS Input Level vs.
Frequency for Specified
Averaging Error
TEMPERATURE (C)
INPUT BIAS CURRENT
100fA
–55
1pA
10pA
100pA
1nA
10nA
–35 –15 5 25 45 65 85 105 125
TPC 15. Pin 2 Input Bias Current
vs. Temperature
CREST FACTOR
ADDITIONAL ERROR (% of Reading)
1
3 CYCLES OF 1kHZ
200mV rms
V
S
= 2.5V
C
C
= 22F
C
F
= 100F
234 5
5
4
3
2
1
0
C
AV
=
10F
22F
33F
100F
220F
TPC 18. Additional Crest Factor
Error for Various Values of C
AV
REV. E
AD737
–7–
INPUT LEVEL (V rms)
ERROR (% of Reading)
–2.5
10m 100m 1 2
–2.0
–1.5
–1.0
–0.5
0
0.5
1.0
f = 1kHz
CAV = 22F, V S = 2.5V
CC = 47F, C F = 4.7F
TPC 19. Error vs. Input Voltage Driving Pin 1
Table I. Error Introduced by an Average Responding Circuit When Measuring Common Waveforms
Average Responding
Circuit Calibrated to Reading Error
Waveform Type Crest Factor True RMS Read RMS Value of Using Average
1 V Peak Amplitude (V
PEAK
/V rms) Value (V) Sine Waves Will Read (V) 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
(98% of Peaks <1 V) 3 0.333 0.295 –11.4
Rectangular 2 0.5 0.278 –44
Pulse Train 10 0.1 0.011 –89
SCR Waveforms
50% Duty Cycle 2 0.495 0.354 –28
25% Duty Cycle 4.7 0.212 0.150 –30
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 ampli-
tude. 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 mF 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 mF line) is around 80 ms. The settling time
corresponding to the new or final input level of 1 mV is approxi-
mately 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
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 absolute value
of a sine wave voltage is 0.636 that of V
PEAK
; the corresponding
rms value is 0.707 times V
PEAK
. 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
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 convert-
ers 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).
REV. E–8–
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 section.
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-coupled 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.
8
COM
+VS
7
6
OUTPUT
5CAV
CURRENT
MODE
ABSOLUTE
VAL U E
1
CC
2
VIN
3
POWER
DOWN
4
–VS
CAV
33F
CF
10F
(OPTIONAL
LPF)
VIN
AC
CC = 10F
+
OPTIONAL RETURN PATH
8k
+VS
0.1F
–VS
0.1F
+
+
DC
RMS
TRANSLINEAR
CORE
COMMON
POSITIVE SUPPLY
NEGATIVE SUPPLY
BIAS
SECTON
FET
OP AMP
IB⬍10pA
8k
Figure 1. AD737 True RMS Circuit (Test 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, C
AV
.
Without C
AV
, 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 mA down to
a mere 30 mA. This feature is selected by tying Pin 3 to the +V
S
terminal. In the average responding connection, all of the aver-
aging is carried out by an RC post filter consisting of an 8 kW
internal scale-factor resistor connected between Pins 6 and 8 and
an external averaging capacitor, C
F
. In the rms circuit, this
additional filtering stage helps reduce any output ripple that was
not removed by the averaging capacitor, C
AV
.
RMS MEASUREMENT—CHOOSING THE OPTIMUM
VALUE FOR C
AV
Since the external averaging capacitor, C
AV
, 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 expo-
nentially 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 C
AV
, a trade-off
between computational accuracy and settling time is required.
RAPID SETTLING TIMES VIA THE AVERAGE
RESPONDING CONNECTION
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 C
F
and the internal 8 kW output scaling resistor.
COM
OUTPUT
CAV
CC
POWER
DOWN
–VS
CF
33F
VIN
POSITIVE SUPPLY +VS
0.1F
–VS
0.1F
COMMON
NEGATIVE SUPPLY
VOUT
8
7
6
5
1
2
3
4
AD737
FULL
WAVE
RECTIFIER
BIAS
SECTION
INPUT
AMPLIFIER 8k
8k
RMS
CORE
+VS
+
Figure 2. AD737 Average Responding Circuit
DC ERROR, OUTPUT RIPPLE, AND AVERAGING ERROR
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 V
OUT
= V
IN
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
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
REV. E
AD737
–9–
averaging capacitor used—no amount of post filtering (i.e., using
a very large C
F
) 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, C
F
.
In most cases, the combined magnitudes of both the dc and
ac error components need to be considered when selecting
appropriate values for capacitors C
AV
and C
F
. This combined
error, representing the maximum uncertainty of the measure-
ment, 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
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 ampli-
tude (Crest Factor = V
PEAK
/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 AD737 for various values of C
AV
.
SELECTING PRACTICAL VALUES FOR INPUT
COUPLING (C
C
), AVERAGING (C
AV
), AND
FILTERING (C
F
) CAPACITORS
Table II provides practical values of C
AV
and C
F
for several
common applications.
The input coupling capacitor, C
C
, in conjunction with the 8 kW
internal input scaling resistor, determines the –3 dB low frequency
roll-off. This frequency, F
L
, is equal to
FTheValue of C in Farads
L
C
=
()( )
1
28000p,
Note that at F
L
, the amplitude error is approximately –30% (–3 dB)
of reading. To reduce this error to 0.5% of reading, choose a
value of C
C
that sets F
L
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 C
C
.
Table II. AD737 Capacitor Selection Chart
Low Frequency Max Settling
Application RMS Input Level Cutoff (–3 dB) Crest Factor C
AV
C
F
Time* to 1%
General-Purpose 0–1 V 20 Hz 5 150 mF10 mF360 ms
RMS Computation 200 Hz 5 15 mF1 mF36 ms
0–200 mV 20 Hz 5 33 mF10 mF360 ms
200 Hz 5 3.3 mF1 mF36 ms
General-Purpose 0–1 V 20 Hz None 33 mF1.2 sec
Average 200 Hz None 3.3 mF120 ms
Responding
0–200 mV 20 Hz None 33 mF1.2 sec
200 Hz None 3.3 mF120 ms
SCR Waveform 0–200 mV 50 Hz 5 100 mF33 mF1.2 sec
Measurement 60 Hz 5 82 mF27 mF1.0 sec
0–100 mV 50 Hz 5 50 mF33 mF1.2 sec
60 Hz 5 47 mF27 mF1.0 sec
Audio
Applications
Speech 0–200 mV 300 Hz 3 1.5 mF0.5 mF18 ms
Music 0–100 mV 20 Hz 10 100 mF68 mF2.4 sec
*Settling time is specified over the stated rms input level with the input signal increasing from zero. Settling times are greater for decreasing amplitude input signals.
REV. E–10–
AD737
Applications Circuits
COM
+V
AD589
1.23V
C
AV
C
C
POWER
DOWN 0.1F
C
C
10F
SWITCH CLOSED
ACTIVATES
POWER-DOWN
MODE. AD737
DRAWS JUST
40A IN THIS
MODE
2V
20V
200V
9M
900k
90k
10k
V
IN
200mV
V
IN
–V
S
8
7
6
5
4
1
2
3
AD737
FULL
WAVE
RECTIFIER
BIAS
SECTION
INPUT
AMPLIFIER
8k
8k
+
RMS
CORE
+
+V
S
+
1F
OUTPUT 1M
–V
S
+V
S
1N4148
1N4148
47k
1W
1F
+
COMMON
33F
REF LOW
REF HIGH
3 1/2 DIGIT AD7136
TYPE CONVERTER
LOW
HIGH
ANALOG
9V
200k
20k
50k
+
1PRV
0.01F
Figure 4. 3 1/2 Digit DVM Circuit
COM
INPUT
C
AV
C
C
POWER
DOWN
C2
0.01F
INPUT SCALE FACTOR ADJ
V
IN
–V
S
OUTPUT
R5
80.6k
+V
S
C
AV
33F
+
OUTPUT
C
F
0.47F
AD737JR
C1
0.47F
+
5V
R2
5k
R1
69.8k
1%
R3
78.7k
0.01F
C3
0.01F
R7
100k
R8
100k
5V
2.5V
AD8541AR
C4
2.2F
C5
1F
5V
R4
5k
OUTPUT ZERO
ADJUST
Figure 5. Battery-Powered Operation for 200 mV Max RMS Full-Scale Input
REV. E
AD737
–11–
COM
+V
S
C
AV
C
C
C
C
10F
V
OUT
V
IN
–V
S
8
7
6
5
SCALE FACTOR
ADJUST
1
2
3
4
AD737
FULL
WAVE
RECTIFIER
BIAS
SECTION
INPUT
AMPLIFIER
8k
8k
+
RMS
CORE
+
C
AV
33F
+
OUTPUT
POWER
DOWN
C
F
10F
100
200
Figure 6. External Scale Factor Trim
COM
+VS
CAV
CC
VIN
–VS
8
7
6
5
NOT
CONNECTED
1
2
3
4
AD737
FULL
WAVE
RECTIFIER
BIAS
SECTION
INPUT
AMPLIFIER
8k
8k
RMS
CORE
+
CAV
+
OUTPUT
POWER
DOWN
CC
10F
1k
+3500
PPM/C
60.4
**RCAL **R1 IREF
10 *11
9
Q2
*Q1, Q2 PART OF
RCA CA3046 OR
SIMILAR NPN
TRANSITOR ARRAY
**R1 + RCAL IN OHMS = 10,000 4.3V
0dB INPUT LEVEL IN VOLTS
AD711 dB OUTPUT
100mV/dB
PRECISION
RESISTOR
CORP
TYPE PT/ST
2k
31.6k
SCALE
FACTOR
TRIM
13 Q1
12
14
*
Figure 7. dB Output Connection
COM
CC
VIN
AD737
FULL
WAVE
RECTIFIER
INPUT
AMPLIFIER
8k
+VS
1
2
3
8
7
6VOUT
499
–VS
+VS
500k
OFFSET ADJUST
1M
1k
1k
SCALE
FACTOR
ADJUST
Figure 8. DC-Coupled V
OS
and Scale Factor Trims
REV. E–12–
AD737
8-Lead Standard Small Outline Package [SOIC]
Narrow Body
(R-8)
Dimensions shown in millimeters and (inches)
0.25 (0.0098)
0.17 (0.0067)
1.27 (0.0500)
0.40 (0.0157)
0.50 (0.0196)
0.25 (0.0099) 45
8
0
1.75 (0.0688)
1.35 (0.0532)
SEATING
PLANE
0.25 (0.0098)
0.10 (0.0040)
85
41
5.00 (0.1968)
4.80 (0.1890)
4.00 (0.1574)
3.80 (0.1497)
1.27 (0.0500)
BSC
6.20 (0.2440)
5.80 (0.2284)
0.51 (0.0201)
0.31 (0.0122)
COPLANARITY
0.10
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
COMPLIANT TO JEDEC STANDARDS MS-012AA
OUTLINE DIMENSIONS
8-Lead Ceramic Dual In-Line Package [CERDIP]
(Q-8)
Dimensions shown in inches and (millimeters)
14
85
0.310 (7.87)
0.220 (5.59)
PIN 1
0.005 (0.13)
MIN
0.055 (1.40)
MAX
0.100 (2.54) BSC
15
0
0.320 (8.13)
0.290 (7.37)
0.015 (0.38)
0.008 (0.20)
SEATING
PLANE
0.200 (5.08)
MAX
0.405 (10.29) MAX
0.150 (3.81)
MIN
0.200 (5.08)
0.125 (3.18)
0.023 (0.58)
0.014 (0.36) 0.070 (1.78)
0.030 (0.76)
0.060 (1.52)
0.015 (0.38)
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)
SEATING
PLANE
0.180
(4.57)
MAX
0.150 (3.81)
0.130 (3.30)
0.110 (2.79) 0.060 (1.52)
0.050 (1.27)
0.045 (1.14)
8
14
5
0.295 (7.49)
0.285 (7.24)
0.275 (6.98)
0.100 (2.54)
BSC
0.375 (9.53)
0.365 (9.27)
0.355 (9.02)
0.150 (3.81)
0.135 (3.43)
0.120 (3.05)
0.015 (0.38)
0.010 (0.25)
0.008 (0.20)
0.325 (8.26)
0.310 (7.87)
0.300 (7.62)
0.022 (0.56)
0.018 (0.46)
0.014 (0.36)
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
COMPLIANT TO JEDEC STANDARDS MO-095AA
0.015
(0.38)
MIN
REV. E
AD737
–13–
Revision History
Location Page
6/03—Data Sheet changed from REV. D to REV. E.
Added AD737JR-5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Universal
Changes to FEATURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Changes to GENERAL DESCRIPTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Changes to SPECIFICATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Changes to ABSOLUTE MAXIMUM RATINGS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Changes to ORDERING GUIDE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Added TPCs 16 through 19 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Changes to Figures 1 and 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Changes to Figure 8 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Updated OUTLINE DIMENSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
12/02—Data Sheet changed from REV. C to REV. D.
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
–14–
–15–
C00828–0–6/03(E)
–16–
