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Low Cost, Low Power,
True RMS-to-DC Converter
AD737
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 3½ digit CMOS ADCs
High input impedance: 10
12
Ω
Low input bias current: 25 pA maximum
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 μA maximum supply current
No external trims needed for specified accuracy
A general-purpose, buffered voltage output version also
available (AD736)
FUNCTIONAL BLOCK DIAGRAM
8kΩ
C
C 1
AD737
FULL-WAVE
RECTIFIER
INPUT
AMPLIFIER
8kΩ
8
COM
V
IN 2
7
+V
S
POWER
3
DOWN
BIAS
SECTION
RMS CORE
6
OUTPUT
Figure 1.
GENERAL DESCRIPTION
The AD737
1
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 μV rms or less, despite variations in tem-
perature 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, which makes
the device highly compatible with high input impedance ADCs.
Requiring only 160 μA 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 that reduces the power-supply standby current to less
than 30 μA.
1
Two signal input terminals are provided in the AD737. A high
impedance (10
12
Ω) FET input interfaces directly with high R
input attenuators, and a low impedance (8 kΩ) input accepts
rms voltages to 0.9 V while operating from the minimum power
supply voltage of ±2.5 V. The two inputs can 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 0°C to 70°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,
8lead packages: PDIP, SOIC_N, and CERDIP.
PRODUCT HIGHLIGHTS
1.
2.
3.
Capable of computing the average rectified value, absolute
value, or true rms value of various input signals.
Only one external component, an averaging capacitor, is
required for the AD737 to perform true rms measurement.
The low power consumption of 0.72 mW makes the
AD737 suitable for battery-powered applications.
Protected under U.S. Patent Number 5,495,245.
Rev. H
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. 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.
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
©2008 Analog Devices, Inc. All rights reserved.
00828-001
–V
S 4
5
C
AV
AD737
TABLE OF CONTENTS
Features .............................................................................................. 1
Functional Block Diagram .............................................................. 1
General Description ......................................................................... 1
Product Highlights ........................................................................... 1
Revision History ............................................................................... 2
Specifications..................................................................................... 3
Absolute Maximum Ratings............................................................ 6
Thermal Resistance ...................................................................... 6
ESD Caution .................................................................................. 6
Pin Configurations and Function Descriptions ........................... 7
Typical Performance Characteristics ............................................. 8
Theory of Operation ...................................................................... 12
Types of AC Measurement ........................................................ 12
DC Error, Output Ripple, and Averaging Error ..................... 13
AC Measurement Accuracy and Crest Factor ........................ 13
Calculating Settling Time.......................................................... 13
Applications Information .............................................................. 14
RMS Measurement—Choosing an Optimum
Value for C
AV
............................................................................... 14
Rapid Settling Times via the Average Responding
Connection.................................................................................. 14
Selecting Practical Values for Capacitors ................................ 14
Scaling Input and Output Voltages .......................................... 14
AD737 Evaluation Board............................................................... 18
Outline Dimensions ....................................................................... 20
Ordering Guide .......................................................................... 22
REVISION HISTORY
10/08—Rev. G to Rev. H
Added Selectable Average or RMS Conversion Section and
Figure 27 .......................................................................................... 14
Updated Outline Dimensions ....................................................... 20
Changes to Ordering Guide .......................................................... 22
12/06—Rev. F to Rev. G
Changes to Specifications ................................................................ 3
Reorganized Typical Performance Characteristics ...................... 8
Changes to Figure 21 ...................................................................... 11
Reorganized Theory of Operation Section ................................. 12
Reorganized Applications Section ................................................ 14
Added Scaling Input and Output Voltages Section.................... 14
Deleted Application Circuits Heading ......................................... 16
Changes to Figure 28 ...................................................................... 16
Added AD737 Evaluation Board Section .................................... 18
Updated Outline Dimensions ....................................................... 20
Changes to Ordering Guide .......................................................... 21
1/05—Rev. E to Rev. F
Updated Format .................................................................. Universal
Added Functional Block Diagram.................................................. 1
Changes to General Description Section ...................................... 1
Changes to Pin Configurations and Function
Descriptions Section ........................................................................ 6
Changes to Typical Performance Characteristics Section ........... 7
Changes to Table 4 .......................................................................... 11
Change to Figure 24 ....................................................................... 12
Change to Figure 27 ....................................................................... 15
Changes to Ordering Guide .......................................................... 18
6/03—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—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
12/99—Rev. B to Rev. C
Rev. H | Page 2 of 24
AD737
SPECIFICATIONS
T
A
= 25°C, ±V
S
= ±5 V except as noted, C
AV
= 33 μF, C
C
= 10 μF, f = 1 kHz, sine wave input applied to Pin 2, unless otherwise specified.
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.
Table 1.
Parameter
ACCURACY
Total Error
Conditions
E
IN
= 0 to 200 mV rms
±V
S
= ±2.5 V
±V
S
= ±2.5 V,
input to Pin 1
E
IN
= 200 mV to 1 V rms
Over
Temperature
AQ and BQ
JN, JR, KN, KR
AN and AR
Vs. Supply
Voltage
E
IN
= 200 mV rms,
±V
S
= ±2.5 V to ±5 V
E
IN
= 200 mV rms,
±V
S
= ±5 V to ±16.5 V
DC coupled,
V
IN
= 600 mV dc
V
IN
= 200 mV dc,
±V
S
= ±2.5 V
E
IN
= 0 mV to
200 mV rms,
@ 100 mV rms
AC coupled,
E
IN
= 100 mV rms, after
correction, ±V
S
= ±2.5 V
E
IN
= 0 mV to
200 mV rms
0
0
−0.18
0.06
1.3
−0.3
0.1
2.5
0
0
−0.18
0.06
1.3
−0.3
0.1
2.5
1.7
0
0.25
0.35
0
0.25
0.35
2.5
0
0
−0.18
0.06
−0.3
0.1
%/V
%/V
POR
POR
POR
AD737A, AD737J
Min
Typ
Max
0.2/0.3
0.4/0.5
AD737B, AD737K
Min
Typ
Max
0.2/0.2
0.2/0.3
0.2/0.3
0.2/0.3
−1.2
±2.0
−1.2
±2.0
0.4/0.5
0.4/0.5
Min
AD737J-5
Typ
Max
Unit
±mV/±POR
1
±mV/±POR
1
±mV/±POR
1
POR
E
IN
= 200 mV rms
E
IN
= 200 mV rms,
±V
S
= ±2.5 V
E
IN
= 200 mV rms,
±V
S
= ±2.5 V
0.5/0.7
0.007
0.014
0.007
0.014
0.3/0.5
0.02
±POR/°C
±POR/°C
±POR/°C
DC Reversal Error
Nonlinearity
2
Input to Pin 1
3
0.02
0.1
POR
Total Error,
External Trim
ADDITIONAL
CREST FACTOR
ERROR
4
For Crest Factors
from 1 to 3
0.1/0.2
0.1/0.2
0.1/0.2
±mV/±POR
C
AV
= C
F
= 100 μF
C
AV
= 22 μF, C
F
= 100 μF,
±V
S
= ±2.5 V, input to
Pin 1
C
AV
= C
F
= 100 μF
0.7
0.7
1.7
%
%
For Crest Factors
from 3 to 5
INPUT
CHARACTERISTICS
High-Z Input (Pin 2)
Signal Range
Continuous
RMS Level
2.5
2.5
%
±V
S
= +2.5 V
±V
S
= +2.8 V/−3.2 V
±V
S
= ±5 V to ±16.5 V
200
1
200
1
200
mV rms
mV rms
V rms
Rev. H | Page 3 of 24
AD737
Parameter
Peak Transient
Input
Conditions
±V
S
= +2.5 V input to
Pin 1
±V
S
= +2.8 V/−3.2 V
±V
S
= ±5 V
±V
S
= ±16.5 V
±V
S
= ±5 V
AD737A, AD737J
Min
Typ
Max
AD737B, AD737K
Min
Typ
Max
Min
±0.6
AD737J-5
Typ
Max
Unit
V
V
V
V
Ω
pA
±0.9
±2.7
±4.0
1012
1
25
±0.9
±2.7
±4.0
1012
1
1012
1
Input Resistance
Input Bias
Current
Low-Z Input
(Pin 1) Signal
Range
Continuous
RMS Level
25
25
±V
S
= +2.5 V
±V
S
= +2.8 V/−3.2 V
±V
S
= ±5 V to ±16.5 V
±V
S
= +2.5 V
±V
S
= +2.8 V/−3.2 V
±V
S
= ±5 V
±V
S
= ±16.5 V
±1.7
±3.8
±11
8
300
1
300
1
±1.7
±1.7
±3.8
±11
8
300
mV rms
mV rms
V rms
V
V
V
V
kΩ
V p-p
Peak Transient
Input
Input Resistance
Maximum
Continuous
Nondestructive
Input
Input Offset
Voltage
5
Over the Rated
Operating
Temperature
Range
Vs. Supply
OUTPUT
CHARACTERISTICS
Output Voltage
Swing
6.4
All supply voltages
9.6
±12
6.4
9.6
±12
6.4
8
9.6
±12
AC coupled
8
±3
30
8
±3
30
8
±3
30
mV
μV/°C
V
S
= ±2.5 V to ±5 V
V
S
= ±5 V to ±16.5 V
No load
±V
S
= +2.8 V/−3.2 V
±V
S
= ±5 V
±V
S
= ±16.5 V
±V
S
= ±2.5 V, input to
Pin 1
DC
−1.6
−3.3
−4
80
50
150
80
50
80
150
μV/V
μV/V
−1.7
−3.4
−5
−1.6
−3.3
−4
−1.7
−3.4
−5
−1.1
–0.9
8
9.6
V
V
V
V
kΩ
Output
Resistance
FREQUENCY
RESPONSE
High-Z Input
(Pin 2)
1% Additional
Error
6.4
8
9.6
6.4
8
9.6
6.4
V
IN
= 1 mV rms
V
IN
= 10 mV rms
V
IN
= 100 mV rms
V
IN
= 200 mV rms
1
6
37
33
1
6
37
33
1
6
37
33
kHz
kHz
kHz
kHz
Rev. H | Page 4 of 24
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