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AD737
Low power, precision, monolithic true rms-to-dc converter with 200 mV Full-scale Input Range
REV.D
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: +2.8 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
FUNCTIONAL BLOCK DIAGRAM
GENERAL DESCRIPTIONThe AD737 is a low power, precision, monolithic true rms-to-dc
converter. It is laser trimmed to provide a maximum error of0.2 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 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.
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.
The AD737 allows the choice of two signal input terminals: a
high impedance (1012 Ω) FET input that directly interfaces with
high Z input attenuators and a low impedance (8 kΩ) 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 0°C to +70°C. 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:
plastic DIP, plastic SOIC, and hermetic CERDIP.
PRODUCT HIGHLIGHTSThe AD737 is 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 many battery-powered applications.
AD737–SPECIFICATIONS
(@ 25�C, �5 V supplies, ac-coupled with 1 kHz sine wave input applied, unless
otherwise noted.)ERROR vs. CREST FACTOR
INPUT CHARACTERISTICS
AD737POWER SUPPLY
NOTESAccuracy is specified with the AD737 connected as shown in Figure 1 with capacitor CC.Nonlinearity is defined as the maximum deviation (in percent error) from a straight line connecting the readings at 0 and 200 mV rms.Error versus crest factor is specified as additional error for a 200 mV rms signal. Crest factor = VPEAK/V rms.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.
AD737
CAUTIONESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000V 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 RATINGS1Supply 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) . . . . . . . . . –65°C to +150°C
Storage Temperature Range (N, R) . . . . . . . –65°C to +125°C
Operating Temperature Range
AD737J/AD737K . . . . . . . . . . . . . . . . . . . . . . 0°C to +70°C
AD737A/AD737B . . . . . . . . . . . . . . . . . . . –40°C to +85°C
Lead Temperature Range (Soldering 60 sec) . . . . . . . . . 300°C
ESD Rating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 500 V
NOTESStresses 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.8-Lead Plastic DIP Package: �JA = 165°C/W
8-Lead CERDIP Package: �JA = 110°C/W
8-Lead Small Outline Package: �JA = 155°C/W
PIN CONFIGURATIONS
Plastic DIP (N-8), CERDIP (Q-8), SOIC (SOIC-8)
ORDERING GUIDE
TPC 1.Additional Error vs. Supply
Voltage
FREQUENCY – kHz
INPUT LEVEL – rms
100�V
1mV
10mV
100mV
10VTPC 4.Frequency Response
Driving Pin 1
TPC 7.Additional Error vs.
Temperature
TPC 2.Maximum Input Level vs.
Supply Voltage
FREQUENCY – kHz
INPUT LEVEL – rms
100�V
1mV
10mV
100mV
10VTPC 5.Frequency Response
Driving Pin 2
TPC 8.DC Supply Current vs.
RMS Input Level
TPC 3.Power-Down Current vs.
Supply Voltage
TPC 6.Additional Error vs. Crest
Factor vs. CAV
TPC 9.–3 dB Frequency vs. RMS
Input Level (Pin 2)
AD737TPC 10.Error vs. RMS Input Voltage
(Pin 2) Using Circuit of Figure 6
TPC 13.Pin 2 Input Bias Current vs.
Supply Voltage
TPC 11.CAV vs. Frequency for
Specified Averaging Error
TPC 14.Settling Time vs. RMS Input
Level for Various Values of CAV
TPC 12.RMS Input Level vs. Fre-
quency for Specified Averaging Error
TPC 15.Pin 2 Input Bias Current vs.
Temperature
