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 10
12
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
C
C 1
AD737
FULL
WAVE
RECTIFIER
INPUT
AMPLIFIER
8
COM
V
IN 2
8k
7
+V
S
POWER
3
DOWN
BIAS
SECTION
RMS CORE
6
OUTPUT
–V
S 4
5
C
AV
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 (10
12
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 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
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.
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.
REV. D
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
Fax: 781/326-8703
© Analog Devices, Inc., 2002
AD [ ANALOG DEVICES ]
