Low Cost, Low Power,
a
True RMS-to-DC Converter
AD736
FUNCTIONAL BLOCK DIAGRAM
8k⍀
FEATURES
Computes:
True rms Value
Average Rectified Value
Absolute Value
Provides:
200 mV Full-Scale Input Range
(Larger Inputs with Input Attenuator)
High Input Impedance of 1012 V
Low Input Bias Current: 25 pA Max
High Accuracy: ؎0.3 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: 200 mA Max Supply Current
Buffered Voltage Output
C
1
2
3
4
8
7
6
5
COM
AD736
C
FULL
WAVE
RECTIFIER
8k⍀
+V
S
V
IN
INPUT
AMPLIFIER
C
OUTPUT
rms CORE
F
BIAS
OUTPUT
AMPLIFIER
SECTION
–V
C
S
AV
No External Trims Needed for Specified Accuracy
AD737—An Unbuffered Voltage Output Version with
Chip Power Down also Available
GENERAL DESCRIPTION
that allows the measurement of 300 mV input level while oper-
ating from the minimum power supply voltage of +2.8 V, –3.2 V.
The two inputs may be used either singly or differentially.
The AD736 is a low power, precision, monolithic true rms-to-dc
converter. It is laser trimmed to provide a maximum error of
±0.3 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 AD736 offers higher accuracy at equal or
lower cost.
The AD736 achieves a 1% of reading error bandwidth exceeding
10 kHz for input amplitudes from 20 mV rms to 200 mV rms
while consuming only 1 mW.
The AD736 is available in four performance grades. The AD736J
and AD736K grades are rated over the commercial temperature
range of 0°C to +70°C. The AD736A and AD736B grades are
rated over the industrial temperature range of –40°C to +85°C.
The AD736 is available in three low cost, 8-lead packages:
plastic miniDIP, plastic SOIC, and hermetic CERDIP.
The AD736 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 AD736 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.
PRODUCT HIGHLIGHTS
1. The AD736 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 AD736 to perform true rms measurement.
3. The low power consumption of 1 mW makes the AD736
suitable for many battery-powered applications.
4. A high input impedance of 1012 Ω eliminates the need for an
external buffer when interfacing with input attenuators.
5. A low impedance input is available for those applications
requiring up to 300 mV rms input signal operating from low
power supply voltages.
The AD736 has its own output buffer amplifier, thereby providing
a great deal of design flexibility. Requiring only 200 µA of power
supply current, the AD736 is optimized for use in portable
multimeters and other battery-powered applications.
The AD736 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
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, norforanyinfringementsofpatentsorotherrightsofthirdpartiesthat
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
www.analog.com
© Analog Devices, Inc., 2002
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