Low Cost, Low Power,
True RMS-to-DC Converter
Data Sheet
AD737
FEATURES
Computes
FUNCTIONAL BLOCK DIAGRAM
COM
8kΩ
8kΩ
C
True rms value
C
C
F
Average rectified value
Absolute value
Provides
ABSOLUTE
VALUE
CIRCUIT
OUTPUT
SQUARER
DIVIDER
V
IN
200 mV full-scale input range (larger inputs with
input scaling)
C
AV
+V
C
S
AV
BIAS
SECTION
Direct interfacing with 3½ digit CMOS ADCs
POWER
DOWN
–V
S
High input impedance: 1012
Ω
Low input bias current: 25 pA maximum
High accuracy: 0.2 mV 0.3% of reading
Figure 1.
RMS conversion with signal crest factors up to 5
Wide power supply range: 2.5 V to 16.5 V
Low power: 25 µA (typical) standby current
No external trims needed for specified accuracy
The AD737 output is negative-going; the AD736 is a positive
output-going version of the same basic device
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 has both high (1012 Ω) and low impedance input
options. The high-Z FET input connects high source impedance
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 two performance grades. The AD737J
and AD737K grades operate 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 grade operates over the
industrial temperature range of −40°C to +85°C. The AD737 is
available in two low cost, 8lead packages: PDIP and SOIC_N.
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.
PRODUCT HIGHLIGHTS
1. Computes average rectified, absolute, or true rms value of a
signal regardless of waveform.
2. Only one external component, an averaging capacitor, is
required for the AD737 to perform true rms measurement.
3. The standby power consumption of 125 μW makes the
AD737 suitable for battery-powered applications.
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. In power-down mode, the standby supply
current in is typically 25 µA.
Rev. I
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
rightsof third parties that may result fromits 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 andregisteredtrademarks 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
Fax: 781.461.3113
www.analog.com
©2012 Analog Devices, Inc. All rights reserved.
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