High Precision, Wideband
RMS-to-DC Converter
a
AD637
FUNCTIONAL BLOCK DIAGRAMS
FEATURES
High Accuracy
Ceramic DIP (D) and
Cerdip (Q) Packages
SOIC (R) Package
0.02% Max Nonlinearity, 0 V to 2 V RMS Input
0.10% Additional Error to Crest Factor of 3
Wide Bandwidth
BUFFER
BUFFER
AD637
AD637
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8
16
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9
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2
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7
14
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10
9
8 MHz at 2 V RMS Input
600 kHz at 100 mV RMS
Computes:
True RMS
Square
ABSOLUTE
VALUE
ABSOLUTE
VALUE
BIAS
BIAS
SECTION
SECTION
SQUARER/DIVIDER
SQUARER/DIVIDER
25kꢁ
25kꢁ
Mean Square
Absolute Value
25kꢁ
25kꢁ
dB Output (60 dB Range)
Chip Select/Power-Down Feature Allows:
Analog “Three-State” Operation
Quiescent Current Reduction from 2.2 mA to 350 ꢀA
Side Brazed DIP, Low Cost Cerdip and SOIC
FILTER
FILTER
8
PRODUCT DESCRIPTION
The AD637 is available in two accuracy grades (J and K) for
The AD637 is a complete high accuracy monolithic rms-to-dc
converter that computes the true rms value of any complex wave-
form. It offers performance that is unprecedented in integrated
circuit rms-to-dc converters and comparable to discrete and
modular techniques in accuracy, bandwidth, and dynamic range.
A crest factor compensation scheme in the AD637 permits mea-
surements of signals with crest factors of up to 10 with less than
1% additional error. The circuit’s wide bandwidth permits the
measurement of signals up to 600 kHz with inputs of 200 mV
rms and up to 8 MHz when the input levels are above 1 V rms.
commercial (0°C to 70°C) temperature range applications;
two accuracy grades (A and B) for industrial (–40°C to +85°C)
applications; and one (S) rated over the –55°C to +125°C tem-
perature range. All versions are available in hermetically sealed,
14-lead side brazed ceramic DIPs as well as low cost cerdip
packages. A 16-lead SOIC package is also available.
PRODUCT HIGHLIGHTS
1. The AD637 computes the true root-mean-square, mean-
square, or absolute value of any complex ac (or ac plus dc)
input waveform and gives an equivalent dc output voltage.
The true rms value of a waveform is more useful than an
average rectified signal since it relates directly to the power of
the signal. The rms value of a statistical signal is also related
to the standard deviation of the signal.
As with previous monolithic rms converters from Analog Devices,
the AD637 has an auxiliary dB output available to the user. The
logarithm of the rms output signal is brought out to a separate
pin, allowing direct dB measurement with a useful range of
60 dB. An externally programmed reference current allows the
user to select the 0 dB reference voltage to correspond to any
level between 0.1 V and 2.0 V rms.
2. The AD637 is laser wafer trimmed to achieve rated
performance without external trimming. The only external
component required is a capacitor that sets the averaging
time period. The value of this capacitor also determines
low-frequency accuracy, ripple level, and settling time.
A chip select connection on the AD637 permits the user to de-
crease the supply current from 2.2 mA to 350 µA during periods
when the rms function is not in use. This feature facilitates the
addition of precision rms measurement to remote or hand-held
applications where minimum power consumption is critical. In
addition when the AD637 is powered down the output goes to a
high impedance state. This allows several AD637s to be tied
together to form a wideband true rms multiplexer.
3. The chip select feature of the AD637 permits the user to
power down the device during periods of nonuse, thereby
decreasing battery drain in remote or hand-held applications.
4. The on-chip buffer amplifier can be used either as an input
buffer or in an active filter configuration. The filter can be
used to reduce the amount of ac ripple, thereby increasing
the accuracy of the measurement.
The input circuitry of the AD637 is protected from overload
voltages that are in excess of the supply levels. The inputs will
not be damaged by input signals if the supply voltages are lost.
REV. F
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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