High Precision, Wideband
RMS-to-DC Converter
AD637
Rev. J
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FEATURES
High accuracy
0.02% maximum nonlinearity, 0 V to 2 V rms input
0.10% additional error to crest factor of 3
Wide bandwidth
8 MHz at 2 V rms input
600 kHz at 100 mV rms
Computes
True rms
Square
Mean square
Absolute value
dB output (60 dB range)
Chip select/power-down feature allows
Analog three-state operation
Quiescent current reduction from 2.2 mA to 350 μA
14-lead SBDIP, 14-lead low cost CERDIP, and 16-lead SOIC_W
FUNCTIONAL BLOCK DIAGRAM
ABSOLUTE
VALUE
DEN INPUT
RMS OUT
dB OUTPUT
BUFF IN
BUFF
OUT
25kΩ
25kΩ
COMMON
CS
OUTPUT
OFFSET
00788-001
SQUARER/
DIVIDER
BIAS
V
IN
C
AV
AD637
Figure 1.
GENERAL DESCRIPTION
The AD637 is a complete, high accuracy, monolithic rms-to-dc
converter that computes the true rms value of any complex
waveform. 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 measurements of signals with crest factors of
up to 10 with less than 1% additional error. The wide band-
width of the AD637 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.
As with previous monolithic rms converters from Analog
Devices, Inc., the AD637 has an auxiliary dB output available to
users. 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.
A chip select connection on the AD637 permits the user to
decrease 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 handheld
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.
The input circuitry of the AD637 is protected from overload
voltages in excess of the supply levels. The inputs are not
damaged by input signals if the supply voltages are lost.
The AD637 is available in accuracy Grade J and Grade K for
commercial temperature range (0°C to 70°C) applications, accuracy
Grade A and Grade B for industrial range (−40°C to +85°C) appli-
cations, and accuracy Grade S rated over the −55°C to +125°C
temperature range. All versions are available in hermetically sealed,
14-lead SBDIP, 14-lead CERDIP, and 16-lead SOIC_W packages.
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 because 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.
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.
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 accuracy.
