Wide-Band RMS-to-DC Converter
0.02% Max Nonlinearity, 0 V to 2 V RMS Input
0.10% Additional Error to Crest Factor of 3
8 MHz at 2 V RMS Input
600 kHz at 100 mV RMS
dB Output (60 dB Range)
Chip Select-Power Down Feature Allows:
Analog “3-State” Operation
Quiescent Current Reduction from 2.2 mA to 350 A
Side-Brazed DIP, Low Cost Cerdip and SOIC
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 inte-
grated 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 per-
mits measurements of signals with crest factors of up to 10 with
less than 1% additional error. The circuit’s wide bandwidth per-
mits 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,
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.
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 facili-
tates 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 out-
put goes to a high impedance state. This allows several AD637s
to be tied together to form a wide-band true rms multiplexer.
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.
FUNCTIONAL BLOCK DIAGRAMS
Ceramic DIP (D) and
Cerdip (Q) Packages
SOIC (R) Package
The AD637 is available in two accuracy grades (J, K) for com-
mercial (0°C to +70°C) temperature range applications; two
accuracy grades (A, B) for industrial (–40°C to +85°C) applica-
tions; and one (S) rated over the –55°C to +125°C temperature
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.
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.
2. The AD637 is laser wafer trimmed to achieve rated perfor-
mance without external trimming. The only external compo-
nent required is a capacitor which sets the averaging time
period. The value of this capacitor also determines low fre-
quency accuracy, ripple level and settling time.
3. The chip select feature of the AD637 permits the user to
power down the device down during periods of nonuse,
thereby, decreasing battery drain in remote or hand-held
4. The on-chip buffer amplifier can be used as either 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.
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
which may result from its use. No license is granted by implication or
otherwise under any patent or patent rights of Analog Devices.
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© Analog Devices, Inc., 1999