Low Level,
a
True RMS-to-DC Converter
AD636
PIN CONNECTIONS &
FEATURES
True RMS-to-DC Conversion
200 mV Full Scale
FUNCTIONAL BLOCK DIAGRAM
Laser-Trimmed to High Accuracy
0.5% Max Error (AD636K)
1.0% Max Error (AD636J)
I
OUT
BUF IN
R
L
14
13
1
2
3
4
5
6
7
V
+V
S
ABSOLUTE
VALUE
IN
Wide Response Capability:
Computes RMS of AC and DC Signals
1 MHz –3 dB Bandwidth: V RMS >100 mV
Signal Crest Factor of 6 for 0.5% Error
dB Output with 50 dB Range
Low Power: 800 A Quiescent Current
Single or Dual Supply Operation
Monolithic Integrated Circuit
Low Cost
10k⍀
NC
NC
+
–
AD636
BUF
AD636
BUF OUT
COMMON
12 NC
SQUARER
DIVIDER
–V
CURRENT
MIRROR
S
10k⍀
11
C
NC
AV
SQUARER
DIVIDER
CURRENT
MIRROR
10
9
dB
BUF OUT
BUF IN
COMMON
dB
+V
S
R
I
L
ABSOLUTE
VALUE
+
10k⍀
BUF
8
–
OUT
C
AV
10k⍀
V
IN
–V
S
NC = NO CONNECT
Available in Chip Form
PRODUCT DESCRIPTION
The AD636 is a low power monolithic IC which performs true
rms-to-dc conversion on low level signals. It offers performance
which is comparable or superior to that of hybrid and modular
converters costing much more. The AD636 is specified for a
signal range of 0 mV to 200 mV rms. Crest factors up to 6 can
be accommodated with less than 0.5% additional error, allowing
accurate measurement of complex input waveforms.
is accurate within ±0.2 mV to ±0.3% of reading. Both versions
are specified for the 0°C to +70°C temperature range, and are
offered in either a hermetically sealed 14-pin DIP or a 10-lead
TO-100 metal can. Chips are also available.
PRODUCT HIGHLIGHTS
1. The AD636 computes the true root-mean-square of a com-
plex ac (or ac plus dc) input signal and gives an equivalent dc
output level. The true rms value of a waveform is a more
useful quantity than the average rectified value since it is a
measure of the power in the signal. The rms value of an
ac-coupled signal is also its standard deviation.
The low power supply current requirement of the AD636, typi-
cally 800 µA, allows it to be used in battery-powered portable
instruments. A wide range of power supplies can be used, from
±2.5 V to ±16.5 V or a single +5 V to +24 V supply. The input
and output terminals are fully protected; the input signal can
exceed the power supply with no damage to the device (allowing
the presence of input signals in the absence of supply voltage)
and the output buffer amplifier is short-circuit protected.
2. The 200 millivolt full-scale range of the AD636 is compatible
with many popular display-oriented analog-to-digital con-
verters. The low power supply current requirement permits
use in battery powered hand-held instruments.
The AD636 includes an auxiliary dB output. This signal is
derived from an internal circuit point which represents the loga-
rithm of the rms output. The 0 dB reference level is set by an
externally supplied current and can be selected by the user
to correspond to any input level from 0 dBm (774.6 mV) to
–20 dBm (77.46 mV). Frequency response ranges from 1.2 MHz
at a 0 dBm level to over 10 kHz at –50 dBm.
3. The only external component required to perform measure-
ments to the fully specified accuracy is the averaging capaci-
tor. The value of this capacitor can be selected for the desired
trade-off of low frequency accuracy, ripple, and settling time.
4. The on-chip buffer amplifier can be used to buffer either the
input or the output. Used as an input buffer, it provides
accurate performance from standard 10 MΩ input attenua-
tors. As an output buffer, it can supply up to 5 milliamps of
output current.
The AD636 is designed for ease of use. The device is factory-
trimmed at the wafer level for input and output offset, positive
and negative waveform symmetry (dc reversal error), and full-
scale accuracy at 200 mV rms. Thus no external trims are re-
quired to achieve full-rated accuracy.
5. The AD636 will operate over a wide range of power supply
voltages, including single +5 V to +24 V or split ±2.5 V to
±16.5 V sources. A standard 9 V battery will provide several
hundred hours of continuous operation.
AD636 is available in two accuracy grades; the AD636J total
error of ±0.5 mV ±0.06% of reading, and the AD636K
REV. B
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.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781/329-4700
Fax: 781/326-8703
World Wide Web Site: http://www.analog.com
© Analog Devices, Inc., 1999
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