Low Level, True RMS-to-DC Converter
AD636
Data Sheet
FEATURES
FUNCTIONAL BLOCK DIAGRAM
True rms-to-dc conversion
200 mV full scale
ABSOLUTE
VALUE
V
IN
Laser-trimmed to high accuracy
0.5% maximum error (AD636K)
1.0% maximum error (AD636J)
Wide response capability
dB
SQUARER
DIVIDER
COM
C
AV
+V
S
+V
S
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 μA quiescent current
Single or dual supply operation
Monolithic integrated circuit
Low cost
CURRENT
MIRROR
10kΩ
R
L
+V
S
I
OUT
BUFFER IN
BUF
10kΩ
BUFFER OUT
40kΩ
AD636
GENERAL DESCRIPTION
–V
S
The AD636 is a low power monolithic IC that performs true
rms-to-dc conversion on low level signals. It offers performance
that 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.
–V
S
Figure 1.
The AD636 is available in two accuracy grades. The total error of the
J-version is typically less than ±0.5 mV ± ±.0% of reading, while
the total error of the AD636K is less than ±0.2 mV to ±0.5% of
reading. Both versions are temperature rated for operation
between 0°C and 70°C and available in ±4-lead SBDIP and ±0-lead
TO-±00 metal can.
The low power supply current requirement of the AD636,
typically 800 μA, is ideal for battery-powered portable
The AD636 computes the true root-mean-square of a complex 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 because it is a measure of the power
in the signal. The rms value of an ac-coupled signal is also its
standard deviation.
instruments. It operates from a wide range of dual and single
power supplies, from ±2.5 V to ±±6.5 V or from +5 V to +24 V.
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.
The AD636 includes an auxiliary dB output derived from an
internal circuit point that represents the logarithm of the rms
output. The 0 dB reference level is set by an externally supplied
current and can be selected to correspond to any input level from
0 dBm (774.6 mV) to −20 dBm (77.46 mV). Frequency response
ranges from ±.2 MHz at 0 dBm to greater than ±0 kHz at −50 dBm.
The 200 mV full-scale range of the AD636 is compatible with
many popular display-oriented ADCs. The low power supply
current requirement permits use in battery-powered hand-held
instruments. An averaging capacitor is the only external
component required to perform measurements to the fully
specified accuracy is. Its value optimizes the trade-off between
low frequency accuracy, ripple, and settling time.
The AD636 is easy to 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. Therefore, no external trims are required to
achieve full-rated accuracy.
An optional on-chip amplifier acts as a buffer for the input or the
output signals. Used in the input, it provides accurate
performance from standard ±0 MΩ input attenuators. As an
output buffer, it sources up to 5 mA.
Rev. E
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rights of third parties that may result from its 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 and registeredtrademarks arethe property of their respective owners.
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Tel: 781.329.4700
Technical Support
©2013 Analog Devices, Inc. All rights reserved.
www.analog.com
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