Data Sheet
FEATURES
True rms-to-dc conversion
200 mV full scale
Laser-trimmed to high accuracy
0.5% maximum error (AD636K)
1.0% maximum error (AD636J)
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 μA quiescent current
Single or dual supply operation
Monolithic integrated circuit
Low cost
Low Level, True RMS-to-DC Converter
AD636
FUNCTIONAL BLOCK DIAGRAM
V
IN
ABSOLUTE
VALUE
COM
C
AV
+V
S
+V
S
SQUARER
DIVIDER
dB
CURRENT
MIRROR
10kΩ
+V
S
BUFFER IN
BUF
10kΩ
R
L
I
OUT
BUFFER OUT
40kΩ
AD636
GENERAL DESCRIPTION
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.
The low power supply current requirement of the AD636,
typically 800 μA, is ideal for battery-powered portable
instruments. It operates from a wide range of dual and single
power supplies, from ±2.5 V to ±16.5 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 1.2 MHz at 0 dBm to greater than 10 kHz at −50 dBm.
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.
Rev. E
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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 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 registered trademarks are the property of their respective owners.
–V
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Figure 1.
The AD636 is available in two accuracy grades. The total error of the
J-version is typically less than ±0.5 mV ± 1.0% 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 14-lead SBDIP and 10-lead
TO-100 metal can.
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.
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.
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 10 MΩ input attenuators. As an
output buffer, it sources up to 5 mA.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781.329.4700
©2013 Analog Devices, Inc. All rights reserved.
Technical Support
www.analog.com
00787-001
–V
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AD636
TABLE OF CONTENTS
Features .............................................................................................. 1
Functional Block Diagram .............................................................. 1
General Description ......................................................................... 1
Revision History ............................................................................... 2
Specifications..................................................................................... 3
Absolute Maximum Ratings ............................................................ 5
ESD Caution .................................................................................. 5
Pin Configurations and Function Descriptions ........................... 6
Typical Performance Characteristics ............................................. 7
Theory of Operation ........................................................................ 8
RMS Measurements ..................................................................... 8
The AD636 Buffer Amplifier ...................................................... 8
Frequency Response ..................................................................... 9
AC Measurement Accuracy and Crest Factor (CF) ................. 9
Data Sheet
Applications..................................................................................... 10
Standard Connection ................................................................. 10
Optional Trims for High Accuracy .......................................... 10
Single-Supply Connection ........................................................ 10
Choosing the Averaging Time Constant ................................. 11
A Complete AC Digital Voltmeter ........................................... 12
A Low Power, High Input, Impedance dB Meter....................... 12
Circuit Description ................................................................ 12
Performance Data .................................................................. 12
Frequency Response ±3 dBm ............................................... 13
Calibration .............................................................................. 13
Outline Dimensions ....................................................................... 14
Ordering Guide .......................................................................... 14
REVISION HISTORY
5/13—Rev. D to Rev. E
Reorganized Layout ............................................................ Universal
Changes to Figure 1 ........................................................................... 1
Change to Table 1 .............................................................................. 4
Added Typical Performance Characteristics Section ................... 7
Added Theory of Operation Section; Changes to Figure 7 and
Figure 8 ............................................................................................... 8
Changed Applying the AD636 Section to Applications Section;
Changes to Figure 9, Figure 10, and Single-Supply Connection
Section ...............................................................................................10
Changes to Figure 11 .......................................................................11
Changes to Figure 13 and A Complete AC Digital Voltmeter
Section ...............................................................................................12
Changes to Figure 17 and Figure 18..............................................13
Changes to Ordering Guide ...........................................................14
11/06—Rev. C to Rev. D
Changes to General Description ..................................................... 1
Changes to Table 1............................................................................. 3
Changes to Ordering Guide .......................................................... 13
1/06—Rev B to Rev. C
Updated Format .................................................................. Universal
Changes to Figure 1 and General Description ..............................1
Deleted Metallization Photograph ..................................................3
Added Pin Configuration and Function Description Section ....6
Updated Outline Dimensions ....................................................... 14
Changes to Ordering Guide .......................................................... 14
8/99—Rev A to Rev. B
Rev. E | Page 2 of 16
Data Sheet
SPECIFICATIONS
@ 25°C, +V
S
= +3 V, and −V
S
= –5 V, unless otherwise noted.
1
Table 1.
Model
TRANSFER FUNCTION
CONVERSION ACCURACY
Total Error, Internal Trim
2, 3
vs. Temperature, 0°C to +70°C
vs. Supply Voltage
DC Reversal Error at 200 mV
Total Error, External Trim
2
ERROR VS. CREST FACTOR
4
Crest Factor 1 to 2
Crest Factor = 3
Crest Factor = 6
AVERAGING TIME CONSTANT
INPUT CHARACTERISTICS
Signal Range, All Supplies
Continuous RMS Level
Peak Transient Inputs
+3 V, −5 V Supply
±2.5 V Supply
±5 V Supply
Maximum Continuous
Nondestructive
Input Level (All Supply Voltages)
Input Resistance
Input Offset Voltage
FREQUENCY RESPONSE
3, 5
Bandwidth for 1% Additional
Error (0.09 dB)
V
IN
= 10 mV
V
IN
= 100 mV
V
IN
= 200 mV
±3 dB Bandwidth
V
IN
= 10 mV
V
IN
= 100 mV
V
IN
= 200 mV
OUTPUT CHARACTERISTICS
3
Offset Voltage, V
IN
= COM
vs. Temperature
vs. Supply
Voltage Swing
+3 V, −5 V Supply
±5 V to ±16.5 V Supply
Output Impedance
±0.1 ± 0.01
±0.2
±0.3 ± 0.3
Min
AD636J
Typ
Max
Min
AD636K
Typ
Max
Unit
AD636
V
OUT
=
avg
×
(
V
IN
)
2
±0.5 ± 1.0
±0.1 ± 0.01
V
OUT
=
avg
×
(
V
IN
)
2
±0.2 ± 0.5
±0.1 ± 0.005
±0.1 ± 0.01
±0.1
± 0.1 ± 0.2
mV ± % of
reading
mV ± % of
reading/°C
mV ± % of
reading/V
% of reading
mV ± % of
reading
Specified Accuracy
−0.2
−0.5
25
Specified Accuracy
−0.2
−0.5
25
% of reading
% of reading
ms/μF of C
AV
0 to 200
±2.8
±2.0
±5.0
0 to 200
±2.8
±2.0
±5.0
mV rms
V p-p
V p-p
V p-p
5.33
6.67
±12
8
±0.5
5.33
6.67
±12
8
±0.2
V p-p
kΩ
mV
14
90
130
100
900
1.5
±0.5
±10
±0.1
0.3
0.3
8
0 to 1.0
0 to 1.0
10
0.3
0.3
8
14
90
130
100
900
1.5
±0.2
±10
±0.1
0 to 1.0
0 to 1.0
10
kHz
kHz
kHz
kHz
kHz
MHz
mV
μV/°C
mV/V
V
V
kΩ
12
Rev. E | Page 3 of 16
12
AD636
Model
dB OUTPUT
Error, V
IN
= 7 mV to 300 mV rms
Scale Factor
Scale Factor Temperature
Coefficient
I
REF
for 0 dB = 0.1 V rms
I
REF
Range
I
OUT
TERMINAL
I
OUT
Scale Factor
I
OUT
Scale Factor Tolerance
Output Resistance
Voltage Compliance
BUFFER AMPLIFIER
Input and Output Voltage Range
Input Offset Voltage, R
S
= 10 kΩ
Input Bias Current
Input Resistance
Output Current
Short-Circuit Current
Small Signal Bandwidth
Slew Rate
6
POWER SUPPLY
Voltage, Rated Performance
Dual Supply
Single Supply
Quiescent Current
7
TEMPERATURE RANGE
Rated Performance
Storage
TRANSISTOR COUNT
1
Data Sheet
Min
AD636J
Typ
±0.3
−3.0
0.33
−0.033
4
Max
±0.5
Min
AD636K
Typ
±0.1
−3.0
0.33
−0.033
4
Max
±0.2
Unit
dB
mV/dB
% of reading/°C
dB/°C
μA
μA
μA/V rms
%
kΩ
V
2
1
8
50
2
1
8
50
−20
8
100
±10
10
−V
S
to
(+V
S
− 2 V)
+20
12
−20
8
100
±10
10
−V
S
to
(+V
S
− 2 V)
+20
12
−V
S
to
(+V
S
− 2 V)
±0.8
100
10
8
(+5 mA,
−130 μA)
20
1
5
+3, −5
+2, −2.5
5
0.80
0
−55
62
±16.5
24
1.00
+70
+150
±2
300
−V
S
to
(+V
S
− 2 V)
±0.5
100
10
8
(+5 mA,
−130 μA)
20
1
5
+3, −5
+2, −2.5
5
0.80
0
−55
62
±16.5
24
1.00
+70
+150
±1
300
V
mV
nA
Ω
mA
MHz
V/μs
V
V
V
mA
°C
°C
All minimum and maximum specifications are guaranteed. Specifications shown in
boldface
are tested on all production units at final electrical test and are used to
calculate outgoing quality levels.
2
Accuracy specified for 0 mV to 200 mV rms, dc or 1 kHz sine wave input. Accuracy is degraded at higher rms signal levels.
3
Measured at Pin 8 of PDIP (I
OUT
), with Pin 9 tied to common.
4
Error vs. crest factor is specified as additional error for a 200 mV rms rectangular pulse train, pulse width = 200 µs.
5
Input voltages are expressed in V rms.
6
With 10 kΩ pull-down resistor from Pin 6 (BUF OUT) to −V
S
.
7
With BUF IN tied to COMMON.
Rev. E | Page 4 of 16
Data Sheet
ABSOLUTE MAXIMUM RATINGS
Table 2.
Parameter
Supply Voltage
Dual Supply
Single Supply
Internal Power Dissipation
1
Maximum Input Voltage
Storage Temperature Range
Operating Temperature Range
Lead Temperature Range (Soldering 60 sec)
ESD Rating
1
AD636
Stresses above those listed under Absolute Maximum Ratings
may cause permanent damage to the device. This is a stress
rating only; functional operation of the device at these or any
other conditions above those indicated in the operational
section of this specification is not implied. Exposure to absolute
maximum rating conditions for extended periods may affect
device reliability.
Ratings
±16.5 V
24 V
500 mW
±12 V
PEAK
−55°C to +150°C
0°C to 70°C
300°C
1000 V
ESD CAUTION
10-Lead TO: θ
JA
= 150°C/W.
14-Lead PDIP: θ
JA
= 95°C/W.
Rev. E | Page 5 of 16
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