Data Sheet
FEATURES
High accuracy
0.02% maximum nonlinearity, 0 V to 2 V rms input
0.1% 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
High Precision, Wideband
RMS-to-DC Converter
AD637
FUNCTIONAL BLOCK DIAGRAM
BUFF IN
+
BUFF OUT
–
25k
Ω
RMS OUT
V
IN
ABSOLUTE
VALUE
SQUARER/
DIVIDER
–
+
DEN
INPUT
25k
Ω
–
+
C
AV
dB OUTPUT
BIAS
00788-001
OUTPUT
OFFSET
COMMON
CS
Figure 1.
GENERAL DESCRIPTION
The
AD637
is a complete, high accuracy, 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. 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 relates to the standard deviation of
the signal.
Superior crest factor compensation permits measurements
of signals with crest factors of up to 10 with less than 1%
additional error. The wide bandwidth 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.
Direct dB value of the rms output with a range of 60 dB is
available on a separate pin. 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 laser wafer trimmed to achieve rated performance
without external trimming. The only external component
required is a capacitor that sets the averaging period. The value
of this capacitor also determines low frequency accuracy, ripple
level, and settling time.
The on-chip buffer amplifier is 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.
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.
Rev. L
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AD637
TABLE OF CONTENTS
Features .............................................................................................. 1
Functional Block Diagram .............................................................. 1
General Description ......................................................................... 1
Revision History ............................................................................... 3
Specifications..................................................................................... 4
Absolute Maximum Ratings .......................................................... 10
ESD Caution ................................................................................ 10
Pin Configurations and Function Descriptions ......................... 11
Theory of Operation ...................................................................... 12
Applications Information .............................................................. 13
Standard Connection ................................................................. 13
Chip Select ................................................................................... 13
Optional Trims for High Accuracy .......................................... 13
Data Sheet
Offset Trim .............................................................................. 14
Scale Factor Trim ................................................................... 14
Choosing the Averaging Time Constant ................................. 14
Frequency Response .................................................................. 16
AC Measurement Accuracy and Crest Factor ........................ 17
Connection for dB Output ........................................................ 17
dB Calibration ............................................................................. 18
Low Frequency Measurements ................................................. 19
Vector Summation ..................................................................... 19
Evaluation Board ............................................................................ 21
Outline Dimensions ....................................................................... 24
Ordering Guide .......................................................................... 25
Rev. L | Page 2 of 25
Data Sheet
REVISION HISTORY
4/15—Rev. K to Rev. L
Changes to Features Section, Figure 1, and General Description
Section ................................................................................................ 1
Changes to Table 1 ............................................................................ 4
Added Table 2; Renumbered Sequentially ..................................... 6
Added Table 3 .................................................................................... 8
Changed Pin NC to Pin NIC (Throughout) ................................11
Changes to Theory of Operation Section and Figure 4 .............12
Changes to Figure 5.........................................................................13
Changes to Optional Trims for High Accuracy Section and
Figure 8; Added Offset Trim Section and Scale Factor Trim
Section...............................................................................................14
Changes to Choosing the Averaging Time Constant and
Figure 11 ...........................................................................................15
Changes to Figure 20 ......................................................................19
Changes to Figure 21 ......................................................................20
Changes to Ordering Guide ...........................................................26
2/11—Rev. J to Rev. K
Changes to Figure 15 ......................................................................11
Changes to Figure 16 ......................................................................12
Changes to Evaluation Board Section and Figure 23 .................16
Added Figure 24; Renumbered Sequentially ...............................17
Changes to Figure 25 Through Figure 29 ....................................17
Changes to Figure 30 ......................................................................18
Added Figure 31 ..............................................................................18
Deleted Table 6; Renumbered Sequentially .................................18
Changes to Ordering Guide ...........................................................20
4/07—Rev. I to Rev. J
Added Evaluation Board Section ..................................................16
Updated Outline Dimensions ........................................................20
10/06—Rev. H to Rev. I
AD637
Changes to Table 1 ............................................................................ 3
Changes to Figure 4 .......................................................................... 7
Changes to Figure 7 .......................................................................... 9
Changes to Figure 16, Figure 18, and Figure 19 .......................... 12
Changes to Figure 20 ...................................................................... 13
12/05—Rev. G to Rev. H
Updated Format ................................................................. Universal
Changes to Figure 1 .......................................................................... 1
Changes to Figure 11 ...................................................................... 10
Updated Outline Dimensions........................................................ 16
Changes to Ordering Guide ........................................................... 17
4/05—Rev. F to Rev. G
Updated Format ................................................................. Universal
Changes to Figure 1 .......................................................................... 1
Changes to General Description ..................................................... 1
Deleted Product Highlights ............................................................. 1
Moved Figure 4 to Page .................................................................... 8
Changes to Figure 5 .......................................................................... 9
Changes to Figure 8 ........................................................................ 10
Changes to Figure 11, Figure 12, Figure 13, and Figure 14 ....... 11
Changes to Figure 19 ...................................................................... 14
Changes to Figure 20 ...................................................................... 14
Changes to Figure 21 ...................................................................... 16
Updated Outline Dimensions........................................................ 17
Changes to Ordering Guide ........................................................... 18
3/02—Rev. E to Rev. F
Edits to Ordering Guide ................................................................... 3
Rev. L | Page 3 of 25
AD637
SPECIFICATIONS
At 25°C, test frequency = 1 kHz, V
IN
units are V
RMS
, VS = ±15 V dc, unless otherwise noted.
Table 1.
Parameter
TRANSFER FUNCTION
CONVERSION ACCURACY
Total Error, Internal Trim
T
MIN
to T
MAX
vs. Supply
DC Reversal
Nonlinearity
Total Error
ERROR VS. CREST FACTOR (CF)
Test Conditions/Comments
Min
AD637J/AD637A
Typ
Max
V
OUT
=
avg
(V
IN
)
2
See Figure 5
V
IN
= 300 mV
V
IN
= −300 mV
−2 V < V
IN
< +2 V
2 V full scale
7 V full scale
External trim
Additional, at 1 V rms
For CF = 1 to 2
For CF = 3
For CF = 10
30
100
±1 ± 0.5
±3.0 ± 0.6
150
300
0.25
0.04
0.05
Unit
Data Sheet
±0.5 ± 0.1
Specified accuracy
±0.1
±1.0
25
mV rms ±% of reading
mV ± % of reading
μV/V
μV/V
% of reading
% of FSR
% of FSR
mV ± % of reading
CF, %
% of reading
% of reading
ms/μF C
AV
AVERAGING TIME CONSTANT
INPUT CHARACTERISTICS
Signal Range
VS = ±15 V
Continuous
Transient
VS = ±5 V
Continuous
Transient
6.4
0 to 7
±15
0 to 4
8
±6
9.6
±0.5
V rms
V p-p
V rms
V p-p
kΩ
mV
kHz
kHz
kHz
kHz
MHz
MHz
Input Resistance
Input Offset Voltage
FREQUENCY RESPONSE
Bandwidth for 1% (0.09 dB) Additional Error
V
IN
= 20 mV
V
IN
= 200 mV
V
IN
= 2 V
V
IN
= 20 mV
V
IN
= 200 mV
V
IN
= 2 V
11
66
200
150
1
8
±1
±0.089
±3 dB
OUTPUT CHARACTERISTICS
Offset Voltage
vs. temperature
Voltage Swing, 2 kΩ load
VS = ±3 V
Output Current
Short-Circuit Current
Resistance
Resistance
0 to 12.0
0 to 2
6
±0.05
13.5
2.2
20
0.5
100
CS high
CS low
mV
mV/°C
V
V
mA
mA
Ω
kΩ
Rev. L | Page 4 of 25
Data Sheet
Parameter
dB OUTPUT
Error
Scale Factor
Scale Factor Tempco
I
REF
for 0 dB = 1 V rms
I
REF
Range
BUFFER AMPLIFIER
Input Output Voltage Range
Input Offset Voltage
Input Current
Input Resistance
Output Current
Short-Circuit Current
Small Signal Bandwidth
Slew Rate
DENOMINATOR INPUT
Input Range
Input Resistance
Offset Voltage
CHIP SELECT (CS)
RMS On Level
RMS Off Level
I
OUT
of Chip Select
Test Conditions/Comments
V
IN
= 7 mV to 7 V rms, 0 dBV
Min
AD637J/AD637A
Typ
Max
±0.5
−3
+0.33
−0.033
20
Unit
AD637
5
1
80
100
dBV
mV/dB
% of reading/°C
dB/°C
µA
µA
V
mV
nA
Ω
mA
mA
MHz
V/µs
V
kΩ
mV
V
V
µA
µA
µs
µs
V
mA
µA
2 kΩ load, to −V
S
−V
S
to (+V
S
− 2.5)
±0.8
±2
±2
±10
8
10
−0.13
+5
20
1
5
0 to 10
25
±0.2
20
30
±0.5
Open or 2.4 < V
C
< +V
S
V
C
< 0.2
V
C
< 0.2
CS low
CS high
10
0
10 + ((25 kΩ) × C
AV
)
10 + ((25 kΩ) × C
AV
)
±3.0
2.2
350
±18
3
450
On Time Constant
Off Time Constant
POWER SUPPLY
Operating Voltage Range
Quiescent Current
Standby Current
Rev. L | Page 5 of 25
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