ANALOG DEVICES AD637 Service Manual

High Precision, Wideband

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FEATURES

High accuracy

0.02% maximum nonlinearity, 0 V to 2 V rms input

0.10% 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
14-lead SBDIP, 14-lead low cost CERDIP, and 16-lead SOIC_W

GENERAL DESCRIPTION

BUFF IN

DEN INPUT

OUTPUT

OFFSET

COMMON

RMS-to-DC Converter

FUNCTIONAL BLOCK DIAGRAM

V

CS

ABSOLUTE

IN

VALUE

25k

SQUARER/

DIVIDE R

BIAS

Figure 1.

25k

AD637

AD637

BUFF
OUT

RMS OUT

C

AV

dB OUTPUT

00788-001

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
integrated 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 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.

As with previous monolithic rms converters from Analog

vices, Inc., the AD637 has an auxiliary dB output available to

De
users. 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

ease the supply current from 2.2 mA to 350 μA during periods

decr
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.

Rev. J

Information furnished by Analog Devices is believed to be accurate and reliable. However, no
responsibility is assumed by Anal og Devices for its use, nor for any infringements of patents or ot her
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.

The input circuitry of the AD637 is protected from overload

oltages in excess of the supply levels. The inputs are not

v
damaged by input signals if the supply voltages are lost.

The AD637 is available in accuracy Grade J and Grade K for

mmercial temperature range (0°C to 70°C) applications, accuracy

co
Grade A and Grade B for industrial range (−40°C to +85°C) appli­cations, 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.

The AD637 computes the true root mean square, mean square,
o

r 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 is also related to the
standard deviation of the signal.

The AD637 is laser wafer trimmed to achieve rated performance

ithout external trimming. The only external component

w
required is a capacitor that sets the averaging time period. The
value of this capacitor also determines low frequency accuracy,
ripple level, and settling time.

The on-chip buffer amplifier can be used either as an input

uffer or in an active filter configuration. The filter can be used

b
to reduce the amount of ac ripple, thereby increasing accuracy.

One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: [phone] www.analog.com
Fax: [phone] ©2007 Analog Devices, Inc. All rights reserved.

AD637

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TABLE OF CONTENTS

Features .............................................................................................. 1

Choosing the Averaging Time Constant....................................9

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

Functional Description ....................................................................7

Standard Connection ................................................................... 8

Chip Select..................................................................................... 8

Optional Trims for High Accuracy............................................ 8

REVISION HISTORY

4/07—Rev. I to Rev. J

Added Evaluation Board Section ................................................. 16

Updated Outline Dimensions....................................................... 20

10/06—Rev. H to Rev. I

hanges to Table 1............................................................................ 3

C

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

pdated Format..................................................................Universal

U

Changes to Figure 1.......................................................................... 1

Changes to Figure 11...................................................................... 10

Updated Outline Dimensions....................................................... 16

Changes to Ordering Guide.......................................................... 17

Frequency Response .................................................................. 11

AC Measurement Accuracy and Crest Factor........................ 12

Connection for dB Output........................................................ 12

dB Calibration............................................................................. 13

Low Frequency Measurements................................................. 14

Vector Summation ..................................................................... 14

Evaluation Board ............................................................................ 16

Outline Dimensions ....................................................................... 19

Ordering Guide .......................................................................... 20

4/05—Rev. F to Rev. G

pdated Format..................................................................Universal

U

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

dits to Ordering Guide...................................................................3

E

Rev. J | Page 2 of 20

AD637

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SPECIFICATIONS

At 25°C and ±15 V dc, unless otherwise noted.

Table 1.

AD637J/AD637A AD637K/AD637B AD637S
Parameter Min Typ Max Min Typ Max Min Typ Max Unit

TRANSFER FUNCTION

CONVERSION ACCURACY

Total Error, Internal Trim2

(Figure 5)
T

to T

MIN

vs. Supply

vs. Supply

DC Reversal

Nonlinearity 2 V Full Scale3
Nonlinearity 7 V Full Scale
Total Error, External Trim ±0.5 ± 0.1 ±0.25 ± 0.05 ±0.5 ± 0.1 mV ± % of

ERROR VS. CREST FACTOR

Crest Factor 1 to 2 Specified accuracy Specified accuracy Specified accuracy
Crest Factor = 3 ±0.1 ±0.1 ±0.1 % of

Crest Factor = 10 ±1.0 ±1.0 ±1.0 % of

AVERAGING TIME CONSTANT 25 25 25 ms/μF C
INPUT CHARACTERISTICS

Signal Range, ±15 V Supply

Continuous RMS Level 0 to 7 0 to 7 0 to 7 V rms
Peak Transient Input ±15 ±15 ±15 V p-p

Signal Range, ±5 V Supply

Continuous RMS Level 0 to 4 0 to 4 0 to 4 V rms
Peak Transient Input ±6 ±6 ±6 V p-p

Maximum Continuous

Nondestructive
Input Level

(All Supply Voltages)
Input Resistance 6.4 8 9.6 6.4 8 9.6 6.4 8 9.6 kΩ
Input Offset Voltage ±0.5 ±0.2 ±0.5 mV

FREQUENCY RESPONSE5

Bandwidth for 1%

Additional Error

(0.09 dB)

VIN = 20 mV 11 11 11 kHz

VIN = 200 mV 66 66 66 kHz

VIN = 2 V 200 200 200 kHz
±3 dB Bandwidth

VIN = 20 mV 150 150 150 kHz

VIN = 200 mV 1 1 1 MHz

VIN = 2 V 8 8 8 MHz

MAX

= 300 mV

+V

IN

= −300 mV

−V

IN

Error at 2 V

4

V

=

OUT

30

100

±15 ±15 ±15 V p-p

1

2

)(V avg × V

IN

±1 ± 0.5 ±0.5 ± 0.2 ±1 ± 0.5

±3.0 ± 0.6 ±2.0 ± 0.3 ±6 ± 0.7

150

300

0.25

0.04

0.05

30

100

OUT

=

2

)(V avg × V

IN

150

300

0.1

0.02

0.05

30

100

OUT

=

)(V avg ×

IN

2

150

300

0.25

0.04

0.05

mV ±% of
re

ading

mV ± % of
reading

μV/V

μV/V

% of
reading

% of FSR
% of FSR

reading

reading

reading

AV

Rev. J | Page 3 of 20

AD637

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AD637J/AD637A AD637K/AD637B AD637S
Parameter Min Typ Max Min Typ Max Min Typ Max Unit

OUTPUT CHARACTERISTICS

Offset Voltage

vs. Temperature ±0.05

Voltage Swing,

0 to 12.0

13.5

±1
±0.089

±15 V Supply, 2 kΩ Load

Voltage Swing,

0 to 2

2.2

±3 V Supply, 2 kΩ Load

Output Current

6

Short-Circuit Current 20 20 20 mA
Resistance

0.5 0.5 0.5 Ω

Chip Select High

Resistance

100 100 100 kΩ

Chip Select Low

dB OUTPUT

Error, VIN 7 mV to 7 V rms,

±0.5 ±0.3 ±0.5 dB

0 dB = 1 V rms
Scale Factor −3 −3 −3 mV/dB
Scale Factor Temperature

+0.33 +0.33 +0.33 % of

Coefficient

−0.033 −0.033 −0.033 dB/°C
I

for 0 dB = 1 V rms 5 20

REF

I

Range 1 100 1 100 1 100 μA

REF

80

BUFFER AMPLIFIER

Input Output

−VS to (+VS − 2.5 V) −VS to (+VS − 2.5 V) −VS to (+VS − 2.5 V) V

Voltage Range
Input Offset Voltage ±0.8
Input Current ±2
Input Resistance 10

8

±2
±10

10
Output Current −0.13 +5 −0.13 +5 −0.13 +5 mA
Short-Circuit Current 20 20 20 mA
Small Signal Bandwidth 1 1 1 MHz
Slew Rate

6

5 5 5 V/μs

DENOMINATOR INPUT

Input Range 0 to 10 0 to 10 0 to 10 V
Input Resistance 20 25 30 20 25 30 20 25 30 kΩ
Offset Voltage ±0.2 ±0.5 ±0.2 ±0.5 ±0.2 ±0.5 mV

CHIP SELECT (CS)

RMS On Level Open or 2.4 V < VC < +V
RMS Off Level VC < 0.2 V VC < 0.2 V VC < 0.2 V
I

of Chip Select

OUT

CS Low 10 10 10 μA

CS High 0 0 0 μA
On Time Constant 10 + ((25 kΩ) × CAV) 10 + ((25 kΩ) × CAV) 10 + ((25 kΩ) × CAV) μs
Off Time Constant 10 + ((25 kΩ) × CAV) 10 + ((25 kΩ) × CAV) 10 + ((25 kΩ) × CAV) μs

POWER SUPPLY

Operating Voltage Range

±3.0

Quiescent Current 2.2
Standby Current 350

1

Specifications shown in bold are tested on all production units at final electrical test. Results from those tests are used to calculate outgoing quality levels.

All minimum and maximum specifications are guaranteed, although only those shown in boldface are tested on all production units.

2

Accuracy specified 0 V rms to 7 V rms dc with AD637 connected, as shown in Figure 5.

3

Nonlinearity is defined as the maximum deviation from the straight line connecting the readings at 10 mV and 2 V.

4

Error vs. crest factor is specified as additional error for 1 V rms.

5

Input voltages are expressed in volts rms. Percent is in % of reading.

6

With external 2 kΩ pull-down resistor tied to −VS.

±18 ±3.0
3
450

±0.04

0 to 12.0

0 to 2

6

5 20

±0.5
±2

S

Open or 2.4 V < VC < +V

2.2
350

±0.5
±0.056

13.5

2.2

80

±1
±5

8

10

S

±18 ±3.0
3
450

±0.04

0 to 12.0

0 to 2

6

13.5 V

2.2 V

mA

5 20

±0.8
±2

8

±1
±0.07

80

±2
±10
Ω

Open or 2.4 V < VC < +V

2.2
350

±18
3
450

S

mV
mV/°C

reading/°C

μA

mV
nA

V
mA
μA

Rev. J | Page 4 of 20

AD637

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ABSOLUTE MAXIMUM RATINGS

Table 2.

Parameter Rating

ESD Rating 500 V

Supply Voltage ±18 V dc

Internal Quiescent Power Dissipation 108 mW

Output Short-Circuit Duration Indefinite

Storage Temperature Range −65°C to +150°C

Lead Temperature (Soldering 10 sec) 300°C

Rated Operating Temperature Range

AD637J, AD637K 0°C to 70°C
AD637A, AD637B −40°C to +85°C
AD637S, 5962-8963701CA −55°C to +125°C

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.

ESD CAUTION

Rev. J | Page 5 of 20

AD637

T

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PIN CONFIGURATIONS AND FUNCTION DESCRIPTIONS

1

BUFF IN

NC

2

COMMON

OUTPUT OFFSET

DEN INPUT

dB OUTPUT

3

AD637

TOP VIEW

4

(Not to Scale)

5

CS

6

7

NC = NO CONNECT

Figure 2. 14-Lead SBDIP/CERDIP Pin Configuration

14

BUFF OUT

V

13

NC

12

+V

11

10

–V

9

RMS OUT

C

8

BUFF IN

1

NC

IN

S

S

AV

00788-002

COMMON

OUTPUT OFFSET

DEN INPUT

dB OUTPUT

2

3

AD637

4

TOP VIEW

(Not to Scale)

5

CS

6

7

NC

8

NC = NO CONNECT

BUFF OU

16

V

15

NC

14

13

+V

12

–V

11

RMS OUT

C

10

NC

9

IN

S

S

AV

00788-003

Figure 3. 16-Lead SOIC_W Pin Configuration

Table 3. 14-Lead SBDIP/CERDIP Pin Function Descriptions Table 4. 16-Lead SOIC_W Pin Function Descriptions

Pin No. Mnemonic Description Pin No. Mnemonic Description

1 BUFF IN Buffer Input 1 BUFF IN Buffer Input
2, 12 NC No Connection
3 COMMON Analog Common
4 OUTPUT OFFSET Output Offset
5 CS Chip Select
6 DEN INPUT Denominator Input
7 dB OUTPUT dB Output
8 C

AV

Averaging Capacitor Connection
9 RMS OUT RMS Output
10 −V
11 +V
13 V

S

S

IN

Negative Supply Rail

Positive Supply Rail

Signal Input
14 BUFF OUT Buffer Output

2, 8, 9, 14 NC No Connection
3 COMMON
4 OUTPUT OFFSET
5 CS
6 DEN INPUT
7 dB OUTPUT
10 C

AV

11 RMS OUT
12 −V
13 +V
15 V

S

S

IN

Analog Common
Output Offset
Chip Select
Denominator Input
dB Output
Averaging Capacitor Connection
RMS Output
Negative Supply Rail
Positive Supply Rail
Signal Input

16 BUFF OUT Buffer Output

Rev. J | Page 6 of 20