Analog Devices AD637 f Datasheet

High Precision, Wideband

a

FEATURES
High Accuracy

0.02% Max 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 A
Side Brazed DIP, Low Cost Cerdip and SOIC

PRODUCT DESCRIPTION

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 circuit’s wide bandwidth 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 Devices,
the AD637 has an auxiliary dB output available to the user. 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 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 hand-held
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 that are in excess of the supply levels. The inputs will
not be damaged by input signals if the supply voltages are lost.

RMS-to-DC Converter

AD637

FUNCTIONAL BLOCK DIAGRAMS

Ceramic DIP (D) and

Cerdip (Q) Packages

BUFFER

1

2

3

BIAS

SECTION

4

5

25k

6

7

SQUARER/DIVIDER

AD637

ABSOLUTE

VALUE

25k

FILTER

14

13

12

11

10

9

8

The AD637 is available in two accuracy grades (J and K) for
commercial (0°C to 70°C) temperature range applications;
two accuracy grades (A and B) for industrial (–40°C to +85°C)
applications; and one (S) rated over the –55°C to +125°C tem­perature range. All versions are available in hermetically sealed,
14-lead side brazed ceramic DIPs as well as low cost cerdip
packages. A 16-lead SOIC package is also available.

PRODUCT HIGHLIGHTS

1. The AD637 computes the true root-mean-square, mean­square, 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 since 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.

2. 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
time period. The value of this capacitor also determines
low-frequency accuracy, ripple level, and settling time.

3. The chip select feature of the AD637 permits the user to
power down the device during periods of nonuse, thereby
decreasing battery drain in remote or hand-held applications.

4. The on-chip buffer amplifier can be 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
the accuracy of the measurement.

SOIC (R) Package

BUFFER

1

2

3

BIAS

SECTION

4

5

25k

6

7

SQUARER/DIVIDER

AD637

ABSOLUTE

VALUE

25k

FILTER

16

15

14

13

12

11

10

98

REV. F

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. 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: [phone] www.analog.com
Fax: [phone] © Analog Devices, Inc., 2002

AD637–SPECIFICATIONS

(@ 25C, and 15 V dc unless otherwise noted.)

Model Min Typ Max Min Typ Max Min Typ Max Unit

TRANSFER FUNCTION

CONVERSION ACCURACY

Total Error, Internal Trim

to T

T

MIN

MAX

vs. Supply, + V
vs. Supply, – V

DC Reversal Error at 2 V 0.25 0.1 0.25 % of Reading
Nonlinearity 2 V Full Scale
Nonlinearity 7 V Full Scale 0.05 0.05 0.05 % of FSR
Total Error, External Trim ±0.5 ± 0.1 ±0.25 ± 0.05 ±0.5 ± 0.1 mV ± % of Reading

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 Reading
Crest Factor = 10 ± 1.0 ±1.0 ± 1.0 % of Reading

1

(Fig. 2)

= +300 mV 30 150 30 150 30 150 µV/V

IN

= –300 mV 100 300 100 300 100 300 µV/V

IN

2

3

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) ±15 ±15 ±15 V p-p
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 RESPONSE

Bandwidth for 1% Additional Error (0.09 dB)

= 20 mV 11 11 11 kHz

V

IN

= 200 mV 66 66 66 kHz

V

IN

= 2 V 200 200 200 kHz

V

IN

±3 dB Bandwidth

= 20 mV 150 150 150 kHz

V

IN

= 200 mV 1 1 1 MHz

V

IN

VIN = 2 V 8 8 8 MHz

OUTPUT CHARACTERISTICS

Offset Voltage

vs. Temperature ±0.05
Voltage Swing, ±15 V Supply,

2 kΩ Load 0 to 12.0 13.5 0 to 12.0 13.5 0 to 12.0 13.5 V
Voltage Swing, ±3 V Supply,

2 kΩ Load 0 to 2 2.2 0 to 2 2.2 0 to 2 2.2 V
Output Current 66 6mA
Short Circuit Current 20 20 20 mA
Resistance, Chip Select “High” 0.5 0.5 0.5 Ω
Resistance, Chip Select “Low” 100 100 100 kΩ

dB OUTPUT

BUFFER AMPLIFIER

DENOMINATOR INPUT

7 mV to 7 V rms, ±0.5 ± 0.3 ±0.5 dB

Error, V

IN

0 dB = 1 V rms
Scale Factor –3 –3 –3 mV/dB
Scale Factor Temperature Coefficient +0.33 +0.33 +0.33 % of Reading/°C

for 0 dB = 1 V rms 5 20 80 52080 52080 µA

I

REF

I

Range 1 100 1 100 1 100 µA

REF

Input Output Voltage Range –V
Input Offset Voltage ±0.8
Input Current ± 2
Input Resistance 10
Output Current (+5 mA, –130 µA) (+5 mA, –130 µA) (+5 mA, –130 µA)
Short Circuit Current 20 20 20 mA
Small Signal Bandwidth 1 1 1 MHz

5

Slew Rate

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

4

AD637J/A AD637K/B AD637S

V avg V

=×

OUT IN

2

()

1 

0.5



3.0  0.6

V avg V

=×

OUT IN

2

()



0.5  0.2



2.0  0.3

V avg V

=×

OUT IN

2

()

1 

0.5 mV ± % of Reading

6 

0.7 mV ± % of Reading

0.04 0.02 0.04 % of FSR



1



0.089 ±0.04



0.5



0.056 ±0.040.07 mV/°C



1 mV

–0.033 –0.033 –0.033 dB/°C

to (+VS – 2.5 V) –VS to (+VS – 2.5 V) –VS to (+VS – 2.5 V) V

S



2 ± 0.5



8

10 ± 2

10



1 ± 0.82 mV



8

5 ± 2



10 nA

8

10

55 5V/µs

AV

Ω

–2–

REV. F

AD637

Model Min Typ Max Min Typ Max Min Typ Max Unit

AD637J/A AD637K/B AD637S

CHIP SELECT PROVISION (CS)
RMS “ON” Level Open or 2.4 V < V
RMS “OFF” Level VC < 0.2 V VC < 0.2 V VC < 0.2 V

of Chip Select

I

OUT

CS “Low” 10 10 10 µA

< +V

C

S

Open or 2.4 V < VC < +V

S

Open or 2.4 V < VC < +V

S

CS “High” Zero Zero Zero
On Time Constant 10 µs + ((25 kΩ) ⫻ C
Off Time Constant 10 µs + ((25 kΩ) ⫻ CAV) 10 µs + ((25 kΩ) ⫻ CAV) 10 µs + ((25 kΩ) ⫻ CAV)

) 10 µs + ((25 kΩ) ⫻ CAV) 10 µs + ((25 kΩ) ⫻ CAV)

AV

POWER SUPPLY
Operating Voltage Range ±3.0

18



3.0



18



3.0



18 V



Quiescent Current 2.2 3 2.2 3 2.2 3 mA
Standby Current 350 450 350 450 350 450 µA

TRANSISTOR COUNT 107 107 107

NOTES

1

Accuracy specified 0–7 V rms dc with AD637 connected as shown in Figure 2.

2

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

3

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

4

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

5

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

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 min and max specifications

are guaranteed, although only those shown in boldface are tested on all production units.

Specifications subject to change without notice.

REV. F

–3–

AD637

WARNING!

ESD SENSITIVE DEVICE

ABSOLUTE MAXIMUM RATINGS

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 Range (Soldering 10 secs) . . . . . . . . 300°C

Rated Operating Temperature Range

AD637J, K . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0°C to 70°C

AD637A, B . . . . . . . . . . . . . . . . . . . . . . . . –40°C to +85°C

AD637S, 5962-8963701CA . . . . . . . . . . . –55°C to +125°C

BUFF OUT

BUFF IN

A5

BUFFER

AMPLIFIER

ORDERING GUIDE

Model Range Description Option

AD637AR –40°C to +85°C SOIC R-16
AD637BR –40°C to +85°C SOIC R-16
AD637AQ – 40°C to +85°C Cerdip Q-14
AD637BQ – 40°C to +85°C Cerdip Q-14
AD637JD 0°C to 70°C Side Brazed Ceramic DIP D-14
AD637JD/+ 0°C to 70°C Side Brazed Ceramic DIP D-14
AD637KD 0°C to 70°C Side Brazed Ceramic DIP D-14
AD637KD/+ 0°C to 70°C Side Brazed Ceramic DIP D-14
AD637JQ 0°C to 70°C Cerdip Q-14
AD637KQ 0°C to 70°C Cerdip Q-14
AD637JR 0°C to 70°C SOIC R-16
AD637JR-REEL 0°C to 70°C SOIC R-16
AD637JR-REEL7 0°C to 70°C SOIC R-16
AD637KR 0°C to 70°C SOIC R-16
AD637SD –55°C to +125°C Side Brazed Ceramic DIP D-14
AD637SD/883B –55°C to +125°C Side Brazed Ceramic DIP D-14
AD637SQ/883B –55°C to +125°C Cerdip Q-14
AD637SCHIPS –55°C to +125°CDie
5962-8963701CA* –55°C to +125°C Cerdip Q-14

*A standard microcircuit drawing is available.

ONE QUADRANT

SQUARER/DIVIDER

Temperature Package Package

I

4

FILTER/AMPLIFIER

24k

A4

CAV

+V

RMS

OUT

S

I

6k

1

Q3

Q4

Q5

A3

125

I

3

AD637

Q1

Q2

A2

24k

ABSOLUTE VALUE VOLTAGE –

CURRENT CONVERTER

6k

V

12k

IN

A1

Figure 1. Simplified Schematic

CAUTION

ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily
accumulate on the human body and test equipment and can discharge without detection.
Although the AD637 features proprietary ESD protection circuitry, permanent damage may
occur on devices subjected to high energy electrostatic discharges. Therefore, proper ESD
precautions are recommended to avoid performance degradation or loss of functionality.

BIAS

24k

dB
OUT

COM

CS

DEN
INPUT

OUTPUT
OFFSET

–V

S

–4–

REV. F

PIN CONFIGURATIONS

AD637

14-Lead DIP

BUFF IN

NC

COMMON

OUTPUT OFFSET

CS

DEN INPUT

dB OUTPUT

1

2

3

AD637

4

TOP VIEW

(Not to Scale)

5

6

7

NC = NO CONNECT

14

BUFF OUT

13

V

12

NC

11

+V

10

–V

9

RMS OUT

8

C

IN

S

S

AV

PIN FUNCTION DESCRIPTIONS

14-Lead DIP

Pin No. Mnemonic Description

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
8C

AV

Averaging Capacitor

Connection
9 RMS OUT rms Output
10 –V

S

Negative Supply

Rail
11 +V
13 V

S

IN

Positive Supply Rail

Signal Input
14 BUFF OUT Buffer Output

16-Lead SOIC

BUFF IN

COMMON

OUTPUT OFFSET

DEN INPUT

dB OUTPUT

1

2

NC

3

AD637

4

TOP VIEW

5

CS

(Not to Scale)

6

7

8

NC

NC = NO CONNECT

16

BUFF OUT

15

V

14

NC

13

+V

12

–V

11

RMS OUT

10

C

9

NC

IN

S

S

AV

16-Lead SOIC

Pin No. Mnemonic Description

1 BUFF IN Buffer Input
2, 8, 9, 14 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
10 C

AV

Averaging Capacitor

Connection
11 RMS OUT rms Output
12 –V

S

Negative Supply

Rail
13 +V
15 V

S

IN

Positive Supply Rail

Signal Input
16 BUFF OUT Buffer Output

REV. F

–5–