Analog Devices AD636KH, AD636KD, AD636JH, AD636JD, AD636JCHIP Datasheet

REV. B

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a

Low Level,

True RMS-to-DC Converter

AD636

PRODUCT DESCRIPTION

The AD636 is a low power monolithic IC which performs true
rms-to-dc conversion on low level signals. It offers performance
which 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, typi-

cally 800 µA, allows it to be used in battery-powered portable

instruments. A wide range of power supplies can be used, from

±2.5 V to ±16.5 V or a single +5 V to +24 V supply. 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. This signal is
derived from an internal circuit point which represents the loga­rithm of the rms output. The 0 dB reference level is set by an
externally supplied current and can be selected by the user
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 a 0 dBm level to over 10 kHz at –50 dBm.

The AD636 is designed for ease of use. The device is factory­trimmed at the wafer level for input and output offset, positive
and negative waveform symmetry (dc reversal error), and full­scale accuracy at 200 mV rms. Thus no external trims are re­quired to achieve full-rated accuracy.

AD636 is available in two accuracy grades; the AD636J total

error of ±0.5 mV ±0.06% of reading, and the AD636K

FEATURES
True RMS-to-DC Conversion
200 mV Full Scale
Laser-Trimmed to High Accuracy

0.5% Max Error (AD636K)

1.0% Max 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 ␮A Quiescent Current
Single or Dual Supply Operation
Monolithic Integrated Circuit
Low Cost
Available in Chip Form

PIN CONNECTIONS &

FUNCTIONAL BLOCK DIAGRAM

is accurate within ±0.2 mV to ±0.3% of reading. Both versions
are specified for the 0°C to +70°C temperature range, and are

offered in either a hermetically sealed 14-pin DIP or a 10-lead
TO-100 metal can. Chips are also available.

PRODUCT HIGHLIGHTS

1. 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 since it is a
measure of the power in the signal. The rms value of an
ac-coupled signal is also its standard deviation.

2. The 200 millivolt full-scale range of the AD636 is compatible
with many popular display-oriented analog-to-digital con­verters. The low power supply current requirement permits
use in battery powered hand-held instruments.

3. The only external component required to perform measure­ments to the fully specified accuracy is the averaging capaci­tor. The value of this capacitor can be selected for the desired
trade-off of low frequency accuracy, ripple, and settling time.

4. The on-chip buffer amplifier can be used to buffer either the
input or the output. Used as an input buffer, it provides

accurate performance from standard 10 MΩ input attenua-

tors. As an output buffer, it can supply up to 5 milliamps of
output current.

5. The AD636 will operate over a wide range of power supply

voltages, including single +5 V to +24 V or split ±2.5 V to
±16.5 V sources. A standard 9 V battery will provide several

hundred hours of continuous operation.

One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: [phone] World Wide Web Site: http://www.analog.com
Fax: [phone] © Analog Devices, Inc., 1999

V

IN

NC

–V

S

C

AV

dB

BUF OUT

BUF IN

+V

S

NC
NC

NC
COMMON

R

L

I

OUT

AD636

14

13

12

11

10

9

8

1

2

3

4

5

6

7

ABSOLUTE

VALUE

SQUARER

DIVIDER

BUF

SQUARER

DIVIDER

ABSOLUTE

VALUE

AD636

BUF OUT

dB

C

AV

BUF IN

R

L

COMMON

+V

S

V

IN

–V

S

I

OUT

CURRENT

MIRROR

+–

+
–

CURRENT

MIRROR

BUF

10kV

10kV

10kV

10kV

NC = NO CONNECT

AD636–SPECIFICATIONS

(@ +25ⴗC, and +VS = +3 V, –VS = –5 V, unless otherwise noted)

REV. B–2–

M

odel AD636J AD636K

Min Typ Max Min Typ Max Units

TRANSFER FUNCTION

V

OUT

= avg. ( V

IN

)

2

V

OUT

= avg. ( V

IN

)

2

CONVERSION ACCURACY

Total Error, Internal Trim

1, 2

ⴞ0.5 ⴞ1.0 ⴞ0.2 ⴞ0.5 mV ±% of Reading
vs. Temperature, 0°C to +70°C ±0.1 ±0.01 ±0.1 ±0.005 mV ±% of Reading/°C
vs. Supply Voltage ±0.1 ±0.01 ±0.1 ±0.01 mV ±% of Reading/V
dc Reversal Error at 200 mV ±0.2 ±0.1 % of Reading

Total Error, External Trim

1

±0.3 ±0.3 ±0.1 ±0.2 mV ±% of Reading

ERROR VS. CREST FACTOR

3

Crest Factor 1 to 2 Specified Accuracy Specified Accuracy
Crest Factor = 3 –0.2 –0.2 % of Reading
Crest Factor = 6 –0.5 –0.5 % of Reading

AVERAGING TIME CONSTANT 25 25 ms/µF CAV

INPUT CHARACTERISTICS

Signal Range, All Supplies

Continuous rms Level 0 to 200 0 to 200 mV rms

Peak Transient Inputs

+3 V, –5 V Supply ±2.8 ±2.8 V pk

±2.5 V Supply ±2.0 ±2.0 V pk
±5 V Supply ±5.0 ±5.0 V pk

Maximum Continuous Nondestructive

Input Level (All Supply Voltages) ±12 ±12 V pk

Input Resistance 5.33 6.67 8 5.33 6.67 8 kΩ
Input Offset Voltage ±0.5 ±0.2 mV

FREQUENCY RESPONSE

2, 4

Bandwidth for 1% Additional Error (0.09 dB)

V

IN

= 10 mV 14 14 kHz

V

IN

= 100 mV 90 90 kHz

V

IN

= 200 mV 130 130 kHz

±3 dB Bandwidth

V

IN

= 10 mV 100 100 kHz

V

IN

= 100 mV 900 900 kHz

VIN = 200 mV 1.5 1.5 MHz

OUTPUT CHARACTERISTICS

2

Offset Voltage, VIN = COM ⴞ0.5 ⴞ0.2 mV

vs. Temperature ±10 ±10 µV/°C
vs. Supply ±0.1 ±0.1 mV/ V

Voltage Swing

+3 V, –5 V Supply 0.3 0 to +1.0 0.3 0 to +1.0 V

±5 V to ±16.5 V Supply 0.3 0 to +1.0 0.3 0 to +1.0 V

Output Impedance 8 10 12 8 10 12 kΩ

dB OUTPUT

Error, V

IN

= 7 mV to 300 mV rms ±0.3 ⴞ0.5 ±0.1 ⴞ0.2 dB

Scale Factor –3.0 –3.0 mV/dB

Scale Factor Temperature Coefficient +0.33 +0.33 % of Reading/°C

–0.033 –0.033 dB/°C

I

REF

for 0 dB = 0.1 V rms 248 2 4 8 µA

I

REF

Range 1 50 1 50 µA

I

OUT

TERMINAL

I

OUT

Scale Factor 100 100 µA/V rms

I

OUT

Scale Factor Tolerance –20 ±10 +20 –20 ±10 +20 %

Output Resistance 8 10 12 8 10 12 kΩ

Voltage Compliance –V

S

to (+V

S

–VS to (+V

S

–2 V) –2 V) V

BUFFER AMPLIFIER

Input and Output Voltage Range –V

S

to (+V

S

–VS to (+V

S

–2 V) –2 V) V

Input Offset Voltage, R

S

= 10k ±0.8 ⴞ2 ±0.5 ⴞ1 mV

Input Bias Current 100 300 100 300 nA
Input Resistance 10

8

10

8

Ω

Output Current (+5 mA, (+5 mA,

–130 µA) –130 µA)

Short Circuit Current 20 20 mA
Small Signal Bandwidth l l MHz
Slew Rate

5

55V/µs

POWER SUPPLY

Voltage, Rated Performance +3, –5 +3, –5 V

Dual Supply +2, –2.5 ±16.5 +2, –2.5 ±16.5 V

Single Supply +5 +24 +5 +24 V

Quiescent Current

6

0.80 1.00 0.80 1.00 mA

ORDERING GUIDE

Temperature Package Package

Model Range Descriptions Options

AD636JD 0°C to +70°C Side Brazed Ceramic DIP D-14
AD636KD 0°C to +70°C Side Brazed Ceramic DIP D-14
AD636JH 0°C to +70°C Header H-10A
AD636KH 0°C to +70°C Header H-10A
AD636J Chip 0°C to +70°C Chip
AD636JD/+ 0°C to +70°C Side Brazed Ceramic DIP D-14

AD636

M

odel AD636J AD636K

Min Typ Max Min Typ Max Units

TEMPERATURE RANGE

Rated Performance 0 +70 0 +70 °C
Storage –55 +150 –55 +150 °C

TRANSISTOR COUNT 62 62

NOTES

1

Accuracy specified for 0 mV to 200 mV rms, dc or 1 kHz sine wave input. Accuracy is degraded at higher rms signal levels.

2

Measured at Pin 8 of DIP (I

OUT

), with Pin 9 tied to common.

3

Error vs. crest factor is specified as additional error for a 200 mV rms rectangular pulse trim, pulse width = 200 µs.

4

Input voltages are expressed in volts rms.

5

With 10 kΩ pull down resistor from Pin 6 (BUF OUT) to –V

S

.

6

With BUF input tied to Common.

Specifications subject to change without notice.

All min and max 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.

ABSOLUTE MAXIMUM RATINGS

1

Supply Voltage

Dual Supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ±16.5 V

Single Supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . +24 V

Internal Power Dissipation

2

. . . . . . . . . . . . . . . . . . . .500 mW

Maximum Input Voltage . . . . . . . . . . . . . . . . . . . . ±12 V Peak

Storage Temperature Range N, R . . . . . . . . . –55°C to +150°C

Operating Temperature Range

AD636J/K . . . . . . . . . . . . . . . . . . . . . . . . . . . 0°C to +70°C

Lead Temperature Range (Soldering 60 sec) . . . . . . . . +300°C

ESD Rating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1000 V

NOTES

1

Stresses above those listed under Absolute Maximum Ratings may cause perma-

nent 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.

2

10-Lead Header: θJA = 150°C/Watt.
14-Lead Side Brazed Ceramic DIP: θJA = 95°C/Watt.

METALIZATION PHOTOGRAPH

Contact factory for latest dimensions.

Dimensions shown in inches and (mm).

COM

10

R

L

9

+V

S

14

1a*

1b*

V

IN

3

–V

S

4

C

AV

5

dB

7 BUF IN
6 BUF OUT

8 I

OUT

0.1315 (3.340)

0.0807

(2.050)

PAD NUMBERS CORRESPOND TO PIN NUMBERS
FOR THE TO-116 14-PIN CERAMIC DIP PACKAGE.

NOTE
*BOTH PADS SHOWN MUST BE CONNECTED TO V

IN

.

STANDARD CONNECTION

The AD636 is simple to connect for the majority of high accu­racy rms measurements, requiring only an external capacitor to
set the averaging time constant. The standard connection is
shown in Figure 1. In this configuration, the AD636 will mea­sure the rms of the ac and dc level present at the input, but will
show an error for low frequency inputs as a function of the filter
capacitor, C

AV

, as shown in Figure 5. Thus, if a 4 µF capacitor

is used, the additional average error at 10 Hz will be 0.1%, at
3 Hz it will be 1%. The accuracy at higher frequencies will be
according to specification. If it is desired to reject the dc input, a
capacitor is added in series with the input, as shown in Fig­ure 3; the capacitor must be nonpolar. If the AD636 is driven
with power supplies with a considerable amount of high frequency
ripple, it is advisable to bypass both supplies to ground with

0.1 µF ceramic discs as near the device as possible. C

F

is an
optional output ripple filter, as discussed elsewhere in this data
sheet.

AD636

14

13

12

11

10

9

8

1

2

3

4

5

6

7

ABSOLUTE

VALUE

SQUARER

DIVIDER

BUF

+
–

CURRENT

MIRROR

10kV

10kV

V

IN

–V

S

+V

S

V

OUT

C

F

(OPTIONAL)

+–

C

AV

SQUARER

DIVIDER

ABSOLUTE

VALUE

AD636

CURRENT

MIRROR

+–

BUF

10kV

10kV

+V

S

V

IN

–V

S

+–

C

AV

V

OUT

C

F

(OPTIONAL)

Figure 1. Standard RMS Connection

REV. B –3–