ad637.pdf

High Precision,

Wide-Band RMS-to-DC Converter

AD637

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 “3-State” Operation

Quiescent Current Reduction from 2.2 mA to 350

FUNCTIONAL BLOCK DIAGRAMS

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 inte-

grated 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 per-

mits measurements of signals with crest factors of up to 10 with

less than 1% additional error. The circuit’s wide bandwidth per-

mits 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 mA during

periods when the rms function is not in use. This feature facili-

tates 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 out-

put goes to a high impedance state. This allows several AD637s

to be tied together to form a wide-band 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.

The AD637 is available in two accuracy grades (J, K) for com-

mercial (0°C to +70°C) temperature range applications; two

accuracy grades (A, B) for industrial (–40°C to +85°C) applica-

tions; and one (S) rated over the –55°C to + 125°C temperature

range. All versions are available in hermetically-sealed, 14-pin

side-brazed ceramic DIPs as well as low cost cerdip packages. A

16-pin SOIC package is also available.

PRODUCT HIGHLIGHTS

1. 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 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 perfor-

mance without external trimming. The only external compo-

nent required is a capacitor which sets the averaging time

period. The value of this capacitor also determines low fre-

quency accuracy, ripple level and settling time.

3. The chip select feature of the AD637 permits the user to

power down the device down during periods of nonuse,

thereby, decreasing battery drain in remote or hand-held

applications.

4. The on-chip buffer amplifier can be used as either 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.

REV. C

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

which 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: 617/329-4700

Fax: 617/326-8703

AD637–SPECIFICATIONS (@ + 25 8 C, and 6 15 V dc unless otherwise noted)

AD637J/A

AD637K/B

AD637S

Model

Min

Typ

Max

Min

Typ

Max

Min

Typ

Max

Units

TRANSFER FUNCTION

V OUT

avg .( V IN ) 2

V OUT

avg .( V IN ) 2

V OUT

avg .( V IN ) 2

CONVERSION ACCURACY

Total Error, Internal Trim 1 (Fig. 2)

6 1 6 0.5

6 0.5 6 0.2

6 1 6 0.5 mV ± % of Reading

T MIN to T MAX

6 3.0 6 0.6

6 2.0 6 0.3

6 6 6 0.7 mV ± % of Reading

vs. Supply, + V IN = +300 mV

150

mV/V

vs. Supply, – V IN = –300 mV

100

300

100

300

100

300

mV/V

DC Reversal Error at 2 V

0.25

0.1

0.25

% of Reading

Nonlinearity 2 V Full Scale 2

0.04

0.02

0.04

% of FSR

Nonlinearity 7 V Full Scale

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 3

Crest Factor 1 to 2

Specified Accuracy

Crest Factor = 3

± 0.1

% of Reading

Crest Factor = 10

± 1.0

% of Reading

AVERAGING TIME CONSTANT

ms/mF C AV

INPUT CHARACTERISTICS

Signal Range, ± 15 V Supply

Continuous RMS Level

0 to 7

V rms

Peak Transient Input

± 15

V p-p

Signal Range, ± 5 V Supply

Continuous rms Level

0 to 4

V rms

Peak Transient Input

± 6

V p-p

Maximum Continuous Nondestructive

Input Level (All Supply Voltages)

± 15

V p-p

Input Resistance

6.4

9.6

6.4

9.6

6.4

9.6

Input Offset Voltage

± 0.5

± 0.2

± 0.5

FREQUENCY RESPONSE 4

Bandwidth for 1% Additional Error (0.09 dB)

V IN = 20 mV

kHz

V IN = 200 mV

kHz

V IN = 2 V

200

kHz

± 3 dB Bandwidth

V IN = 20 mV

150

kHz

V IN = 200 mV

MHz

V IN = 2 V

MHz

OUTPUT CHARACTERISTICS

Offset Voltage

6 1

6 0.5

6 1

vs. Temperature

± 0.05

6 0.089

± 0.04

6 0.056

± 0.04

6 0.07

mV/°C

Voltage Swing, ± 15 V Supply,

2 kW Load

0 to + 12.0 +13.5

0 to +12.0 +13.5

Voltage Swing, ± 3 V Supply,

2 kW Load

0 to +2

+2.2

0 to +2

+2.2

0 to +2

+2.2

Output Current

Short Circuit Current

Resistance, Chip Select “High”

0.5

Resistance, Chip Select “Low”

100

dB OUTPUT

Error, V IN 7 mV to 7 V rms, 0 dB = 1 V rms

± 0.5

± 0.3

± 0.5

Scale Factor

–3

mV/dB

Scale Factor Temperature Coefficient

+0.33

% of Reading/°C

–0.033

dB/°C

I REF for 0 dB = 1 V rms

I REF Range

100

BUFFER AMPLIFIER

Input Output Voltage Range

–V S to (+V S

– 2.5 V)

Input Offset Voltage

± 0.8

6 2

± 0.5

6 1

± 0.8

6 2

Input Current

± 2

6 10

± 2

6 5

± 2

6 10

Input Resistance

10 8

Output Current

(+5 mA,

–130 mA)

Short Circuit Current

Small Signal Bandwidth

MHz

Slew Rate 5

V/ms

DENOMINATOR INPUT

Input Range

0 to +10

Input Resistance

Offset Voltage

± 0.2

± 0.5

± 0.2

± 0.5

± 0.2

± 0.5

CHIP SELECT PROVISION (CS)

RMS “ON” Level

Open or +2.4 V < V C < +V S

RMS “OFF” Level

V C < +0.2 V

I OUT of Chip Select

CS “LOW”

CS “HIGH”

Zero

On Time Constant

10 ms + ((25 kW) ´ C AV )

Off Time Constant

10 ms + ((25 kW) ´ C AV )

POWER SUPPLY

Operating Voltage Range

6 3.0

6 18

6 3.0

6 18

6 3.0

6 18

Quiescent Current

2.2

Standby Current

350

450

350

450

350

450

TRANSISTOR COUNT

107

–2–

REV. C

AD637

pull down resistor tied to –V S .

Specifications subject to change without notice.

Specifications shown in boldface 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.

ABSOLUTE MAXIMUM RATINGS

ESD Rating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 500 V

Supply Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

ORDERING GUIDE

18 V dc

Internal Quiescent Power Dissipation . . . . . . . . . . . . 108 mW

Output Short-Circuit Duration . . . . . . . . . . . . . . . . . Indefinite

Storage Temperature Range . . . . . . . . . . . . –65

Temperature

Package

Model 1, 2

Range

Description

Option

C to +150

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

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

Lead Temperature Range (Soldering 10 secs) . . . . . . . +300

Rated Operating Temperature Range

AD637J, K . . . . . . . . . . . . . . . . . . . . . . . . . . . 0

C to +70

AD637A, B . . . . . . . . . . . . . . . . . . . . . . . . –40

C to +85

AD637S, 5962-8963701CA . . . . . . . . . . . –55

C to +125

Q-14

NOTES

1 “S” grade chips are also available.

2 A Standard Microcircuit Drawing, 5962-8963701CA, is also available.

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.

WARNING!

ESD SENSITIVE DEVICE

REV. C

–3–

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 k

AD637

FUNCTIONAL DESCRIPTION

The AD637 embodies an implicit solution of the rms equation

that overcomes the inherent limitations of straightforward rms

computation. The actual computation performed by the AD637

follows the equation

Figure 1 is a simplified schematic of the AD637, it is subdivided

into four major sections; absolute value circuit (active rectifier),

square/divider, filter circuit and buffer amplifier. The input volt-

age V IN which can be ac or dc is converted to a unipolar current

I1 by the active rectifier A1, A2. I1 drives one input of the

squarer divider which has the transfer function

V rms

Avg

V IN 2

V rms

Figure 2. Standard RMS Connection

The performance of the AD637 is tolerant of minor variations in

the power supply voltages, however, if the supplies being used

exhibit a considerable amount of high frequency ripple it is

advisable to bypass both supplies to ground through a 0.1

I 1 2

I 3

The output current of the squarer/divider, I4 drives A4 which

forms a low-pass filter with the external averaging capacitor. If

the RC time constant of the filter is much greater than the long-

est period of the input signal than A4s output will be propor-

tional to the average of I4. The output of this filter amplifier is

used by A3 to provide the denominator current I3 which equals

Avg. I4 and is returned to the squarer/divider to complete the

implicit rms computation.

I 4

ceramic disc capacitor placed as close to the device as possible.

The output signal range of the AD637 is a function of the sup-

ply voltages, as shown in Figure 3. The output signal can be

used buffered or nonbuffered depending on the characteristics

of the load. If no buffer is needed, tie buffer input (Pin 1) to

common. The output of the AD637 is capable of driving 5 mA

into a 2 k

load without degrading the accuracy of the device.

I 4 =

Avg

I 1 2

I 4

I 1 rms

and

V OUT = V IN rms

If the averaging capacitor is omitted, the AD637 will compute the

absolute value of the input signal. A nominal 5 pF capacitor should

be used to insure stability. The circuit operates identically to that of

the rms configuration except that I3 is now equal to I4 giving

I 1 2

I 4

I 4 = I 1

The denominator current can also be supplied externally by pro-

viding a reference voltage, V REF , to Pin 6. The circuit operates

identically to the rms case except that I3 is now proportional to

V REF . Thus:

I 4

Figure 3. AD637 max V OUT vs. Supply Voltage

CHIP SELECT

The AD637 includes a chip select feature which allows the user

to decrease the quiescent current of the device from 2.2 mA to

350 mA. This is done by driving the CS, Pin 5, to below 0.2 V

dc. Under these conditions, the output will go into a high im-

pedance state. In addition to lowering power consumption, this

feature permits bussing the outputs of a number of AD637s to

form a wide bandwidth rms multiplexer. If the chip select is not

being used, Pin 5 should be tied high.

I 1 2

I 3

I 4

Avg

and

V IN 2

V DEN

This is the mean square of the input signal.

STANDARD CONNECTION

The AD637 is simple to connect for a majority of rms measure-

ments. In the standard rms connection shown in Figure 2, only

a single external capacitor is required to set the averaging time

constant. In this configuration, the AD637 will compute the

true rms of any input signal. An averaging error, the magnitude

of which will be dependent on the value of the averaging capaci-

tor, will be present at low frequencies. For example, if the filter

capacitor C AV , is 4 mF this error will be 0.1% at 10 Hz and in-

creases to 1% at 3 Hz. If it is desired to measure only ac signals,

the AD637 can be ac coupled through the addition of a non-

polar capacitor in series with the input as shown in Figure 2.

V O

OPTIONAL TRIMS FOR HIGH ACCURACY

The AD637 includes provisions to allow the user to trim out

both output offset and scale factor errors. These trims will result

in significant reduction in the maximum total error as shown in

Figure 4. This remaining error is due to a nontrimmable input

offset in the absolute value circuit and the irreducible non-

linearity of the device.

The trimming procedure on the AD637 is as follows:

l. Ground the input signal, V IN and adjust R1 to give 0 V out-

put from Pin 9. Alternatively R1 can be adjusted to give the

correct output with the lowest expected value of V IN .

–4–

REV. C

AD637

2. Connect the desired full scale input to V IN , using either a dc

or a calibrated ac signal, trim R3 to give the correct output at

Pin 9, i.e., 1 V dc should give l.000 V dc output. Of course, a

2 V peak-to-peak sine wave should give 0.707 V dc output.

Remaining errors are due to the nonlinearity.

This ripple can add a significant amount of uncertainty to the

accuracy of the measurement being made. The uncertainty can

be significantly reduced through the use of a post filtering net-

work or by increasing the value of the averaging capacitor.

The dc error appears as a frequency dependent offset at the

output of the AD637 and follows the equation:

0.16 + 6. 4 t

2 f 2 in % of reading

Since the averaging time constant, set by C AV , directly sets the

time that the rms converter “holds” the input signal during

computation, the magnitude of the dc error is determined only

by C AV and will not be affected by post filtering.

Figure 4. Max Total Error vs. Input Level AD637K

Internal and External Trims

Figure 5. Optional External Gain and Offset Trims

CHOOSING THE AVERAGING TIME CONSTANT

The AD637 will compute the true rms value of both dc and ac

input signals. At dc the output will track the absolute value of

the input exactly; with ac signals the AD637’s output will ap-

proach the true rms value of the input. The deviation from the

ideal rms value is due to an averaging error. The averaging error

is comprised of an ac and dc component. Both components are

functions of input signal frequency f , and the averaging time

constant t (t: 25 ms/mF of averaging capacitance). As shown in

Figure 6, the averaging error is defined as the peak value of the

ac component, ripple, plus the value of the dc error.

The peak value of the ac ripple component of the averaging er-

ror is defined approximately by the relationship:

Figure 7. Comparison of Percent DC Error to the Percent

Peak Ripple over Frequency Using the AD637 in the Stan-

dard RMS Connection with a 1 ´ m F C AV

The ac ripple component of averaging error can be greatly

reduced by increasing the value of the averaging capacitor.

There are two major disadvantages to this: first, the value of the

averaging capacitor will become extremely large and second, the

settling time of the AD637 increases in direct proportion to the

value of the averaging capacitor (Ts = 115 ms/mF of averaging

capacitance). A preferable method of reducing the ripple is

through the use of the post filter network, shown in Figure 8.

This network can be used in either a one or two pole configura-

tion. For most applications the single pole filter will give the

best overall compromise between ripple and settling time.

6.3 t f

in % of reading where (t > 1/f)

Figure 6. Typical Output Waveform for a Sinusoidal Input

Figure 8. Two Pole Sallen-Key Filter

REV. C

–5–

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