AD637S_技术文档

High Precision,

a

Wide-Band RMS-to-DC Converter

AD637

FUNCTIONAL BLOCK DIAGRAMS

FEATURES

High Accuracy

Ceramic DIP (D) and

SOIC (R) Package

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 ␮A

Side-Brazed DIP, Low Cost Cerdip and SOIC

Cerdip (Q) Packages

BUFFER

AD637

BUFFER

AD637

1

2

3

4

5

6

7

8

16

15

14

13

12

11

10

9

1

2

3

4

5

6

7

14

13

12

11

10

9

ABSOLUTE

VALUE

ABSOLUTE

VALUE

BIAS

SECTION

BIAS

SQUARER/DIVIDER

SECTION

SQUARER/DIVIDER

25k⍀

FILTER

8

PRODUCT DESCRIPTION

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-lead

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

16-lead SOIC package is also available.

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.

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.

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.

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.

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 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.

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.

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.

REV. E

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: 781/329-4700

Fax: 781/326-8703

World Wide Web Site: http://www.analog.com

(@ +25؇C, and ؎15 V dc unless otherwise noted)

AD637–SPECIFICATIONS

AD637J/A

AD637K/B

AD637S

Typ

Model

Min

Typ

Max

Min

Typ

Max

Min

Max

Units

2

TRANSFER FUNCTION

V_OUT

=

avg . (V_IN

)

V_OUT

=

avg . (V_IN

)

V_OUT

=

avg . (V_IN)

CONVERSION ACCURACY

Total Error, Internal Trim¹(Fig. 2)

؎1 ؎ 0.5

؎3.0 ؎ 0.6

150

؎0.5 ؎ 0.2

؎2.0 ؎ 0.3

150

؎1 ؎ 0.5 mV ± % of Reading

؎6 ؎ 0.7 mV ± % of Reading

T

_MINto T_MAX

vs. Supply, + V_IN= +300 mV

vs. Supply, – V_IN= –300 mV

DC Reversal Error at 2 V

30

100

30

100

30

100

150

µV/V

300

µV/V

0.25

0.04

0.05

0.1

0.02

0.25

0.04

0.05

% of Reading

% of FSR

mV ± % of Reading

Nonlinearity 2 V Full Scale²

Nonlinearity 7 V Full Scale

Total Error, External Trim

0.05

± 0.25 ± 0.05

±0.5 ± 0.1

± 0.5 ± 0.1

ERROR VS. CREST FACTOR³

Crest Factor 1 to 2

Crest Factor = 3

Specified Accuracy

±0.1

±1.0

± 0.1

± 1.0

± 0.1

± 1.0

% of Reading

Crest Factor = 10

AVERAGING TIME CONSTANT

25

ms/µF C_AV

INPUT CHARACTERISTICS

Signal Range, ±15 V Supply

Continuous RMS Level

Peak Transient Input

Signal Range, ±5 V Supply

Continuous rms Level

0 to 7

0 to 4

0 to 7

0 to 4

0 to 7

V rms

V p-p

±15

±6

± 15

± 6

± 15

0 to 4

V rms

V p-p

Peak Transient Input

± 6

Maximum Continuous Nondestructive

Input Level (All Supply Voltages)

Input Resistance

±15

9.6

±0.5

± 15

9.6

± 0.2

± 15

9.6

± 0.5

V p-p

kΩ

mV

6.4

8

6.4

8

6.4

8

Input Offset Voltage

FREQUENCY RESPONSE⁴

Bandwidth for 1% Additional Error (0.09 dB)

V

_IN= 20 mV

_IN= 200 mV

_IN= 2 V

11

66

200

11

66

200

11

66

200

kHz

±3 dB Bandwidth

V

_IN= 20 mV

_IN= 200 mV

150

1

8

150

1

8

150

1

8

kHz

MHz

V_IN= 2 V

OUTPUT CHARACTERISTICS

Offset Voltage

؎1

؎0.089

؎0.5

؎0.056

؎1

؎0.07

mV

mV/°C

vs. Temperature

±0.05

± 0.04

Voltage Swing, ±15 V Supply,

2 kΩ Load

0 to +12.0 +13.5

V

Voltage Swing, ±3 V Supply,

2 kΩ Load

Output Current

Short Circuit Current

Resistance, Chip Select “High”

Resistance, Chip Select “Low”

0 to +2

6

+2.2

0 to +2

6

+2.2

0 to +2

6

+2.2

V

mA

Ω

20

0.5

100

20

0.5

100

20

0.5

100

kΩ

dB OUTPUT

Error, V_IN7 mV to 7 V rms, 0 dB = 1 V rms

Scale Factor

±0.5

–3

± 0.3

–3

± 0.5

–3

dB

mV/dB

Scale Factor Temperature Coefficient

+0.33

–0.033

20

+0.33

–0.033

20

+0.33

–0.033

20

% of Reading/°C

dB/°C

µA

I

_REFfor 0 dB = 1 V rms

5

1

80

100

5

1

80

100

5

1

80

100

I_REFRange

µA

BUFFER AMPLIFIER

Input Output Voltage Range

–V_Sto (+V_S

– 2.5 V)

–V_Sto (+V_S

– 2.5 V)

–V_Sto (+V_S

– 2.5 V)

V

Input Offset Voltage

Input Current

Input Resistance

Output Current

±0.8

±2

؎2

؎10

± 0.5

± 2

؎1

؎5

± 0.8

± 2

؎2

؎10

mV

nA

Ω

10⁸

(+5 mA,

–130 µA)

Short Circuit Current

Small Signal Bandwidth

Slew Rate⁵

20

1

5

20

1

5

20

1

5

mA

MHz

V/µs

DENOMINATOR INPUT

Input Range

Input Resistance

Offset Voltage

0 to +10

25

±0.2

0 to +10

25

± 0.2

0 to +10

25

± 0.2

V

kΩ

mV

20

30

±0.5

20

30

± 0.5

20

30

± 0.5

CHIP SELECT PROVISION (CS)

RMS “ON” Level

RMS “OFF” Level

I_OUTof Chip Select

CS “LOW”

Open or +2.4 V < V_C< +V_S

V_C< +0.2 V

Open or +2.4 V < V_C< +V_S

V_C< +0.2 V

Open or +2.4 V < V_C< +V_S

V_C< +0.2 V

10

Zero

10

Zero

10

Zero

µA

CS “HIGH”

On Time Constant

Off Time Constant

10 µs + ((25 kΩ) × C_AV

)

10 µs + ((25 kΩ) × C_AV

)

10 µs + ((25 kΩ) × C_AV

)

POWER SUPPLY

Operating Voltage Range

Quiescent Current

Standby Current

؎3.0

2.2

؎18

3

450

؎3.0

2.2

؎18

3

450

؎3.0

؎18

3

450

V

mA

µA

2.2

350

TRANSISTOR COUNT

107

–2–

REV. E

AD637

NOTES

¹Accuracy specified 0-7 V rms dc with AD637 connected as shown in Figure 2.

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

³Error vs. crest factor is specified as additional error for 1 V rms.

⁴Input voltages are expressed in volts rms. % are in % of reading.

⁵With external 2 kΩ 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

ORDERING GUIDE

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

Temperature

Range

Package

Description

Package

Option

Model

AD637AR

AD637BR

AD637AQ

AD637BQ

AD637JD

AD637JD/+

AD637KD

AD637KD/+

AD637JQ

–40°C to +85°C SOIC

–40°C to +85°C Cerdip

0°C to +70°C

R-16

Q-14

Side Brazed Ceramic DIP D-14

Cerdip

SOIC

Q-14

R-16

AD637KQ

AD637JR

AD637JR-REEL

AD637JR-REEL7 0°C to +70°C

AD637KR

0°C to +70°C

AD637SD

–55°C to +125°C Side Brazed Ceramic DIP D-14

AD637SD/883B

AD637SQ/883B

AD637SCHIPS

–55°C to +125°C Cerdip

0°C to +70°C Die

Q-14

5962-8963701CA* –55°C to +125°C Cerdip

Q-14

*A standard microcircuit drawing is available.

FILTER/AMPLIFIER

CAV

+V

BUFF OUT

ONE QUADRANT

SQUARER/DIVIDER

24k⍀

S

BUFF IN

BUFFER

AMPLIFIER

A5

RMS

OUT

A4

I

4

dB

OUT

I

1

24k⍀

COM

Q4

Q1

ABSOLUTE VALUE VOLTAGE –

CURRENT CONVERTER

CS

Q5

BIAS

DEN

INPUT

I

24k⍀

Q2

Q3

A3

3

6k⍀

OUTPUT

OFFSET

A2

12k⍀

125⍀

AD637

V

IN

A1

–V

S

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. E

–3–

AD637

FUNCTIONAL DESCRIPTION

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

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

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

BUFFER

AD637

1

NC

14

13

V_IN₂

V rms = Avg

V rms

OPTIONAL

AC COUPLING

CAPACITOR

ABSOLUTE

VALUE

2

3

V

IN

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_INwhich 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

12 NC

11

BIAS

SECTION

SQUARER/DIVIDER

+V

4

5

6

7

S

25k⍀

–V

10

S

25k⍀

I₁²

I₃

V

3

V

=

9

8

IN

O

C

I₄

=

AV

FILTER

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.

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 µF

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

2

I₁

I₄

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 kΩ load without degrading the accuracy of the device.

I₄= Avg

= I₁rms

and

V_OUT= V_INrms

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

20

15

10

5

I₁²

I₄

=

I₄= I₁

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:

I₁²

I₄= Avg

I₃

0

and

0

؎3

؎5

؎10

؎15

؎18

2

SUPPLY VOLTAGE – DUAL SUPPLY – Volts

V_IN

V_O

=

V_DEN

Figure 3. AD637 Max V_OUTvs. Supply Voltage

CHIP SELECT

This is the mean square of the input signal.

STANDARD CONNECTION

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 µA. 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.

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 µF 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,

–4–

REV. E

AD637

OPTIONAL TRIMS FOR HIGH ACCURACY

functions of input signal frequency f, and the averaging time

constant τ (τ: 25 ms/µF 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 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 peak value of the ac ripple component of the averaging er-

ror is defined approximately by the relationship:

50

6.3 τf

The trimming procedure on the AD637 is as follows:

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

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

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

E

O

IDEAL

O

E

correct output with the lowest expected value of V_IN

.

DC ERROR = AVERAGE OF OUTPUT–IDEAL

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.

AVERAGE ERROR

DOUBLE-FREQUENCY

RIPPLE

TIME

5.0

Figure 6. Typical Output Waveform for a Sinusoidal Input

AD637K MAX

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.

2.5

INTERNAL TRIM

AD637K

EXTERNAL TRIM

0

The dc error appears as a frequency dependent offset at the

output of the AD637 and follows the equation:

1

in % of reading

0.16 + 6.4τ²f ²

2.5

AD637K: 0.5mV ؎0.2%

0.25mV ؎0.05%

EXTERNAL

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_AVand will not be affected by post filtering.

5.0

0

0.5

1.0

1.5

2.0

INPUT LEVEL – Volts

100

Figure 4. Max Total Error vs. Input Level AD637K

Internal and External Trims

BUFFER

AD637

10

1

14

13

12

11

10

9

R4

147⍀

ABSOLUTE

VALUE

2

3

PEAK RIPPLE

V

IN

+V

S

1.0

BIAS

SECTION

OUTPUT

OFFSET

ADJUST

SQUARER/DIVIDER

R1

50k⍀

+V

–V

4

5

S

DC ERROR

R2

1M⍀

25k⍀

S

–V

S

0.1

10

25k⍀

100

1k

10k

6

7

+

V rms

OUT

SINEWAVE INPUT FREQUENCY – Hz

C

AV

FILTER

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 × µF C_AV

8

R3

1k⍀

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/µF 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.

SCALE FACTOR ADJUST,

؎2%

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

REV. E

–5–

AD637

100

10

100

10

BUFFER

AD637

RMS

BUFFER

OUTPUT

BUFFER INPUT

1

14

SIGNAL

INPUT

NC

ABSOLUTE

VALUE

13

2

3

ANALOG COM

+

12 NC

11

C3

BIAS

SECTION

1.0

OUTPUT

OFFSET

SQUARER/DIVIDER

+V

S

4

5

25k⍀

VALUES FOR C AND

AV

CHIP

SELECT

–V

S

10

1% SETTLING TIME

0.1

FOR STATED % OF READING

AVERAGING ERROR*

ACCURACY ؎2% DUE TO

COMPONENT TOLERANCE

25k⍀

DENOMINATOR

INPUT

9

+

6

7

* %dc ERROR + %RIPPLE (Peak)

C

AV

FILTER

0.01

8

0.01

100k

dB

1

10

100

1k

10k

INPUT FREQUENCY – Hz

Figure 9a.

R

24k⍀

X

24k⍀

100

10

100

VALUES OF C , C2 AND

AV

+

C2

FOR 1 POLE

FILTER, SHORT

AND

1% SETTLING TIME FOR

STATED % OF READING

AVERAGING ERROR*

R

X

REMOVE C3

FOR 1 POLE POST FILTER

10

* %dc ERROR + % PEAK RIPPLE

ACCURACY ؎20% DUE TO

COMPONENT TOLERANCE

Figure 8. Two Pole Sallen-Key Filter

Figure 9a shows values of C_AVand the corresponding averaging

error as a function of sine-wave frequency for the standard rms

connection. The 1% settling time is shown on the right side of

the graph.

1.0

Figure 9b shows the relationship between averaging error, signal

frequency settling time and averaging capacitor value. This

graph is drawn for filter capacitor values of 3.3 times the averag-

ing capacitor value. This ratio sets the magnitude of the ac and

dc errors equal at 50 Hz. As an example, by using a 1 µF averag-

ing capacitor and a 3.3 µF filter capacitor, the ripple for a 60 Hz

input signal will be reduced from 5.3% of reading using the

averaging capacitor alone to 0.15% using the single pole filter.

This gives a factor of thirty reduction in ripple and yet the set-

tling time would only increase by a factor of three. The values of

C_AVand C2, the filter capacitor, can be calculated for the desired

value of averaging error and settling time by using Figure 9b.

0.1

0.01

100k

0.01

1

10

100

1k

10k

INPUT FREQUENCY – Hz

Figure 9b.

100

10

100

10

VALUES OF C , C2 AND C3

AV

AND 1% SETTLING TIME FOR

STATED % OF READING

AVERAGING ERROR*

2 POLL SALLEN-KEY FILTER

* %dc ERROR + % PEAK RIPPLE

ACCURACY ؎20% DUE TO

COMPONENT TOLERANCE

The symmetry of the input signal also has an effect on the mag-

nitude of the averaging error. Table I gives practical component

values for various types of 60 Hz input signals. These capacitor

values can be directly scaled for frequencies other than 60 Hz,

i.e., for 30 Hz double these values, for 120 Hz they are halved.

1.0

0.1

For applications that are extremely sensitive to ripple, the two pole

configuration is suggested. This configuration will minimize

capacitor values and settling time while maximizing performance.

0.01

100k

0.01

1

10

100

1k

10k

Figure 9c can be used to determine the required value of C_AV

C2 and C3 for the desired level of ripple and settling time.

,

INPUT FREQUENCY – Hz

Figure 9c.

–6–

REV. E

AD637

Table I. Practical Values of C_AVand C2 for Various Input

Waveforms

AC MEASUREMENT ACCURACY AND CREST FACTOR

Crest factor is often overlooked in determining the accuracy of

an ac measurement. Crest factor is defined as the ratio of the

peak signal amplitude to the rms value of the signal (C.F. = Vp/

V rms). Most common waveforms, such as sine and triangle

waves, have relatively low crest factors (≤2). Waveforms which

resemble low duty cycle pulse trains, such as those occurring in

switching power supplies and SCR circuits, have high crest

factors. For example, a rectangular pulse train with a 1% duty

Recommended C and C2

AV

Values for 1% Averaging

Error@60Hz with T = 16.6ms

Minimum

Absolute Value

Circuit Waveform

and Period

Input Waveform

and Period

R

؋

C

1%

Settling

Time

AV

Recommended Recommended

Time

Constant

Standard

Value C_AV

Standard

Value C2

1/2T

T

1/2T

0.47␮F

0.82␮F

1.5␮F

2.7␮F

181ms

325ms

A

0V

Symmetrical Sine Wave

T

η

cycle has a crest factor of 10 (C.F. = 1

).

T

B

C

D

100␮s

T

␩ = DUTY CYCLE =

0V

Sine Wave with dc Offset

Vp

e₀

␩

CF = 1/

0

T

e

IN

(rms) = 1 Volt rms

100␮F

10(T – T )

2

T

2

T

6.8␮F

5.6␮F

22␮F

18␮F

2.67sec

2.17sec

2

0V

10

Pulse Train Waveform

T

C

AV

= 22␮F

T

2

10(T – 2T )

2

T

2

0V

1.0

0.1

CF = 10

FREQUENCY RESPONSE

The frequency response of the AD637 at various signal levels is

shown in Figure 10. The dashed lines show the upper frequency

limits for 1%, 10% and ±3 dB of additional error. For example,

note that for 1% additional error with a 2 V rms input the high-

est frequency allowable is 200 kHz. A 200 mV signal can be

measured with 1% error at signal frequencies up to 100 kHz.

CF = 3

0.01

1

10

100

1000

PULSEWIDTH – ␮s

10

Figure 11. AD637 Error vs. Pulsewidth Rectangular Pulse

7V RMS INPUT

2V RMS INPUT

Figure 12 is a curve of additional reading error for the AD637

for a 1 volt rms input signal with crest factors from 1 to 11. A

rectangular pulse train (pulsewidth 100 µs) was used for this test

since it is the worst-case waveform for rms measurement (all

1V RMS INPUT

1

1%

10%

؎3dB

100mV RMS INPUT

0.1

+1.5

+1.0

+0.5

0

0.01

10mV RMS INPUT

1k

10k

100k

1M

10M

INPUT FREQUENCY – Hz

Figure 10. Frequency Response

To take full advantage of the wide bandwidth of the AD637 care

must be taken in the selection of the input buffer amplifier. To

insure that the input signal is accurately presented to the con-

verter, the input buffer must have a –3 dB bandwidth that is

wider than that of the AD637. A point that should not be over-

looked is the importance of slew rate in this application. For

example, the minimum slew rate required for a 1 V rms 5 MHz

sine-wave input signal is 44 V/µs. The user is cautioned that this

is the minimum rising or falling slew rate and that care must be

exercised in the selection of the buffer amplifier as some amplifi-

ers exhibit a two-to-one difference between rising and falling slew

rates. The AD845 is recommended as a precision input buffer.

+0.5

POSITIVE INPUT PULSE

C

= 22␮F

AV

–1.0

–1.5

1

2

3

4

5

6

7

8

9

10

11

CREST FACTOR

Figure 12. Additional Error vs. Crest Factor

REV. E

–7–

AD637

2.0

1.8

1.6

DB CALIBRATION

1. Set V_IN= 1.00 V dc or 1.00 V rms

2. Adjust R1 for 0 dB out = 0.00 V

3. Set V_IN= 0.1 V dc or 0.10 V rms

4. Adjust R2 for dB out = – 2.00 V

1.4

1.2

CF = 10

CF = 7

1.0

0.8

Any other dB reference can be used by setting V_INand R1

accordingly.

0.6

0.4

LOW FREQUENCY MEASUREMENTS

If the frequencies of the signals to be measured are below

10 Hz, the value of the averaging capacitor required to deliver

even 1% averaging error in the standard rms connection be-

comes extremely large. The circuit shown in Figure 15 shows an

alternative method of obtaining low frequency rms measure-

ments. The averaging time constant is determined by the prod-

uct of R and C_AV1, in this circuit 0.5 s/µF of C_AV. This circuit

permits a 20:1 reduction in the value of the averaging capacitor,

permitting the use of high quality tantalum capacitors. It is

suggested that the two pole Sallen-Key filter shown in the dia-

gram be used to obtain a low ripple level and minimize the value

of the averaging capacitor.

0.2

0.0

CF = 3

0.5

1.0

– V rms

1.5

2.0

V

IN

Figure 13. Error vs. RMS Input Level for Three Common

Crest Factors

the energy is contained in the peaks). The duty cycle and peak

amplitude were varied to produce crest factors from l to 10

while maintaining a constant 1 volt rms input amplitude.

CONNECTION FOR dB OUTPUT

Another feature of the AD637 is the logarithmic or decibel out-

put. The internal circuit which computes dB works well over a

60 dB range. The connection for dB measurement is shown in

Figure 14. The user selects the 0 dB level by setting R1 for the

proper 0 dB reference current (which is set to exactly cancel the

log output current from the squarer/divider circuit at the desired

0 dB point). The external op amp is used to provide a more

convenient scale and to allow compensation of the +0.33%/°C

temperature drift of the dB circuit. The special T.C. resistor R3

is available from Tel Labs in Londenderry, New Hampshire

(model Q-81) and from Precision Resistor Inc., Hillside, N.J.

(model PT146).

If the frequency of interest is below 1 Hz, or if the value of the

averaging capacitor is still too large, the 20:1 ratio can be

increased. This is accomplished by increasing the value of R. If

this is done it is suggested that a low input current, low offset

voltage amplifier like the AD548 be used instead of the internal

buffer amplifier. This is necessary to minimize the offset error

introduced by the combination of amplifier input currents and

the larger resistance.

R2

dB SCALE

FACTOR

ADJUST

33.2k⍀

SIGNAL

INPUT

5k⍀

+V

S

BUFFER

AD637

R3

60.4⍀

BUFFER

OUTPUT

BUFFER INPUT

1

14

13

12

11

10

9

*

1k⍀

AD707JN

SIGNAL

INPUT

ABSOLUTE

VALUE

NC

2

3

4

5

6

7

COMPENSATED

dB OUTPUT

+ 100mV/dB

ANALOG COM

NC

–V

S

BIAS

OUTPUT

OFFSET

SECTION

SQUARER/DIVIDER

+V

S

25k⍀

CHIP

–V

S

SELECT

RMS OUTPUT

25k⍀

DENOMINATOR

INPUT

+

1␮F

dB

FILTER

8

C

AV

10k⍀

+V

S

*1k⍀ + 3500ppm

R1

500k⍀

TC RESISTOR TEL LAB Q81

PRECISION RESISTOR PT146

OR EQUIVALENT

+2.5 VOLTS

AD508J

0dB ADJUST

Figure 14. dB Connection

–8–

REV. E

AD637

+V

S

1␮F

NOTE: VALUES CHOSEN TO GIVE 0.1%

AVERAGING ERROR @ 1Hz

3.3M⍀ 3.3M⍀

1␮F

AD548JN

BUFFER

AD637

FILTERED

V rms OUTPUT

1

2

3

4

5

6

7

14

13

12

11

10

9

–V

S

SIGNAL

INPUT

ABSOLUTE

VALUE

NC

6.8M⍀

+V

NC

S

BIAS

SECTION

1000pF

1M⍀

OUTPUT

OFFSET

ADJUST

SQUARER/DIVIDER

+V

S

50k⍀

25k⍀

–V

S

–V

S

2

25k⍀

V

IN

V rms

+

100␮F

FILTER

8

C

AV

1%

499k⍀

R

C

3.3␮F

AV1

Figure 15. AD637 as a Low Frequency RMS Converter

EXPANDABLE

VECTOR SUMMATION

Vector summation can be accomplished through the use of two

AD637s as shown in Figure 16. Here the averaging capacitors

are omitted (nominal 100 pF capacitors are used to insure

stability of the filter amplifier), and the outputs are summed as

shown. The output of the circuit is

BUFFER

AD637

1

2

3

14

13

12

11

10

9

ABSOLUTE

VALUE

V

IN

X

2

V_O= V_X²+V_Y

BIAS

SECTION

SQUARER/DIVIDER

This concept can be expanded to include additional terms by

feeding the signal from Pin 9 of each additional AD637 through

a 10 kΩ resistor to the summing junction of the AD711, and ty-

ing all of the denominator inputs (Pin 6) together.

+V

S

4

5

25k⍀

–V

S

25k⍀

6

7

If C_AVis added to IC1 in this configuration, the output is

100pF

5pF

FILTER

AD637

8

². If the averaging capacitor is included on both

V_X²+V_Y

10k⍀

BUFFER

2

V_X²+V_Y

IC1 and IC2, the output will be

.

1

2

3

14

13

12

11

10

9

AD711K

This circuit has a dynamic range of 10 V to 10 mV and is lim-

ited only by the 0.5 mV offset voltage of the AD637. The useful

bandwidth is 100 kHz.

ABSOLUTE

VALUE

V

IN

X

10k⍀

BIAS

SECTION

SQUARER/DIVIDER

+V

4

5

S

20k⍀

25k⍀

–V

S

25k⍀

6

7

100pF

FILTER

8

2

V

=

V

+ V

V

OUT

X

Figure 16. AD637 Vector Sum Configuration

REV. E

–9–

AD637

OUTLINE DIMENSIONS

Dimensions shown in inches and (mm).

TO-116 Package

(D-14)

Cerdip Package

(Q-14)

0.005 (0.13) MIN

0.098 (2.49) MAX

0.005 (0.13) MIN

14

0.098 (2.49) MAX

8

14

8

0.310 (7.87)

0.220 (5.59)

7

0.220 (5.59)

7

1

0.320 (8.13)

0.290 (7.37)

0.320 (8.13)

0.290 (7.37)

PIN 1

0.060 (1.52)

0.785 (19.94) MAX

0.060 (1.52)

0.015 (0.38)

0.785 (19.94) MAX

0.015 (0.38)

0.200 (5.08)

MAX

0.200 (5.08)

0.125 (3.18)

0.200 (5.08)

MAX

0.150

(3.81)

MAX

0.150

(3.81)

MIN

0.200 (5.08)

0.125 (3.18)

0.015 (0.38)

0.008 (0.20)

SEATING

PLANE

0.100

(2.54)

BSC

SEATING

PLANE

0.023 (0.58)

0.014 (0.36)

0.070 (1.78)

0.023 (0.58)

0.100

(2.54)

BSC

0.070 (1.78)

0.008 (0.20)

15°

0°

0.030 (0.76)

0.014 (0.36)

0.030 (0.76)

SOIC Package

(R-16)

0.4133 (10.50)

0.3977 (10.00)

16

9

0.2992 (7.60)

0.2914 (7.40)

0.4193 (10.65)

0.3937 (10.00)

1

8

PIN 1

0.1043 (2.65)

0.0926 (2.35)

0.050 (1.27)

BSC

0.0291 (0.74)

0.0098 (0.25)

؋

45؇

8؇

0؇

0.0192 (0.49)

0.0138 (0.35)

0.0118 (0.30)

0.0040 (0.10)

SEATING

PLANE

0.0500 (1.27)

0.0157 (0.40)

0.0125 (0.32)

0.0091 (0.23)

–10–

REV. E


型号：	AD637S
厂家：	ADI
描述：	High Precision, Wide-Band RMS-to-DC Converter 高精度，宽波段RMS至DC转换器转换器
文件：	总10页 (文件大小：163K)
中文：	中文翻译
下载：	下载PDF数据表文档文件

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