AD736 [ADI]

Low Cost, Low Power, True RMS-to-DC Converter; 低成本，低功耗，真RMS至DC转换器


型号：	AD736
厂家：	ADI
描述：	Low Cost, Low Power, True RMS-to-DC Converter 低成本，低功耗，真RMS至DC转换器转换器
文件：	总8页 (文件大小：220K)
中文：	中文翻译
下载：	下载PDF数据表文档文件

Low Cost, Low Power,

True RMS-to-DC Converter

AD736

FUNCTIO NAL BLO CK D IAGRAM

FEATURES

COMPUTES

True RMS Value

Average Rectified Value

Absolute Value

PROVIDES

200 m V Full-Scale Input Range

(Larger Inputs w ith Input Attenuator)

High Input Im pedance of 10¹²

⍀

Low Input Bias Current: 25 pA m ax

High Accuracy: ؎0.3 m V ؎0.3% of Reading

RMS Conversion w ith Signal Crest Factors Up to 5

Wide Pow er Supply Range: +2.8 V, –3.2 V to ؎16.5 V

Low Pow er: 200 ␮A m ax Supply Current

Buffered Voltage Output

No External Trim s Needed for Specified Accuracy

AD737—An Unbuffered Voltage Output Version w ith

Chip Pow er Dow n Is Also Available

which allows the measurement of 300 mV input levels, while

operating from the minimum power supply voltage of +2.8 V,

–3.2 V. The two inputs may be used either singly or differentially.

T he AD736 achieves a 1% of reading error bandwidth exceeding

10 kHz for input amplitudes from 20 mV rms to 200 mV rms

while consuming only 1 mW.

P RO D UCT D ESCRIP TIO N

T he AD736 is a low power, precision, monolithic true

rms-to-dc converter. It is laser trimmed to provide a maximum

error of ±0.3 mV ±0.3% of reading with sine-wave inputs. Fur-

thermore, it maintains high accuracy while measuring a wide

range of input waveforms, including variable duty cycle pulses

and triac (phase) controlled sine waves. T he low cost and small

physical size of this converter make it suitable for upgrading the

performance of non-rms “precision rectifiers” in many applica-

tions. Compared to these circuits, the AD736 offers higher ac-

curacy at equal or lower cost.

T he AD736 is available in four performance grades. T he

AD736J and AD736K grades are rated over the commercial tem-

perature range of 0°C to +70°C. T he AD736A and AD736B

grades are rated over the industrial temperature range of –40°C

to +85°C.

T he AD736 is available in three low-cost, 8-pin packages: plastic

mini-DIP, plastic SO and hermetic cerdip.

T he AD736 can compute the rms value of both ac and dc input

voltages. It can also be operated ac coupled by adding one ex-

ternal capacitor. In this mode, the AD736 can resolve input sig-

nal levels of 100 µV rms or less, despite variations in

temperature or supply voltage. High accuracy is also maintained

for input waveforms with crest factors of 1 to 3. In addition,

crest factors as high as 5 can be measured (while introducing

only 2.5% additional error) at the 200 mV full-scale input level.

P RO D UCT H IGH LIGH TS

1. T he AD736 is capable of computing the average rectified

value, absolute value or true rms value of various input

signals.

2. Only one external component, an averaging capacitor, is

required for the AD736 to perform true rms measurement.

3. T he low power consumption of 1 mW makes the AD736

suitable for many battery powered applications.

4. A high input impedance of 10¹²

T he AD736 has its own output buffer amplifier, thereby provid-

ing a great deal of design flexibility. Requiring only 200 µA of

power supply current, the AD736 is optimized for use in por-

table multimeters and other battery powered applications.

Ω eliminates the need for an

external buffer when interfacing with input attenuators.

5. A low impedance input is available for those applications

requiring up to 300 mV rms input signal operating from low

power supply voltages.

T he AD736 allows the choice of two signal input terminals: a

high impedance (10¹²Ω) FET input which will directly interface

with high Z input attenuators and a low impedance (8 kΩ) input

REV. C

Inform ation furnished by Analog Devices is believed to be accurate and

reliable. However, no responsibility is assum ed by Analog Devices for its

use, nor for any infringem ents of patents or other rights of third parties

which m ay result from its use. No license is granted by im plication or

otherwise under any patent or patent rights of Analog Devices.

One Technology Way, P.O. Box 9106, Norw ood, MA 02062-9106, U.S.A.

Tel: 617/ 329-4700

Fax: 617/ 326-8703

(@ +25؇C ؎5 V supplies, ac coupled with 1 kHz sine-wave input applied unless

otherwise noted.)

AD736–SPECIFICATIONS

AD 736J/A

Typ

AD 736K/B

Typ

Model

Conditions

Min

Max

Min

Max

Units

V_OUT

Avg.(V_IN

)

V_OUT

Avg.(V_IN

)

T RANSFER FUNCT ION

CONVERSION ACCURACY

T otal Error, Internal T rim¹

All Grades

1 kHz Sine Wave

ac Coupled Using C_C

0–200 mV rms

0.3/0.3 0.5/0.5

0.2/0.2 0.3/0.3

±mV/±% of Reading

200 mV–1 V rms

–1.2

؎2.0

–1.2

؎2.0

% of Reading

_MIN–T _MAX

A&B Grades

J&K Grades

@ 200 mV rms

0.7/0.7

0.5/0.5

±mV/±% of Reading

±% of Reading/°C

0.007

vs. Supply Voltage

@ 200 mV rms Input

V_S= ±5 V to ±16.5 V

V_S= ±5 V to ±3 V

+0.06

–0.18

1.3

+0.25

0.1/0.5

+0.1

–0.3

2.5

+0.06

–0.18

1.3

+0.25

0.1/0.3

+0.1

–0.3

2.5

%/V

dc Reversal Error, dc Coupled @ 600 mV dc

% of Reading

±mV/±% of Reading

Nonlinearity², 0 mV–200 mV @ 100 mV rms

+0.35

T otal Error, External T rim

0–200 mV rms

ERROR vs. CREST FACT OR³

Crest Factor 1 to 3

Crest Factor = 5

C_AV, C_F= 100 µF

0.7

2.5

0.7

2.5

% Additional Error

INPUT CHARACT ERIST ICS

High Impedance Input (Pin 2)

Signal Range

Continuous rms Level

Peak T ransient Input

Input Resistance

V_S= +2.8 V, –3.2 V

V_S= ±5 V to ±16.5 V

V_S= +2.8 V, –3.2 V

V_S= ±5 V

200

mV rms

V rms

؎0.9

؎4.0

؎0.9

؎4.0

±2.7

V_S= ±16.5 V

10¹²

Ω

Input Bias Current

V_S= ±3 V to ±16.5 V

Low Impedance Input (Pin 1)

Signal Range

Continuous rms Level

Peak T ransient Input

Input Resistance

V_S= +2.8 V, –3.2 V

V_S= ±5 V to ±16.5 V

V_S= +2.8 V, –3.2 V

V_S= ±5 V

300

mV rms

V rms

kΩ

±1.7

±3.8

±11

±1.7

±3.8

±11

V_S= ±16.5 V

6.4

9.6

6.4

9.6

Maximum Continuous

Nondestructive Input

Input Offset Voltage⁴

J&K Grades

A&B Grades

vs. T emperature

vs. Supply

All Supply Voltages

ac Coupled

±12

V p-p

؎3

µV/°C

µV/V

V_S= ±5 V to ±16.5 V

V_S= ±5 V to ±3 V

150

vs. Supply

OUT PUT CHARACT ERIST ICS

Output Offset Voltage

J&K Grades

A&B Grades

vs.T emperature

±0.1

؎0.5

±0.1

؎0.3

µV/°C

µV/V

vs. Supply

V_S= ±5 V to ±16.5 V

V_S= ±5 V to ±3 V

130

Output Voltage Swing

2 kΩ Load

No Load

Output Current

Short-Circuit Current

Output Resistance

V_S= +2.8 V, –3.2 V

V_S= ±5 V

V_S= ±16.5 V

0 to +1.6 +1.7

0 to +3.6 +3.8

0 to +1.6

0 to +3.6

0 to +4

+1.7

+3.8

Ω

0 to +4

+12

0.2

@ dc

FREQUENCY RESPONSE

High Impedance Input (Pin 2)

For 1% Additional Error

V_IN= 1 mV rms

Sine-Wave Input

kHz

V_IN= 10 mV rms

V_IN= 100 mV rms

V_IN= 200 mV rms

–2–

REV. C

AD736

AD 736J/A

Typ

AD 736K/B

Typ

Model

±3 dB Bandwidth

Conditions

Min

Max

Min

Max

Units

Sine-Wave Input

V_IN= 1 mV rms

V_IN= 10 mV rms

V_IN= 100 mV rms

V_IN= 200 mV rms

170

190

170

190

kHz

FREQUENCY RESPONSE

Low Impedance Input (Pin 1)

For 1% Additional Error

V_IN= 1 mV rms

Sine-Wave Input

kHz

V_IN= 10 mV rms

V_IN= 100 mV rms

V_IN= 200 mV rms

±3 dB Bandwidth

V_IN= l mV rms

V_IN= 10 mV rms

V_IN= 100 mV rms

V_IN= 200 mV rms

350

460

350

460

kHz

POWER SUPPLY

OperatingVoltageRange

Quiescent Current

+2.8, –3.2 ±5

160

±16.5

200

+2.8, –3.2 ±5

160

±16.5

200

Volts

µA

Zero Signal

200 mV rms, No Load

Sine-Wave Input

230

270

230

270

µA

T EMPERAT URE RANGE

Operating, Rated Performance

Commercial (0°C to +70°C)

Industrial (–40°C to +85°C)

AD736J

AD736A

AD736K

AD736B

NOT ES

^lAccuracy is specified with the AD736 connected as shown in Figure 16 with capacitor C _C

²Nonlinearity is defined as the maximum deviation (in percent error) from a straight line connecting the readings at 0 and 200 mV rms. Output offset voltage is adjusted to zero.

³Error vs. Crest Factor is specified as additional error for a 200 mV rms signal. C.F. = V_PEAK/V rms.

⁴DC offset does not limit ac resolution.

Specifications are 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.

ABSO LUTE MAXIMUM RATINGS¹

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

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

NOT ES

Supply Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ±16.5 V

Internal Power Dissipation². . . . . . . . . . . . . . . . . . . . . 200 mW

Input Voltage . . . . . . . . . . . . . . . . . . . . . . . ±V_S

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

Differential Input Voltage . . . . . . . . . . . . . . . . . . +V_Sand –V_S

Storage T emperature Range (Q) . . . . . . –65°C to +150°C

Storage T emperature Range (N, R) . . . . . –65°C to +125°C

Operating T emperature Range

¹Stresses above those listed under “Absolute Maximum Ratings” may cause

permanent damage to the device. T his is a stress rating only and 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 .

²8-Pin Plastic Package: θ_JA= 165°C/W

8-Pin Cerdip Package: θ_JA= 110°C/W

8-Pin Small Outline Package: θ_JA= 155°C/W

AD736J/K . . . . . . . . . . . . . . . . . . . . . . . . . . . 0°C to +70°C

AD736A/B . . . . . . . . . . . . . . . . . . . . . . . . . . –40°C to +85°C

O RD ERING GUID E

P IN CO NFIGURATIO N

8-P in Mini-D IP (N-8), 8-P in SO IC (R-8),

8-P in Cer dip (Q -8)

Tem perature

Range

P ackage

D escription

P ackage

O ption

Model

AD736JN

AD736KN

AD736JR

AD736KR

AD736AQ

AD736BQ

0°C to +70°C

–40°C to +85°C Cerdip

Plastic Mini-DIP N-8

Plastic SOIC

SO-8

Q-8

AD736JR-REEL

AD736JR-REEL-7

AD736KR-REEL

0°C to +70°C

Plastic SOIC

SO-8

Plastic SOIC

AD736KR-REEL-7 0°C to +70°C

REV. C

–3–

AD736–Typical Characteristics

Figure 2. Maxim um Input Level

vs. SupplyVoltage

Figure 3. Peak Buffer Output vs.

Supply Voltage

Figure 1. Additional Error vs.

Supply Voltage

Figure 4. Frequency Response

Driving Pin 1

Figure 5. Frequency Response

Driving Pin 2

Figure 6. Additional Error vs.

Crest Factor vs. C_AV

Figure 9. –3 dB Frequency vs.

RMS Input Level (Pin2)

Figure 7. Additional Error vs.

Tem perature

Figure 8. DC Supply Current vs.

RMS lnput Level

–4–

REV. C

Typical Characteristics–

AD736

Figure 10. Error vs. RMS Input

Voltage (Pin 2), Output Buffer Off-

set Is Adjusted To Zero

Figure 11. C_AVvs. Frequency for

Specified Averaging Error

Figure 12. RMS Input Level vs.

Frequency for Specified Averag-

ing Error

Figure 14. Settling Tim e vs. RMS

Input Level for Various

Values of C_AV

Figure 15. Pin 2 Input Bias Cur-

rent vs. Tem perature

Figure 13. Pin 2 Input Bias Current

vs. Supply Voltage

CALCULATING SETTLING TIME USING FIGURE 14

T he graph of Figure 14 may be used to closely approximate the

time required for the AD736 to settle when its input level is re-

duced in amplitude. T he net time required for the rms converter

to settle will be the difference between two times extracted from

the graph – the initial time minus the final settling time. As an

example, consider the following conditions: a 33 µF averaging

capacitor, an initial rms input level of 100 mV and a final (re-

duced) input level of 1 mV. From Figure 14, the initial settling

time (where the 100 mV line intersects the 33 µF line) is around

80 ms.

T he settling time corresponding to the new or final input level

of 1 mV is approximately 8 seconds. T herefore, the net time for

the circuit to settle to its new value will be 8 seconds minus

80 ms which is 7.92 seconds. Note that, because of the smooth

decay characteristic inherent with a capacitor/diode combina-

tion, this is the total settling time to the final value (i.e., not the

settling time to 1%, 0.1%, etc., of final value). Also, this graph

provides the worst case settling time, since the AD736 will settle

very quickly with increasing input levels.

REV. C

–5–

AD736

tions: input amplifier, full-wave rectifier, rms core, output am-

plifier and bias sections. T he FET input amplifier allows

both a high impedance, buffered input (Pin 2) or a low imped-

ance, wide-dynamic-range input (Pin 1). T he high impedance

input, with its low input bias current, is well suited for use with

high impedance input attenuators.

TYP ES O F AC MEASUREMENT

T he AD736 is capable of measuring ac signals by operating as

either an average responding or a true rms-to-dc converter. As

its name implies, an average responding converter computes the

average absolute value of an ac (or ac and dc) voltage or current

by full wave rectifying and low-pass filtering the input signal;

this will approximate the average. T he resulting output, a dc

“average” level, is then scaled by adding (or reducing) gain; this

scale factor converts the dc average reading to an rms equivalent

value for the waveform being measured. For example, the aver-

age absolute value of a sine-wave voltage is 0.636 that of V_PEAK

the corresponding rms value is 0.707 times V_PEAK. T herefore,

for sine-wave voltages, the required scale factor is 1.11 (0.707

divided by 0.636).

T he output of the input amplifier drives a full wave precision

rectifier, which in turn, drives the rms core. It is in the core that

the essential rms operations of squaring, averaging and square

rooting are performed, using an external averaging capacitor,

;

_AV. Without C_AV, the rectified input signal travels through the

core unprocessed, as is done with the average responding con-

nection (Figure 17).

A final subsection, an output amplifier, buffers the output from

the core and also allows optional low-pass filtering to be per-

formed via external capacitor, C_F, connected across the feed-

back path of the amplifier. In the average responding

connection, this is where all of the averaging is carried out. In

the rms circuit, this additional filtering stage helps reduce any

output ripple which was not removed by the averaging capaci-

In contrast to measuring the “average” value, true rms measure-

ment is a “universal language” among waveforms, allowing the

magnitudes of all types of voltage (or current) waveforms to be

compared to one another and to dc. RMS is a direct measure of

the power or heating value of an ac voltage compared to that of

dc: an ac signal of 1 volt rms will produce the same amount of

heat in a resistor as a 1 volt dc signal.

tor, C_AV

Mathematically, the rms value of a voltage is defined (using a

simplified equation) as:

V rms = Avg.(V ²)

T his involves squaring the signal, taking the average, and then

obtaining the square root. T rue rms converters are “smart recti-

fiers”: they provide an accurate rms reading regardless of the

type of waveform being measured. However, average responding

converters can exhibit very high errors when their input signals

deviate from their precalibrated waveform; the magnitude of the

error will depend upon the type of waveform being measured.

As an example, if an average responding converter is calibrated

to measure the rms value of sine-wave voltages, and then is used

to measure either symmetrical square waves or dc voltages, the

converter will have a computational error 11% (of reading)

higher than the true rms value (see T able I).

AD 736 TH EO RY O F O P ERATIO N

As shown by Figure 16, the AD736 has five functional subsec-

Figure 16. AD736 True RMS Circuit

Table I. Error Introduced by an Average Responding Circuit When Measuring Com m on Waveform s

Waveform Type

1 Volt P eak

Am plitude

Crest Factor

(V_{P EAK}/V rm s)

True rm s Value

Average Responding

Circuit Calibrated to

Read rm s Value of

% of Reading Error*

Using Average

Responding Circuit

Sine Waves Will Read

Undistorted

Sine Wave

1.414

0.707 V

Symmetrical

Square Wave

1.00

1.73

1.00 V

1.11 V

+11.0%

–3.8%

Undistorted

T riangle Wave

0.577 V

0.555 V

Gaussian

Noise (98% of

Peaks <1 V)

0.333 V

0.295 V

–11.4%

Rectangular

Pulse T rain

0.5 V

0.1 V

0.278 V

0.011 V

–44%

–89%

SCR Waveforms

50% Duty Cycle

25% Duty Cycle

4.7

0.495 V

0.212 V

0.354 V

0.150 V

–28%

–30%

Average RespondingValue – True rmsValue

*% of Reading Error =

×100%

True rmsValue

–6–

REV. C

AD736

As shown, the dc error is the difference between the average of

the output signal (when all the ripple in the output has been

removed by external filtering) and the ideal dc output. T he dc

error component is therefore set solely by the value of averaging

capacitor used-no amount of post filtering (i.e., using a very

large C_F) will allow the output voltage to equal its ideal value.

T he ac error component, an output ripple, may be easily re-

moved by using a large enough post filtering capacitor, C_F.

RMS MEASUREMENT – CH O O SING TH E O P TIMUM

VALUE FO R C_AV

Since the external averaging capacitor, C_AV, “holds” the recti-

fied input signal during rms computation, its value directly af-

fects the accuracy of the rms measurement, especially at low

frequencies. Furthermore, because the averaging capacitor ap-

pears across a diode in the rms core, the averaging time constant

will increase exponentially as the input signal is reduced. T his

means that as the input level decreases, errors due to nonideal

averaging will reduce while the time it takes for the circuit to

settle to the new rms level will increase. T herefore, lower input

levels allow the circuit to perform better (due to increased aver-

aging) but increase the waiting time between measurements.

Obviously, when selecting C_AV, a trade-off between computa-

tional accuracy and settling time is required.

In most cases, the combined magnitudes of both the dc and ac

error components need to be considered when selecting appro-

priate values for capacitors C_AVand C_F. T his combined error,

representing the maximum uncertainty of the measurement is

termed the “averaging error” and is equal to the peak value of

the output ripple plus the dc error.

As the input frequency increases, both error components de-

crease rapidly: if the input frequency doubles, the dc error and

ripple reduce to 1/4 and 1/2 their original values, respectively,

and rapidly become insignificant.

AC MEASUREMENT ACCURACY AND CREST FACTO R

T he crest factor of the input waveform is often overlooked when

determining the accuracy of an ac measurement. Crest factor is

defined as the ratio of the peak signal amplitude to the rms am-

plitude (C.F. = V_PEAK/V rms). Many common waveforms, such

as sine and triangle waves, have relatively low crest factors (≤2).

Other waveforms, such as low duty cycle pulse trains and SCR

waveforms, have high crest factors. T hese types of waveforms

require a long averaging time constant (to average out the long

time periods between pulses). Figure 6 shows the additional

error vs. crest factor of the AD736 for various values of C_AV

SELECTING P RACTICAL VALUES FO R INP UT

CO UP LING (C _C), AVERAGING (C _AV) AND FILTERING

(C_F) CAP ACITO RS

T able II provides practical values of C_AVand C_Ffor several

common applications.

Figure 17. AD736 Average Responding Circuit

Table II. AD 737 Capacitor Selection Chart

Application

rm s

Input

Level

Low

Max

C_AV

C_F

Settling

Tim e*

to 1%

RAP ID SETTLING TIMES VIA TH E AVERAGE

RESP O ND ING CO NNECTIO N (FIGURE 17)

Because the average responding connection does not use the

C_AVaveraging capacitor, its settling time does not vary with in-

put signal level; it is determined solely by the RC time constant

of C_Fand the internal 8 kΩ resistor in the output amplifier’s

feedback path.

Frequency Crest

Cutoff

(–3dB)

Factor

General Purpose 0–1 V

rms Computation

20 Hz

200 Hz

150 µF 10 µF 360 ms

15 µF 1 µF 36 ms

0–200 mV 20 Hz

200 Hz

33 µF 10 µF 360 ms

3.3 µF 1 µF 36 ms

General Purpose 0–1 V

Average

Responding

20 Hz

200 Hz

None 33 µF 1.2 sec

None 3.3 µF 120 ms

D C ERRO R, O UTP UT RIP P LE, AND AVERAGING

ERRO R

Figure 18 shows the typical output waveform of the AD736 with

a sine-wave input applied. As with all real-world devices, the

ideal output of V_OUT= V_INis never exactly achieved; instead,

the output contains both a dc and an ac error component.

0–200 mV 20 Hz

200 Hz

None 33 µF 1.2 sec

None 3.3 µF 120 ms

SCR Waveform

Measurement

0–200 mV 50 Hz

60 Hz

100 µF 33 µF 1.2 sec

82 µF 27 µF 1.0 sec

0–100 mV 50 Hz

60 Hz

50 µF 33 µF 1.2 sec

47 µF 27 µF 1.0 sec

Audio

Applications

Speech

Music

0–200 mV 300 Hz

0–100 mV 20 Hz

1.5 µF 0.5 µF 18 ms

100 µF 68 µF 2.4 sec

*Settling time is specified over the stated rms input level with the input signal increasing

from zero. Settling times will be greater for decreasing amplitude input signals.

Figure 18. Output Waveform for Sine-Wave Input Voltage

REV. C

–7–

AD736

T he input coupling capacitor, C_C, in conjunction with the 8 kΩ

internal input scaling resistor, determine the –3 dB low fre-

quency rolloff. T his frequency, F_L, is equal to:

Note that at F_L, the amplitude error will be approximately

–30% (–3 dB) of reading. T o reduce this error to 0.5% of read-

ing, choose a value of C_Cthat sets F_Lat one tenth the lowest

frequency to be measured.

F_L

2π(8,000)(TheValue of C_Cin Farads )

In addition, if the input voltage has more than 100 mV of dc

offset, than the ac coupling network shown in Figure 21 should

be used in addition to capacitor C_C.

Applications Circuits

Figure 22. Battery Powered Option

Figure 19. AD736 with a High Im pedance Input Attenuator

Figure 23. Low Z, AC Coupled Input Connection

O UTLINE D IMENSIO NS

D imensions shown in inches and (mm).

Figure 20. Differential Input Connection

Figure 21. External Output V_OSAdjustm ent

–8–

REV. C

AD736 [ADI]

相关型号：

AD736-EVALZ

AD7366

AD7366-5

AD7366-5ARUZ

AD7366-5ARUZ-REEL7

AD7366-5BRUZ

AD7366-5BRUZ-REEL7

AD7366-5_15

AD7366ARUZ

AD7366ARUZ-REEL7

AD7366BRUZ

AD7366BRUZ-5