Datasheet AD736 Datasheet (ANALOG DEVICES)

Low Cost, Low Power,

FEATURES

Computes

True rms value Average rectified value Absolute value

Provides

200 mV full-scale input range (larger inputs with input

attenuator) High input impedance: 10 Low input bias current: 25 pA maximum High accuracy: ±0.3 mV ± 0.3% of reading RMS conversion with signal crest factors up to 5 Wide power supply range: +2.8 V, −3.2 V to ±16.5 V Low power: 200 mA maximum supply current Buffered voltage output No external trims needed for specified accuracy

AD737—an unbuffered voltage output version with

chip power-down also available

GENERAL DESCRIPTION

The AD736 is a low power, precision, monolithic true rms-todc converter. It is laser trimmed to provide a maximum error of ±0.3 mV ± 0.3% of reading with sine wave inputs. Furthermore, it maintains high accuracy while measuring a wide range of input waveforms, including variable duty-cycle pulses and triac (phase)-controlled sine waves. The low cost and small size of this converter make it suitable for upgrading the performance of non-rms precision rectifiers in many applications. Compared to these circuits, the AD736 offers higher accuracy at an equal or lower cost.

The AD736 can compute the rms value of both ac and dc input voltages. It can also be operated as an ac-coupled device by adding one external capacitor. In this mode, the AD736 can resolve input signal levels of 100 V 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 (introducing only 2.5% additional error) at the 200 mV full-scale input level.

The AD736 has its own output buffer amplifier, thereby providing a great deal of design flexibility. Requiring only 200 µA of power supply current, the AD736 is optimized for use in portable multimeters and other battery-powered applications.

12

Ω

True RMS-to-DC Converter

AD736

FUNCTIONAL BLOCK DIAGRAM

8k

1

C

FULL

WAVE

V

2

IN

AMPLIFI ER

C

3

F

–V

S

4

BIAS

SECTION

RECTIFIER

INPUT

rms CORE

Figure 1.

AD736

8k

OUTPUT

AMPLIFI ER

The AD736 allows the choice of two signal input terminals: a

12

high impedance FET input (10

Ω) that directly interfaces with High-Z input attenuators and a low impedance input (8 kΩ) that 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 can be used either single ended or differentially.

The AD736 has a 1% reading error bandwidth that exceeds 10 kHz for the input amplitudes from 20 mV rms to 200 mV rms while consuming only 1 mW.

The AD736 is available in four performance grades. The AD736J and AD736K grades are rated over the 0°C to +70°C and −20°C to +85°C commercial temperature ranges. The AD736A and AD736B grades are rated over the −40°C to +85°C industrial temperature range. The AD736 is available in three low cost, 8-lead packages: PDIP, SOIC, and CERDIP.

PRODUCT HIGHLIGHTS

1. The 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. The low power consumption of 1 mW makes the AD736

suitable for many battery-powered applications.

12

4. A high input impedance of 10

external buffer when interfacing with input attenuators.

5. A low impedance input is available for those applications that

require an input signal up to 300 mV rms operating from low power supply voltages.

Ω eliminates the need for an

8

COM

+V

7

6

OUTPUT

C

5

S

AV

00834-001

Rev. H

Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Anal og Devices for its use, nor for any infringements of patents or ot her rights of third parties that may result from its use. Specifications subject to change without notice. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices. Trademarks and registered trademarks are the property of their respective owners.

AD736

REVISION HISTORY

2/07—Rev. G to Rev. H

Updated Layout.......................................................................9 to 12

Added Applications Section......................................................... 13

Inserted Figure 21 to Figure 24; Renumbered Sequentially..... 13

Deleted Figure 25........................................................................... 15

Added Evaluation Board Section ................................................ 16

Inserted Figure 29 to Figure 34; Renumbered Sequentially..... 16

Inserted Figure 35; Renumbered Sequentially........................... 17

Added Table 6................................................................................. 17

2/06—Rev. F to Rev. G

Updated Format.................................................................Universal

Changes to Features......................................................................... 1

Added Table 3................................................................................... 6

Changes to Figure 21 and Figure 22............................................ 14

Changes to Figure 23, Figure 24, and Figure 25 ........................ 15

Updated Outline Dimensions...................................................... 16

Changes to Ordering Guide......................................................... 17

5/04—Rev. E to Rev. F

Changes to Specifications............................................................... 2

Replaced Figure 18 ........................................................................10

Updated Outline Dimensions...................................................... 16

Changes to Ordering Guide......................................................... 16

RMS Measurement—Choosing the Optimum Value for C

....................................................................................................... 11

Rapid Settling Times via the Average Responding

Connection.................................................................................. 12

DC Error, Output Ripple, and Averaging Error..................... 12

AC Measurement Accuracy and Crest Factor............................ 12

Applications..................................................................................... 13

Connecting the Input................................................................. 13

Selecting Practical Values for Input Coupling (CC), Averaging (C

), and Filtering (C

AV

Evaluation Board ............................................................................ 16

Outline Dimensions ....................................................................... 18

Ordering Guide .......................................................................... 19

4/03—Rev. D to Rev. E

Changes to General Description .................................................1

Changes to Specifications.............................................................3

Changes to Absolute Maximum Ratings....................................4

Changes to Ordering Guide.........................................................4

11/02—Rev. C to Rev. D

Changes to Functional Block Diagram.......................................1

Changes to Pin Configuration.....................................................3

Figure 1 Replaced ..........................................................................6

Changes to Figure 2.......................................................................6

Changes to Application Circuits Figures 4 to 8.........................8

Outline Dimensions Updated......................................................8

) Capacitors......................................... 14

F

AV

Rev. H | Page 2 of 20

AD736

SPECIFICATIONS

At 25°C ± 5 V supplies, ac-coupled with 1 kHz sine wave input applied, unless otherwise noted. Specifications in bold are tested on all production units at final electrical test. Results from those tests are used to calculate outgoing quality levels.

Table 1.

AD736J/AD736A AD736K/AD736B Parameter Conditions Min Typ Max Min Typ Max Unit

TRANSFER FUNCTION V

= √Avg (V

OUT

CONVERSION ACCURACY 1 kHz sine wave

Total Error, Internal Trim

All Grades 0 mV rms to 200 mV rms 0.3/0.3

200 mV to 1 V rms −1.2

T

to T

MIN

MAX

A and B Grades @ 200 mV rms J and K Grades @ 200 mV rms 0.007

vs. Supply Voltage

@ 200 mV rms Input VS = ±5 V to ±16.5 V

V

DC Reversal Error, DC-Coupled @ 600 mV dc Nonlinearity2, 0 mV to 200 mV @ 100 mV rms

1

Using C

C

0.5/0.5 ±2.0

0.7/0.7

+0.06

−0.18

+0.1 0

−0.3 0

1.3 2.5

0.25

0.35 0

= ±5 V to ±3 V

S

0 0

0

Total Error, External Trim 0 mV rms to 200 mV rms 0.1/0.5 0.1/0.3 ±mV/±% of reading

ERROR VS. CREST FACTOR

3

Crest Factor = 1 to 3 CAV, CF = 100 μF 0.7 0.7 % additional error Crest Factor = 5 CAV, CF = 100 μF 2.5 2.5 % additional error

INPUT CHARACTERISTICS

High Impedance Input

Signal Range (Pin 2)

Continuous RMS Level VS = +2.8 V, −3.2 V

V

Peak Transient Input VS = +2.8 V, −3.2 V V V

= ±5 V to ±16.5 V

S

±0.9

= ±5 V

S

= ±16.5 V

S

±4.0

Input Resistance 10

±2.7

12

200 1

10

Input Bias Current VS = ±3 V to ±16.5 V 1 25 1 25 pA

Low Impedance Input

Signal Range (Pin 1)

Continuous RMS Level VS = +2.8 V, –3.2 V 300 300 mV rms V

= ±5 V to ±16.5 V 1 1 V rms

S

Peak Transient Input VS = +2.8 V, −3.2 V ±1.7 ±1.7 V V V

= ±5 V ±3.8 ±3.8 V

S

= ±16.5 V ±11 ±11 V

S

Input Resistance 6.4 8 9.6 6.4 8 9.6 kΩ

Maximum Continuous

All supply voltages ±12 ±12 V p-p

Nondestructive Input

Input Offset Voltage

J and K Grades

A and B Grades

4

±3 ±3

vs. Temperature 8 30 8 30 μV/°C

vs. Supply VS = ±5 V to ±16.5 V 50 V

= ±5 V to ±3 V 80 80 μV/V

S

150

2

)

IN

0.2/0.2

−1.2

0.007

+0.06

−0.18

1.3 2.5 % of reading

0.25

±0.9

±4.0

V ±2.7 V V

50

0.3/0.3 ±2.0

0.5/0.5

+0.1

−0.3

0.35

200 1

12

Ω

±3 ±3

150

±mV/±% of reading % of reading

±mV/±% of reading ±% of reading/°C

%/V %/V

% of reading

mV rms V rms

mV mV

μV/V

Rev. H | Page 3 of 20

AD736

AD736J/AD736A AD736K/AD736B Parameter Conditions Min Typ Max Min Typ Max Unit

OUTPUT CHARACTERISTICS

Output Offset Voltage

J and K Grades ±0.1 A and B Grades

±0.5 ±0.5

vs. Temperature 1 20 1 20 μV/°C vs. Supply VS = ±5 V to ±16.5 V 50

V

= ±5 V to ±3 V 50 50 μV/V

S

130

Output Voltage Swing

2 kΩ Load VS = +2.8 V, −3.2 V

0 to

1.7

1.6

V

= ±5 V

S

0 to

3.8

3.6

V

No Load VS = ±16.5 V

= ±16.5 V

S

0 to 4 0 to 4

5

12 Output Current 2 2 mA Short-Circuit Current 3 3 mA Output Resistance @ dc 0.2 0.2 Ω

FREQUENCY RESPONSE

High Impedance Input (Pin 2)

Sine wave input

for 1% Additional Error

VIN = 1 mV rms 1 1 kHz VIN = 10 mV rms 6 6 kHz VIN = 100 mV rms 37 37 kHz VIN = 200 mV rms 33 33 kHz

±3 dB Bandwidth Sine wave input

VIN = 1 mV rms 5 5 kHz VIN = 10 mV rms 55 55 kHz VIN = 100 mV rms 170 170 kHz VIN = 200 mV rms 190 190 kHz

Low Impedance Input (Pin 1)

Sine wave input

for 1% Additional Error

VIN = 1 mV rms 1 1 kHz VIN = 10 mV rms 6 6 kHz VIN = 100 mV rms 90 90 kHz VIN = 200 mV rms 90 90 kHz

±3 dB Bandwidth Sine wave input

VIN = 1 mV rms 5 5 kHz VIN = 10 mV rms 55 55 kHz VIN = 100 mV rms 350 350 kHz VIN = 200 mV rms 460 460 kHz

POWER SUPPLY

Operating Voltage Range +2.8, −3.2 ± 5 ±16.5 +2.8, −3.2 ± 5 ±16.5 V Quiescent Current Zero signal 160

200

200 mV rms, No Load Sine wave input 230 270 230 270 μA

TEMPERATURE RANGE

Operating, Rated Performance

Commercial 0°C to 70°C AD736JN, AD736JR AD736KN, AD736KR Industrial −40°C to +85°C AD736AQ, AD736AR AD736BQ, AD736BR

1

Accuracy is specified with the AD736 connected as shown in Figure 18 with Capacitor CC.

2

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

3

Error vs. crest factor is specified as additional error for a 200 mV rms signal. Crest factor = V

4

DC offset does not limit ac resolution.

PEAK

/V rms.

±0.1

50

0 to

1.7 V

±0.3 ±0.3

130

1.6 0 to

3.8 V

3.6

0 to 4 0 to 4

160

5 V 12 V

200

mV mV

μV/V

μA

Rev. H | Page 4 of 20

AD736

ABSOLUTE MAXIMUM RATINGS

Table 2.

Parameter Rating

Supply Voltage ±16.5 V Internal Power Dissipation Input Voltage ±V Output Short-Circuit Duration Indefinite Differential Input Voltage +VS and –V Storage Temperature Range (Q) –65°C to +150°C Storage Temperature Range (N, R) –65°C to +125°C Lead Temperature (Soldering, 60 sec) 300°C ESD Rating 500 V

1

8-Lead PDIP: θJA = 165°C/W, 8-Lead CERDIP: θJA = 110°C/W, and

8-Lead SOIC: θJA = 155°C/W.

1

200 mW

S

Stresses above those listed under Absolute Maximum Ratings may cause permanent 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.

ESD CAUTION

Rev. H | Page 5 of 20

AD736

–V

PIN CONFIGURATION AND FUNCTION DESCRIPTIONS

Table 3. Pin Function Descriptions

Pin No. Mnemonic Description

1 C

C

Coupling Capacitor. If dc coupling is desired at Pin 2, connect a coupling capacitor to this pin. If the coupling at Pin 2 is ac, connect this pin to ground. Note that this pin is also an input, with an input impedance of 8 kΩ.

Such an input is useful for applications with high input voltages and low supply voltages. 2 V 3 C 4 −V 5 C

IN

F

S

AV

High Input Impedance Pin.

Connect an Auxiliary Low-Pass Filter Capacitor from the Output.

Negative Supply Voltage if Dual Supplies Are Used, or Ground if Connected to a Single-Supply Source.

Connect the Averaging Capacitor Here. 6 OUTPUT DC Output Voltage. 7 +V

S

Positive Supply Voltage. 8 COM Common.

1

C

AD736

2

V

IN

TOP VIEW

C

3

(Not to Scale)

F

4

S

Figure 2. Pin Configuration

8

COM

7

+V

OUTPUT

6

C

5

S

AV

00834-025

Rev. H | Page 6 of 20

AD736

V

TYPICAL PERFORMANCE CHARACTERISTICS

0.7

0.5

0.3

0.1

–0.1

–0.3

ADDITIONAL ERROR (% of Reading)

VIN = 200mV rms 1kHz SINE WAVE C

= 100µF

AV

C

= 22µF

F

0

100mV

10mV

INPUT LEVEL (rms)

10

SINE WAVE INPUT, VS=±5V, C

= 22µF, CF = 4.7µF, CC = 22µF

AV

1V

1mV

1% ERROR

–3dB

10% ERROR

–0.5

04286121410 16

SUPPLY VOLTAGE (±V)

Figure 3. Additional Error vs. Supply Voltage

16

DC-COUPLED

14

12

10

8

6

4

PEAK INPUT BEF ORE CLIP PING (V)

2

0

04286121410 16

PIN 1

PIN 2

SUPPLY VOLTAGE (±V)

Figure 4. Maximum Input Level vs. Supply Voltage

16

1kHz SINE WAVE INPUT

14

12

10

100µV

0.1 1 10010 1000

00834-002

–3dB FRE QUENCY ( kHz)

00834-005

Figure 6. Frequency Response Driving Pin 1

10

SINE WAVE INPUT, VS=±5V, C

= 22µF, CF = 4.7µF, CC = 22µF

AV

1V

100mV

10mV

INPUT LEVEL (rms)

1mV

100µV

0.1 1 10010 1000

00834-003

1% ERROR

–3dB FRE QUENCY ( kHz)

–3dB

10% ERROR

00834-006

Figure 7. Frequency Response Driving Pin 2

6

3ms BURST OF 1kHz = 3 CYCLES 200mV rms SIGNAL

5

V

= ±5V

S

C

= 22µF

C

= 100µF

F

4

CAV = 10µF

CAV = 33µF

8

6

4

PEAK BUFFER O UTPUT (V)

2

0

0246 1081214

SUPPLY VOLTAGE (±V)

Figure 5. Peak Buffer Output vs. Supply Voltage

16

00834-004

3

2

1

ADDITIONAL ERROR (% of Reading)

CAV = 250µF

0

12345

CREST FACTO R (V

PEAK

CAV = 100µF

/V rms)

Figure 8. Additional Error vs. Crest Factor with Various Values of C

00834-007

AV

Rev. H | Page 7 of 20

AD736

V

0.8

VIN = 200mV rms 1kHz SINE WAVE

0.6 C

= 100mF

AV

C

= 22mF

F

0.4

V

= ±5V

S

0.2

0

–0.2

–0.4

ADDITIONAL ERROR (% of Reading )

–0.6

–0.8

–60 –20–40 200 60 80 100 12040 140

TEMPERATURE (° C)

Figure 9. Additional Error vs. Temperature

600

VIN = 200mV rms 1kHz SINE WAVE C

= 100µF

AV

500

C

= 22µF

F

V

= ±5V

S

400

300

00834-008

1.0

0.5

0

–0.5

–1.0

ERROR (% of Read ing)

–1.5

VIN = SINE WAVE @ 1kHz

–2.0

C

= 22µF, CC = 47µF,

AV

C

= 4.7µF, VS = ±5V

F

–2.5

10mV 100mV 1V 2V

INPUT LEVEL (rms)

Figure 12. Error vs. RMS Input Voltage (Pin 2),

Output Buffer Offset Is Adjusted to Zero

100

(µF)

10

AV

C

–0.5%

VIN = 200mV rms C

= 47µF

C

= 47µF

F

V

= ±5V

S

00834-011

DC SUPPLY CURRENT (µA)

200

100

0 0.2 0.4 0.6 0.8 1.0

rms INPUT LEVEL (V)

Figure 10. DC Supply Current vs. rms Input Level

10m

VIN = 1kHz SINE WAVE INPUT AC-COUPLED V

= ±5V

S

1mV

INPUT LEVEL (rms)

100µV

10µV

100 1k 10k 100k

–3dB FREQUENCY (Hz)

Figure 11. RMS Input Level (Pin 2) vs. −3 dB Frequency

–1%

1

10 100 1k

00834-009

Figure 13. C

AV

FREQUENCY (Hz)

vs. Frequency for Specified Averaging Error

00834-012

1

–1%

100mV

10mV

INPUT LEVEL (rms)

1mV

1 10 100 1k

00834-010

FREQUENCY (Hz)

–0.5%

VIN SINE WAVE AC-COUPLED C

= 10µF, CC = 47µF,

AV

C

= 47µF, VS = ±5V

F

00834-013

Figure 14. RMS Input Level vs. Frequency for Specified Averaging Error

Rev. H | Page 8 of 20

AD736

V

A

4.0

10n

3.5

3.0

2.5

2.0

INPUT BIAS CURRENT (pA)

1.5

1.0 024 68 121410 16

SUPPLY VOLTAGE (±V)

Figure 15. Pin 2 Input Bias Current vs. Supply Voltage

1

100mV

CAV = 10µF

10mV

INPUT LEVEL (rms)

1mV

CAV = 33µF

VS = 5V C

= 22µF

C

= 0µF

F

CAV = 100µF

1nA

100pA

10pA

INPUT BIAS CURRE NT

1pA

100fA

–55 –35 –15 5 25 65 85 10545 125

00834-014

TEMPERATURE ( °C)

00834-016

Figure 17. Pin 2 Input Bias Current vs. Temperature

100µV

1ms 10ms 100ms 1s 10s 100s

SETTLING TIME

Figure 16. RMS Input Level for Various Values of C

vs. Settling Time

AV

00834-015

Rev. H | Page 9 of 20

AD736

A

THEORY OF OPERATION

C

10µF

C =

+

DC

OPTIONAL RETURN PATH

FWR

CURRENT

MODE

ABSOLU TE

VALUE

C

1

C

8k

V

IN

V

2

IN

INPUT

AMPLIFIER

I

<10pA

B

AD736

OUTPUT

AMPLIFIER

8k

COM

8

7

0.1µF

+V

S

BIAS

3

SECTION

rms

TRANSLINEAR

CORE

–V

S

4

0.1µF

TO COM PIN

Figure 18. AD736 True RMS Circuit

As shown by Figure 18, the AD736 has five functional subsections: the input amplifier, full-wave rectifier (FWR), rms core, output amplifier, and bias section. The FET input amplifier allows both a high impedance, buffered input (Pin 2) and a low impedance, wide dynamic range input (Pin 1). The high impedance input, with its low input bias current, is well suited for use with high impedance input attenuators.

The output of the input amplifier drives a full-wave precision rectifier that, in turn, drives the rms core. The essential rms operations of squaring, averaging, and square rooting are performed in the core using an external averaging capacitor, C

. Without CAV, the rectified input signal travels through the

AV

core unprocessed, as is done with the average responding connection (see

Figure 19).

A final subsection, an output amplifier, buffers the output from the core and allows optional low-pass filtering to be performed via the external capacitor, C

, which is connected across the

F

feedback 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 that was not removed by the averaging capacitor, C

AV

C

33µF

C

10µF

.

A

F

+

(OPTIO NAL)

+

6

C

5

rms OUTPUT

AV

0834-017

TYPES OF AC MEASUREMENT

The AD736 is capable of measuring ac signals by operating as either an average responding converter 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 approximates the average. The resulting output, a dc average level, is 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 average absolute value of a sine wave voltage is 0.636 times V

; the corresponding rms value is 0.707 × V

PEAK

sine wave voltages, the required scale factor is 1.11 (0.707/0.636).

In contrast to measuring the average value, true rms measurement 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 a dc voltage; an ac signal of 1 V rms produces the same amount of heat in a resistor as a 1 V dc signal.

. Therefore, for

PEAK

Rev. H | Page 10 of 20

AD736

Mathematically, the rms value of a voltage is defined (using a simplified equation) as

2

rms VAvgV =

()

This involves squaring the signal, taking the average, and then obtaining the square root. True rms converters are smart rectifiers; 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 depends on the type of waveform being measured. For 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 has a computational error 11% (of reading) higher than the true rms value (see

Table 4 ).

CALCULATING SETTLING TIME USING FIGURE 16

Figure 16 can be used to closely approximate the time required for the AD736 to settle when its input level is reduced in amplitude. The net time required for the rms converter to settle is 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, a 100 mV initial rms input level, and a final (reduced) 1 mV input level. From

Figure 16, the initial settling time (where the 100 mV line

intersects the 33 µF line) is approximately 80 ms.

The settling time corresponding to the new or final input level of 1 mV is approximately 8 seconds. Therefore, the net time for the circuit to settle to its new value is 8 seconds minus 80 ms, which is 7.92 seconds. Note that because of the smooth decay characteristic inherent with a capacitor/diode combination, this is the total settling time to the final value (that is, not the settling time to 1%, 0.1%, and so on, of the final value). In addition, this graph provides the worst-case settling time because the AD736 settles very quickly with increasing input levels.

RMS MEASUREMENT—CHOOSING THE OPTIMUM VALUE FOR C

Because the external averaging capacitor, CAV, holds the rectified input signal during rms computation, its value directly affects the accuracy of the rms measurement, especially at low frequencies. Furthermore, because the averaging capacitor appears across a diode in the rms core, the averaging time constant increases exponentially as the input signal is reduced. This means that as the input level decreases, errors due to nonideal averaging decrease, and the time required for the circuit to settle to the new rms level increases. Therefore, lower input levels allow the circuit to perform better (due to increased averaging) but increase the waiting time between measurements. Obviously, when selecting C accuracy and settling time is required.

AV

, a trade-off between computational

AV

Table 4. Error Introduced by an Average Responding Circuit when Measuring Common Waveforms

Average Responding Circuit

Crest Factor

Waveform Type 1 V Peak Amplitude

Undistorted Sine Wave 1.414 0.707 0.707 0 Symmetrical Square Wave 1.00 1.00 1.11 +11.0 Undistorted Triangle Wave 1.73 0.577 0.555 −3.8 Gaussian Noise (98% of Peaks <1 V) 3 0.333 0.295 −11.4 Rectangular 2 0.5 0.278 −44 Pulse Train 10 0.1 0.011 −89 SCR Waveforms

50% Duty Cycle 2 0.495 0.354 −28 25% Duty Cycle 4.7 0.212 0.150 −30

(V

PEAK

/V rms)

True RM S Value (V)

Calibrated to Read RMS Value of Sine Waves (V)

% of Reading Error Using Average Responding Circuit

Rev. H | Page 11 of 20

AD736

RAPID SETTLING TIMES VIA THE AVERAGE RESPONDING CONNECTION

Because the average responding connection shown in Figure 19 does not use the C not vary with the input signal level. It is determined solely by the RC time constant of C output amplifier’s feedback path.

8k

C

1

C

V

IN

2

V

IN

C

F

3

–V

S

–V

S C

4

averaging capacitor, its settling time does

AV

and the internal 8 kΩ resistor in the

F

C

10µF

+

(OPTIO NAL)

AD736

FULL

INPUT

AMPLIFIER

BIAS

SECTION

WAVE

RECTIFIER

rms

CORE

8k

OUTPUT

AMPLI FIER

8

COM

+V

S

7

OUTPUT

6

5

AV

+V

S

V

OUT

In most cases, the combined magnitudes of both the dc and ac error components need to be considered when selecting appropriate values for Capacitor C

and Capacitor CF. This

AV

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.

E

O

IDEAL

E

O

DC ERROR = EO – EO (IDEAL)

DOUBLE-FREQ UENCY

RIPPLE

AVERAGE E

= E

O

TIME

00834-019

Figure 20. Output Waveform for Sine Wave Input Voltage

As the input frequency increases, both error components decrease rapidly; if the input frequency doubles, the dc error and ripple reduce to one quarter and one half of their original values, respectively, and rapidly become insignificant.

+

C

F

33µF

POSITIVE SUPPLY

COMMON

NEGATIVE SUPPLY

0.1µF

+V

S

–V

S

Figure 19. AD736 Average Responding Circuit

DC ERROR, OUTPUT RIPPLE, AND AVERAGING ERROR

Figure 20 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 the output contains both a dc and an ac error component.

As shown in

Figure 20, the dc error is the difference between the average of the output signal (when all the ripple in the output is removed by external filtering) and the ideal dc output. The dc error component is therefore set solely by the value of the averaging capacitor used. No amount of post filtering (that is, using a very large C ideal value. The ac error component, an output ripple, can be easily removed by using a large enough post filtering capacitor, C

= VIN is never achieved exactly. Instead,

OUT

) allows the output voltage to equal its

F

AC MEASUREMENT ACCURACY AND CREST FACTOR

The 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 amplitude (crest factor = V such as sine and triangle waves, have relatively low crest factors

00834-018

(≤2). Other waveforms, such as low duty-cycle pulse trains and SCR waveforms, have high crest factors. These types of waveforms require a long averaging time constant (to average out the long periods between pulses). the crest factor of the AD736 for various values of C

.

/V rms). Many common waveforms,

PEAK

Figure 8 shows the additional error vs.

.

AV

Rev. H | Page 12 of 20

AD736

APPLICATIONS

CONNECTING THE INPUT

The inputs of the AD736 resemble an op amp, with noninverting and inverting inputs. The input stages are JFETs accessible at Pin 1 and Pin 2. Designated as the high impedance input, Pin 2 is connected directly to a JFET gate. Pin 1 is the low impedance input because of the scaling resistor connected to the gate of the second JFET. This gate-resistor junction is not externally accessible and is servo-ed to the voltage level of the gate of the first JFET, as in a classic feedback circuit. This action results in the typical 8 kΩ input impedance referred to ground or reference level.

This input structure provides four input configurations as shown in Figure 21 and Figure 22 show the high impedance configurations, and used to extend the input voltage range.

Figure 21, Figure 22, Figure 23, and Figure 24.

Figure 23 and Figure 24 show the low impedance connections

COM

+V

C

8

7

+V

S

AV

S

6

VOUT

DC

5

1M

1

C

AD736

2

V

IN

3

4

–V

OUTPUTC

F

S

Figure 23. Low-Z AC-Coupled Input Connection

Figure 24. Low-Z DC-Coupled Input Connection

1

C

AD736

2

V

IN

3

C

4

–V

S

1

C

2

V

3

C

4

–V

S

F

S

C

IN

F

AD736

S

C

AV

C

OUTPUT

AV

COM

+V

C

AV

COM

+V

OUTPUT

C

8

7

S

+V

S

6

VOUT

5

S

AV

DC

00834-028

8

7

+V

S

6

VOUT

5

DC

00834-029

C

AV

–V

S

00834-026

Figure 21. High-Z AC-Coupled Input Connection (Default)

1

C

AD736

2

V

IN

3

C

F

4

–V

S

C

AV

–V

S

COM

+V

OUTPUT

C

8

7

S

+V

S

6

VOUT

5

AV

DC

00834-027

Figure 22. High-Z DC-Coupled Input Connection

Rev. H | Page 13 of 20

AD736

A

G

SELECTING PRACTICAL VALUES FOR INPUT COUPLING (C (C

) CAPACITORS

F

Tabl e 5 provides practical values of CAV and CF for several common applications.

The input coupling capacitor, C 8 kΩ internal input scaling resistor, determine the −3 dB low frequency roll-off. This frequency, F

=

F

L

), AVERAGING (CAV), AND FILTERING

C

, in conjunction with the

C

, is equal to

L

1

)(8000)(2π

FaradsinCofValue

C

Note that at F (

–3 dB) of the reading. To reduce this error to 0.5% of the

reading, choose a value of C lowest frequency to be measured.

In addition, if the input voltage has more than 100 mV of dc offset, then the ac-coupling network shown in be used in addition to C

Table 5. Capacitor Selection Chart

Low Frequency

Application RMS Input Level

Cutoff (−3 dB)

General-Purpose RMS Computation 0 V to 1 V 20 Hz 5 150 10 360 ms 200 Hz 5 15 1 36 ms 0 mV to 200 mV 20 Hz 5 33 10 360 ms 200 Hz 5 3.3 1 36 ms General Purpose 0 V to 1 V 20 Hz None 33 1.2 sec Average 200 Hz None 3.3 120 ms Responding 0 mV to 200 mV 20 Hz None 33 1.2 sec 200 Hz None 3.3 120 ms SCR Waveform Measurement 0 mV to 200 mV 50 Hz 5 100 33 1.2 sec 60 Hz 5 82 27 1.0 sec 0 mV to 100 mV 50 Hz 5 50 33 1.2 sec 60 Hz 5 47 27 1.0 sec Audio Applications

Speech 0 mV to 200 mV 300 Hz 3 1.5 0.5 18 ms Music 0 mV to 100 mV 20 Hz 10 100 68 2.4 sec

1

Settling time is specified over the stated rms input level with the input signal increasing from zero. Settling times are greater for decreasing amplitude input signals.

OPTIONAL

C COUPLIN

CAPACITOR

V

IN

9M

900k

90k

10k

0.01µF

1kV

200mV

2V

20V

200V

47k

1W

–V

S

+V

–V

S

1N4148

S

8k

C

1

C

V

IN

2

C

F

3

–V

S

4

INPUT

AMPLIFIER

BIAS

SECTION

C

10µF +

(OPTIO NAL)

FULL

WAVE

RECTIFIER

rms

CORE

, the amplitude error is approximately −30%

L

that sets FL at one-tenth of the

C

Figure 27 should

.

C

Max Crest Fac tor

AD736

8k

OUTPUT

AMPLIFIER

C (μF)

8

COM

+V

S

7

OUTPUT

6

C

AV

5

CF

AV

+V

S

1µF

OUTPUT

(μF)

Settling Time

1

to 1%

+

C

1µF

AV

33µF

C

10µF

+

F

(OPTIONAL)

00834-020

Figure 25. AD736 with a High Impedance Input Attenuator

Rev. H | Page 14 of 20

AD736

C

–IN

+IN

INPUT IMPEDANCE: 10

DC-COUPLED

V

IN

0.1µF

AC-COUPLED

+V

S

1M

–V

S

0.1µF

V

IN

1M

V

S

2

AD711

3

2

–V

39M

OUTPUT V

OS

ADJUST

S

1M

C

V

IN

C

–V

C

10µF

C

+

V

||10pF

C

–V

8k

1

IN

2

F

3

S

4

INPUT

AMPLIFIER

BIAS

SECTION

FULL

WAVE

RECTIFIER

rms

CORE

AD736

8k

OUTPUT

AMPLIFI ER

8

+V

S

7

OUTPUT

6

C

AV

5

COM

1µF

+V

S

OUTPUT

6

12

+

C

1µF

33µF

C

10µF

AV

+

F

(OPTIONAL)

00834-021

Figure 26. Differential Input Connection

C

10µF +

(OPTIO NAL)

8k

C

1

C

V

IN

2

C

F

3

–V

S

4

INPUT

AMPLIFIER

BIAS

SECTION

FULL

WAVE

RECTIFIE R

rms

CORE

AD736

8k

OUTPUT

AMPLIFI ER

8

COM

+V

S

7 +V

1µF

OUTPUT

6

C

AV

5

S

OUTPUT

+

C

1µF

Figure 27. External Output V

C

10µF

+

C

8k

1

FULL

WAVE

2

AMPLIFIER

F

3

BIAS

SECTION

S

4

INPUT

RECTIFIER

rms

CORE

AV

33µF

+

C

F

(OPTIONAL)

10µF

OS

AD736

8k

OUTPUT

AMPLIFI ER

Adjustment

COM

8

+V

7

OUTPUT

6

C

AV

5

00834-022

V

S

2

100k

4.7µF

9V

4.7µF

+

100k

33µF

+

C

F

(OPTIO NAL)

10µF

00834-023

Figure 28. Battery-Powered Option

Rev. H | Page 15 of 20

AD736

EVALUATION BOARD

An evaluation board, AD736-EVALZ, is available for experimentation or becoming familiar with rms-to-dc converters. Figure 29 is a photograph of the board, and Figure 30 is the top silkscreen showing the component locations. Figure 33, and Figure 34 show the layers of copper, and Figure 35 shows the schematic of the board configured as shipped. The board is designed for multipurpose applications and can be used for the AD737 as well.

Figure 29. AD736 Evaluation Board

Figure 31, Figure 32,

Figure 31. Evaluation Board—Component-Side Copper

00834-030

00834-033

Figure 30. Evaluation Board—Component-Side Silkscreen

As shipped, the board is configured for dual supplies and high impedance input. Optional jumper locations enable low impedance and dc input connections. Using the low impedance input (Pin 1) often enables higher input signals than otherwise possible. A dc connection enables an ac plus dc measurement, but care must be taken so that the opposite polarity input is not dc-coupled to ground.

Figure 35 shows the board schematic with all movable jumpers. The jumper positions in black are default connections; the dottedoutline jumpers are optional connections. The board is tested prior to shipment and only requires a power supply connection and a precision meter to perform measurements.

Tabl e 6 is the bill of materials for the AD736 evaluation board.

Figure 32. Evaluation Board—Secondary-Side Copper

00834-032

Figure 33. Evaluation Board—Internal Power Plane

00834-034

00834-035

00834-036

Figure 34. Evaluation Board—Internal Ground Plane

Rev. H | Page 16 of 20

AD736

–V

V

GND1 GND2 GND3 GND4

W1 DC

COUP

VIN

J1

+V

CIN

0.1µF

S

GND

NORM

PD

FILT

LO-Z IN

P2 HI-Z SEL

IN

R1

1M

SEL

J3

W4

W2

HI-Z

C4

0.1µF

C1

10µF

25V

LO-Z

1

C

AD736

2

V

IN

3

C

F

4

–V

S

–V

S

CAV 33µF

16V+

CF2

+

S

–V

+V

S

W3

AC COUP

C

COM

+V

OUT

C

AV

S

+

C2 10µF 25V

S

R3 0

R4 0

8

C6

0.1µF

7

S

6

5

CAV

+V

S

VOUT

CF1

J2

00834-032

Figure 35. Evaluation Board Schematic

Table 6. Evaluation Board Bill of Materials

Qty Name Description Reference Designator Manufacturer Mfg. Part Number

1 Test loop Red +V 1 Test loop Green −V

S

Components Corp. TP-104-01-02

Components Corp. TP-104-01-05 2 Capacitors Tantalum 10 μF, 25 V C1, C2 Nichicon Corp. F931E106MCC 3 Capacitors 0.1 μF, 16 V, 0603, X7R C4, C6, CIN KEMET Corp. C0603C104K4RACTU 1 Capacitor Tantalum 33 μF, 16V, 20%, 6032 CAV Nichicon Corp. F931C336MCC 5 Test loops Purple CAV, HI Z, LO Z, VIN, VOUT Components Corp. TP-104-01-07 1 Integrated circuit RMS-to-dc converter DUT Analog Devices, Inc. AD736JRZ 4 Test loops Black GND1, GND2, GND3, GND4 Components Corp. TP-104-01-00 2 Connectors BNC, right angle J1, J2 AMP 227161-1 1 Header 6-pin, 2 × 3 J3 3M 929836-09-03 1 Header 3-pin P2 Molex, Inc. 22-10-2031 1 Resistor 1 MΩ, 1/10 W, 1%, 0603 R1 Panasonic Corp. ERJ3EKF1004V 2 Resistors 0 Ω, 5%, 0603 R3, R4 Panasonic Corp. ERJ3GEY0R00V 4 Headers 2-pin, 0.1" center W1, W2, W3, W4 Molex, Inc. 22-10-2021

Rev. H | Page 17 of 20

AD736

OUTLINE DIMENSIONS

0.400 (10.16)

0.365 (9.27)

0.355 (9.02)

0.210 (5.33)

0.150 (3.81)

0.130 (3.30)

0.115 (2.92)

0.022 (0.56)

0.018 (0.46)

0.014 (0.36)

MAX

8

1

0.100 (2.54)

0.070 (1.78)

0.060 (1.52)

0.045 (1.14)

BSC

5

4

0.280 (7. 11)

0.250 (6.35)

0.240 (6.10)

0.015 (0.38) MIN

SEATING PLANE

0.005 (0.13) MIN

0.060 (1.52) MAX

0.015 (0.38) GAUGE

PLANE

0.325 (8.26)

0.310 (7.87)

0.300 (7.62)

0.430 (10.92) MAX

0.195 (4.95)

0.130 (3.30)

0.115 (2.92)

0.014 (0.36)

0.010 (0.25)

0.008 (0.20)

CONTROLLING DIMENSIONSARE IN INCHES; MILLIMETER DIMENSIONS (IN PARENTHESES) ARE ROUNDED-OFF INCH EQUIVALENTS FOR REFERENCE ON LY AND ARE NOT APPRO PRIATE FOR USE IN DESIGN. CORNER LEADS MAY BE CONFIGURED AS WHOLE OR HALF LEADS.

Figure 36. 8-Lead Plastic Dual In-Line Package [PDIP]

0.005 (0.13)

0.200 (5.08) MAX

0.200 (5.08)

0.125 (3.18)

0.023 (0.58)

0.014 (0.36)

CONTROLLING DIMENSIONS ARE IN INCHES; MILLIMETER DIMENSIONS (IN PARENTHESES) ARE ROUNDED-OFF INCH EQUIVALENTS FOR REFERENCE ONLY AND ARE NOT APPROPRIATE FOR USE IN DESIGN.

Figure 37. 8-Lead Ceramic Dual In-Line Package [CERDIP]

Dimensions shown in inches and (millimeters)

0.055 (1.40)

MIN

14

0.100 (2.54) BSC

0.405 (10.29) MAX

MAX

58

0.070 (1.78)

0.030 (0.76)

0.310 (7.87)

0.220 (5.59)

0.060 (1.52)

0.015 (0.38)

0.150 (3.81) MIN

SEATING PLANE

(Q-8)

0.320 (8.13)

0.290 (7.37)

15°

0°

COMPLIANT TO JEDEC STANDARDS MS-001

Narrow Body (N-8)

Dimensions shown in inches and (millimeters)

4.00 (0.1574)

3.80 (0.1497)

0.25 (0.0098)

0.10 (0.0040)

COPLANARI TY

0.10

0.015 (0.38)

0.008 (0.20)

SEATING

PLANE

CONTROL LING DIMENSI ONS ARE IN MIL LIMET ERS; IN CH DIMENSI ONS (IN PARENTHESES) ARE ROUNDED-OFF MILLIMETER EQUIVALENTS FOR REFERENCE ONLY AND ARE NOT APPROPRIATE FOR USE IN DESIGN.

Figure 38. 8-Lead Standard Small Outline Package [SOIC_N]

070606-A

5.00 (0.1968)

4.80 (0.1890)

85

1

1.27 (0.0500)

COMPLIANT TO JEDE C STANDARDS MS-012-A A

BSC

6.20 (0. 2441)

5.80 (0. 2284)

4

1.75 (0.0688)

1.35 (0.0532)

0.51 (0.0201)

0.31 (0.0122)

8° 0°

0.25 (0.0098)

0.17 (0.0067)

0.50 (0. 0196)

0.25 (0. 0099)

Narrow Body (R-8)

Dimensions shown in millimeters and (inches)

1.27 (0. 0500)

0.40 (0. 0157)

⋅ 45°

012407-A

Rev. H | Page 18 of 20

AD736

ORDERING GUIDE

Model Temperature Range Package Description Package Option

AD736AQ –40°C to +85°C 8-Lead CERDIP Q-8 AD736BQ –40°C to +85°C 8-Lead CERDIP Q-8 AD736AR –40°C to +85°C 8-Lead SOIC_N R-8 AD736AR-REEL –40°C to +85°C 8-Lead SOIC_N R-8 AD736AR-REEL7 –40°C to +85°C 8-Lead SOIC_N R-8 AD736ARZ AD736ARZ-R7 AD736ARZ-RL AD736BR –40°C to +85°C 8-Lead SOIC_N R-8 AD736BR-REEL –40°C to +85°C 8-Lead SOIC_N R-8 AD736BR-REEL7 –40°C to +85°C 8-Lead SOIC_N R-8 AD736BRZ AD736BRZ-R7 AD736BRZ-RL AD736JN 0°C to +70°C 8-Lead PDIP N-8 AD736JNZ AD736KN 0°C to +70°C 8-Lead PDIP N-8 AD736KNZ AD736JR 0°C to +70°C 8-Lead SOIC_N R-8 AD736JR-REEL 0°C to +70°C 8-Lead SOIC_N R-8 AD736JR-REEL7 0°C to +70°C 8-Lead SOIC_N R-8 AD736JRZ AD736JRZ-RL AD736JRZ-R7 AD736KR 0°C to +70°C 8-Lead SOIC_N R-8 AD736KR-REEL 0°C to +70°C 8-Lead SOIC_N R-8 AD736KR-REEL7 0°C to +70°C 8-Lead SOIC_N R-8 AD736KRZ AD736KRZ-RL AD736KRZ-R7 AD736-EVALZ

1

Z = RoHS compliant part.

1

–40°C to +85°C 8-Lead SOIC_N R-8 –40°C to +85°C 8-Lead SOIC_N R-8 –40°C to +85°C 8-Lead SOIC_N R-8

0°C to +70°C 8-Lead PDIP N-8

0°C to +70°C 8-Lead SOIC_N R-8 0°C to +70°C 8-Lead SOIC_N R-8 0°C to +70°C 8-Lead SOIC_N R-8

0°C to +70°C 8-Lead SOIC_N R-8 0°C to +70°C 8-Lead SOIC_N R-8 0°C to +70°C 8-Lead SOIC_N R-8 Evaluation Board

Rev. H | Page 19 of 20

AD736

NOTES

Rev. H | Page 20 of 20

Datasheet AD736 Datasheet (ANALOG DEVICES)

Specifications and Main Features

Frequently Asked Questions

User Manual

FEATURES

GENERAL DESCRIPTION

FUNCTIONAL BLOCK DIAGRAM

PRODUCT HIGHLIGHTS

TABLE OF CONTENTS

REVISION HISTORY

SPECIFICATIONS

ABSOLUTE MAXIMUM RATINGS

ESD CAUTION

PIN CONFIGURATION AND FUNCTION DESCRIPTIONS

TYPICAL PERFORMANCE CHARACTERISTICS

THEORY OF OPERATION

TYPES OF AC MEASUREMENT

CALCULATING SETTLING TIME USING FIGURE 16

RAPID SETTLING TIMES VIA THE AVERAGE RESPONDING CONNECTION

DC ERROR, OUTPUT RIPPLE, AND AVERAGING ERROR

AC MEASUREMENT ACCURACY AND CREST FACTOR

APPLICATIONS

CONNECTING THE INPUT

EVALUATION BOARD

OUTLINE DIMENSIONS

ORDERING GUIDE