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Low Cost, Low Power,

True RMS-to-DC Converter

Data Sheet

AD737

Rev. I

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FEATURES

Computes

True rms value

Average rectified value

Absolute value

Provides

200 mV full-scale input range (larger inputs with

input scaling)

Direct interfacing with 3½ digit CMOS ADCs

High input impedance: 10

Ω

Low input bias current: 25 pA maximum

High accuracy: ±0.2 mV ± 0.3% of reading

RMS conversion with signal crest factors up to 5

Wide power supply range: ±2.5 V to ±16.5 V

Low power: 25 µA (typical) standby current

No external trims needed for specified accuracy

The AD737 output is negative-going; the AD736 is a positive

output-going version of the same basic device

FUNCTIONAL BLOCK DIAGRAM

00828-001

COM

OUTPUT

–V

8kΩ

BIAS

SECTION

ABSOLUTE

VALUE

CIRCUIT

SQUARER

DIVIDER

POWER

DOWN

8kΩ

Figure 1.

GENERALDESCRIPTION

The AD737 is a low power, precision, monolithic, true rms-to-

dc converter. It is laser trimmed to provide a maximum error of

±0.2 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

physical size of this converter make it suitable for upgrading

the performance of non-rms precision rectifiers in many

applications. Compared to these circuits, the AD737 offers

higher accuracy at equal or lower cost.

The AD737 can compute the rms value of both ac and dc input

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

external capacitor. In this mode, the AD737 can resolve input

signal levels of 100 µV rms or less, despite variations in tem-

perature 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.

The AD737 has no output buffer amplifier, thereby significantly

reducing dc offset errors occurring at the output, which makes

the device highly compatible with high input impedance ADCs.

Requiring only 160 µA of power supply current, the AD737 is

optimized for use in portable multimeters and other battery-

powered applications. In power-down mode, the standby supply

current in is typically 25 µA.

The AD737 has both high (10

Ω) and low impedance input

options. The high-Z FET input connects high source impedance

input attenuators, and a low impedance (8 kΩ) input accepts

rms voltages to 0.9 V while operating from the minimum power

supply voltage of ±2.5 V. The two inputs can be used either

single ended or differentially.

The AD737 achieves 1% of reading error bandwidth, exceeding

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

while consuming only 0.72 mW.

The AD737 is available in two performance grades. The AD737J

and AD737K grades operate over the commercial temperature

range of 0°C to 70°C. The AD737JR-5 is tested with supply

voltages of ±2.5 V dc. The AD737A grade operates over the

industrial temperature range of −40°C to +85°C. The AD737 is

available in two low cost, 8lead packages: PDIP and SOIC_N.

PRODUCT HIGHLIGHTS

1. Computes average rectified, absolute, or true rms value of a

signal regardless of waveform.

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

required for the AD737 to perform true rms measurement.

3. The standby power consumption of 125 μW makes the

AD737 suitable for battery-powered applications.

Summary of content (24 pages)

PAGE 1
Low Cost, Low Power, True RMS-to-DC Converter AD737 Data Sheet FEATURES FUNCTIONAL BLOCK DIAGRAM CC 8kΩ 8kΩ COM CF ABSOLUTE VALUE CIRCUIT VIN SQUARER DIVIDER OUTPUT CAV +VS POWER DOWN CAV BIAS SECTION –VS 00828-001 Computes True rms value Average rectified value Absolute value Provides 200 mV full-scale input range (larger inputs with input scaling) Direct interfacing with 3½ digit CMOS ADCs High input impedance: 1012 Ω Low input bias current: 25 pA maximum High accuracy: ±0.2 mV ± 0.
PAGE 2
AD737 Data Sheet TABLE OF CONTENTS Features .............................................................................................. 1 DC Error, Output Ripple, and Averaging Error ..................... 13 Functional Block Diagram .............................................................. 1 AC Measurement Accuracy and Crest Factor ........................ 13 General Description ......................................................................... 1 Calculating Settling Time..............
PAGE 3
Data Sheet AD737 SPECIFICATIONS TA = 25°C, ±VS = ±5 V except as noted, CAV = 33 µF, CC = 10 µF, f = 1 kHz, sine wave input applied to Pin 2, unless otherwise specified. Specifications shown in boldface are tested on all production units at final electrical test. Results from these tests are used to calculate outgoing quality levels. Table 1. Parameter ACCURACY Total Error Test Conditions/ Comments Min EIN = 0 to 200 mV rms ±VS = ±2.5 V 0.2/0.3 ±VS = ±2.
PAGE 4
AD737 Parameter Peak Transient Input Input Resistance Input Bias Current Low-Z Input (Pin 1) Signal Range Continuous RMS Level Peak Transient Input Data Sheet Test Conditions/ Comments ±VS = +2.5 V input to Pin 1 ±VS = +2.8 V/−3.2 V ±VS = ±5 V ±VS = ±16.5 V Min OUTPUT CHARACTERISTICS Output Voltage Range Output Resistance FREQUENCY RESPONSE High-Z Input (Pin 2) 1% Additional Error Min ±0.9 AD737K Typ Max AD737J-5 Typ Max 1012 1 ±4.0 1012 1 25 1012 1 25 ±VS = +2.5 V ±VS = +2.8 V/−3.
PAGE 5
Data Sheet Parameter 3 dB Bandwidth Low-Z Input (Pin 1) 1% Additional Error 3 dB Bandwidth POWER-DOWN MODE Disable Voltage Input Current, PD Enabled POWER SUPPLY Operating Voltage Range Current AD737 Test Conditions/ Comments VIN = 1 mV rms VIN = 10 mV rms VIN = 100 mV rms VIN = 200 mV rms Min AD737A, AD737J Typ Max 5 55 170 190 Min AD737K Typ Max 5 55 170 190 Min AD737J-5 Typ Max 5 55 170 190 Unit kHz kHz kHz kHz VIN = 1 mV rms 1 1 1 kHz VIN = 10 mV rms VIN = 40 mV rms VIN = 100 mV rms VIN
PAGE 6
AD737 Data Sheet ABSOLUTE MAXIMUM RATINGS THERMAL RESISTANCE Table 2. Parameter Supply Voltage Internal Power Dissipation Input Voltage Pin 1 Pin 2 to Pin 8 Output Short-Circuit Duration Differential Input Voltage Storage Temperature Range Lead Temperature, Soldering (60 sec) ESD Rating θJA is specified for the worst-case conditions, that is, a device soldered in a circuit board for surface-mount packages. Rating ±16.5 V 200 mW Table 3.
PAGE 7
Data Sheet AD737 PIN CONFIGURATIONS AND FUNCTION DESCRIPTIONS 8 COM 7 +VS POWER DOWN 3 6 OUTPUT TOP VIEW –VS 4 (Not to Scale) 5 CAV CC 1 VIN 2 POWER DOWN 3 –VS 4 Figure 2. SOIC_N Pin Configuration (R-8) Mnemonic CC VIN POWER DOWN –VS CAV OUTPUT +VS COM COM 7 +VS TOP VIEW (Not to Scale) 6 OUTPUT 5 CAV Figure 3. PDIP Pin Configuration (N-8) Table 4. Pin Function Descriptions Pin No. 1 2 3 4 5 6 7 8 8 AD737 Description Coupling Capacitor for Indirect DC Coupling. RMS Input.
PAGE 8
AD737 Data Sheet TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C, ±VS = ±5 V (except AD737J-5, where ±VS = ±2.5 V), CAV = 33 µF, CC = 10 µF, f = 1 kHz, sine wave input applied to Pin 2, unless otherwise specified. 10V VIN = 200mV rms CAV = 100µF CF = 22µF CAV = 22µF, CF = 4.7µF, CC = 22µF 1V INPUT LEVEL (rms) 0.5 0.3 0.1 0 –0.1 100mV 1% ERROR 10mV –3dB 1mV –0.3 0 2 4 10 6 8 SUPPLY VOLTAGE (±V) 12 14 100µV 0.1 16 Figure 4. Additional Error vs.
PAGE 9
Data Sheet AD737 1.0 VIN = 200mV rms CAV = 100µF CF = 22µF 0.5 0.4 0.2 0 –0.2 –0.4 –0.8 –60 –40 –20 0 20 40 60 80 TEMPERATURE (°C) 100 120 –1.5 CAV = 22µF, CC = 47µF, CF = 4.7µF 100mV INPUT LEVEL (rms) 1V 2V Figure 13. Error vs. RMS Input Level Using Circuit in Figure 29 100 500 VIN = 200mV rms CC = 47µF CF = 47µF AVERAGING CAPACITOR (µF) 400 300 200 100 10 –0.5% 00828-012 –1% 0 0.2 0.4 0.6 RMS INPUT LEVEL (V) 0.8 1 10 1.0 Figure 11. DC Supply Current vs.
PAGE 10
AD737 Data Sheet 4.0 10nA 1nA INPUT BIAS CURRENT 3.0 2.5 2.0 10pA 1pA 00828-017 1.5 1.0 100pA 0 2 4 6 8 10 SUPPLY VOLTAGE (±V) 12 14 100fA –55 16 Figure 16. Input Bias Current vs. Supply Voltage 00828-019 INPUT BIAS CURRENT (pA) 3.5 –35 –15 5 25 45 65 TEMPERATURE (°C) 85 105 125 Figure 18. Input Bias Current vs. Temperature 1V 10V VS = ±2.5V, CAV = 22µF, CF = 4.
PAGE 11
Data Sheet AD737 1.0 10V VS = ±2.5V, CAV = 22µF, CF = 4.7µF, CC = 22µF 0.5 ERROR (% of Reading) INPUT LEVEL (rms) 1V 100mV 0.5% 10mV 0 –0.5 –1.0 –1.5 00828-021 –3dB 1% 100µV 0.1 1 10 FREQUENCY (kHz) 100 1000 Figure 20. Error Contours Driving Pin 1 CAV = 10µF CAV = 22µF CAV = 33µF 3 CAV = 100µF 2 CAV = 220µF 1 0 00828-022 ADDITIONAL ERROR (% of Reading) 4 1 2 3 CREST FACTOR 4 CAV = 22µF, VS = ±2.5V CC = 47µF, CF = 4.7µF –2.5 10mV 100mV INPUT LEVEL (rms) 1V Figure 22.
PAGE 12
AD737 Data Sheet THEORY OF OPERATION external averaging capacitor, CF. In the rms circuit, this additional filtering stage reduces any output ripple that was not removed by the averaging capacitor. As shown in Figure 23, the AD737 has four functional subsections: an input amplifier, a full-wave rectifier, an rms core, and a bias section. The FET input amplifier allows a high impedance, buffered input at Pin 2 or a low impedance, wide dynamic range input at Pin 1.
PAGE 13
Data Sheet AD737 DC ERROR, OUTPUT RIPPLE, AND AVERAGING ERROR AC MEASUREMENT ACCURACY AND CREST FACTOR Figure 24 shows the typical output waveform of the AD737 with a sine wave input voltage applied. As with all real-world devices, the ideal output of VOUT = VIN is never exactly achieved; instead, the output contains both a dc and an ac error component. The crest factor of the input waveform is often overlooked when determining the accuracy of an ac measurement.
PAGE 14
AD737 Data Sheet APPLICATIONS INFORMATION 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 is connected across a diode in the rms core, the averaging time constant (τAV) increases exponentially as the input signal decreases.
PAGE 15
Data Sheet AD737 Battery Operation All the level-shifting for battery operation is provided by the 3½ digit converter, shown in Figure 27. Alternatively, an external op amp adds flexibility by accommodating nonzero common-mode voltages and providing output scaling and offset to zero. When an external operational amplifier is used, the output polarity is positive going. Figure 28 shows an op amp used in a single-supply application.
PAGE 16
AD737 Data Sheet 20kΩ 1µF +VS + AD589 1PRV 0.01µF VIN + CC 10µF CC 200mV 8kΩ 1 1.23V COM AD737 8 1N4148 9MΩ FULL-WAVE RECTIFIER VIN 2V 2 900kΩ 20V 90kΩ 47kΩ 1W POWER DOWN –VS 10kΩ REF HIGH REF LOW +V 7 COMMON OUTPUT BIAS SECTION 3 50kΩ DIGIT ICL7136 TYPE CONVERTER +VS 8kΩ INPUT AMPLIFIER 1N4148 200V 200kΩ 31/2 1MΩ 0.1µF 1µF 9V ANALOG CAV RMS CORE 4 + LOW 6 HIGH 5 + + –VS 33µF 00828-027 SWITCH CLOSED ACTIVATES POWER-DOWN MODE.
PAGE 17
Data Sheet AD737 13 CC 10µF CC + 8kΩ FULL-WAVE RECTIFIER 2 INPUT AMPLIFIER 1kΩ 3500PPM/°C 8kΩ PRECISION RESISTOR CORP TYPE PT/ST 60.4Ω 8 NC 7 +VS COM VIN 14 12 * AD737 1 Q1 SCALE FACTOR TRIM 2kΩ 31.6kΩ OUTPUT POWER 3 DOWN BIAS SECTION 2 6 AD711 –VS CAV RMS CORE 4 3 * 10 Q2 11 + IREF R1** 9 00828-030 NC = NO CONNECT *Q1, Q2 PART OF RCA CA3046 OR SIMILAR NPN TRANSISTOR ARRAY. 4.3V **R1 + R CAL IN Ω = 10,000 × 0dB INPUT LEVEL IN V Figure 30.
PAGE 18
AD737 Data Sheet AD737 EVALUATION BOARD 00828-033 An evaluation board, AD737-EVALZ, is available for experiments or for becoming familiar with rms-to-dc converters. Figure 32 is a photograph of the board; Figure 34 to Figure 37 show the signal and power plane copper patterns. The board is designed for multipurpose applications and can be used for the AD736 as well. Although not shipped with the board, an optional socket that accepts the 8lead surface-mount package is available from Enplas Corp.
PAGE 19
Data Sheet AD737 –VS GND1 GND2 GND3 GND4 +VS C1 + 10µF 25V W1 DC COUP LO-Z W4 LO-Z IN + C2 10µF 25V –VS +VS W3 AC COUP R3 0Ω + CC J1 CIN 0.1µF DUT P2 HI-Z SEL HI-Z AD737 1 IN COM CC 8 2 V IN GND 7 +VS 3 POWER DOWN OUTPUT 6 W2 R1 1MΩ +VS 4 –VS J3 PD FILT NORM –VS SEL PIN3 CAV R4 0Ω +VS C6 0.1µF 5 CAV VOUT J2 CF1 CAV 33 µF 16V + C4 0.1µF CF2 Figure 38. AD737 Evaluation Board Schematic Rev.
PAGE 20
AD737 Data Sheet OUTLINE DIMENSIONS 5.00 (0.1968) 4.80 (0.1890) 1 5 6.20 (0.2441) 5.80 (0.2284) 4 1.27 (0.0500) BSC 0.25 (0.0098) 0.10 (0.0040) 1.75 (0.0688) 1.35 (0.0532) 0.51 (0.0201) 0.31 (0.0122) COPLANARITY 0.10 SEATING PLANE 0.50 (0.0196) 0.25 (0.0099) 45° 8° 0° 0.25 (0.0098) 0.17 (0.0067) 1.27 (0.0500) 0.40 (0.
PAGE 21
Data Sheet AD737 ORDERING GUIDE Model 1 AD737ANZ AD737ARZ AD737JNZ AD737JRZ AD737JRZ-R7 AD737JRZ-RL AD737JRZ-5 AD737JRZ-5-R7 AD737JRZ-5-RL AD737KR-REEL AD737KR-REEL7 AD737KRZ-RL AD737KRZ-R7 AD737-EVALZ 1 Temperature Range −40°C to +85°C −40°C to +85°C 0°C to 70°C 0°C to 70°C 0°C to 70°C 0°C to 70°C 0°C to 70°C 0°C to 70°C 0°C to 70°C 0°C to 70°C 0°C to 70°C 0°C to 70°C 0°C to 70°C Package Description 8-Lead Plastic Dual In-Line Package [PDIP] 8-Lead Standard Small Outline Package [SOIC_N] 8-Lead Plastic
PAGE 22
AD737 Data Sheet NOTES Rev.
PAGE 23
Data Sheet AD737 NOTES Rev.