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 analog-to-digital
converters (ADCs)
High input impedance: 1012 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
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 the AD737 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 computes the rms value of both ac and dc input
voltages, and is ac-coupled by adding an input capacitor. In this
mode, the AD737 resolves input signal levels of 100 μV rms or
less, despite variations in temperature or supply voltage. High
accuracy is maintained for input waveforms with crest factors of
1 to 3 and crest factors at 2.5% or less with respect to full-scale
input level.
The AD737 has both high (1012 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 of up 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, 8lead packages: PDIP and SOIC_N.
PRODUCT HIGHLIGHTS
The AD737 has no output buffer amplifier, thereby significantly
reducing dc offset errors occurring at the output and making
the device highly compatible with high input impedance ADCs.
1.
Requiring only 160 μA of power supply current, the AD737 is
optimized for use in portable multimeters and other batterypowered applications. In power-down mode, the standby supply
current in is typically 25 μA.
3.
Rev. J
2.
Computes the average rectified, absolute, or true rms value
of a signal regardless of waveform.
Only one external component, an averaging capacitor, is
required for the AD737 to perform true rms measurement.
The standby power consumption of 125 μW makes the
AD737 suitable for battery-powered applications.
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AD737
Data Sheet
TABLE OF CONTENTS
Features .............................................................................................. 1
DC Error, Output Ripple, and Averaging Error .................... 14
Functional Block Diagram .............................................................. 1
AC Measurement Accuracy and Crest Factor ........................ 14
General Description ......................................................................... 1
Calculating Settling Time .......................................................... 14
Product Highlights ........................................................................... 1
Applications Information .............................................................. 15
Revision History ............................................................................... 3
RMS Measurement—Choosing an Optimum Value for CAV 15
Specifications..................................................................................... 4
Absolute Maximum Ratings............................................................ 7
Rapid Settling Times via the Average Responding
Connection.................................................................................. 15
Thermal Resistance ...................................................................... 7
Selecting Practical Values for Capacitors ................................ 15
ESD Caution .................................................................................. 7
Scaling Input and Output Voltages .......................................... 15
Pin Configurations and Function Descriptions ........................... 8
Additional Information ............................................................. 16
Typical Performance Characteristics ............................................. 9
AD737 Evaluation Board............................................................... 19
Theory of Operation ...................................................................... 13
Outline Dimensions ....................................................................... 21
Types of AC Measurement ........................................................ 13
Ordering Guide .......................................................................... 22
Rev. J | Page 2 of 24
Data Sheet
AD737
REVISION HISTORY
10/15—Rev. I to Rev. J
Changes to General Description Section ....................................... 1
Changes to Table 4 ............................................................................ 8
Updated Typical Performance Characteristics Section Format;
Reordered Figures ............................................................................. 9
Changes to Figure 4 to Figure 6....................................................... 9
Change to Types of AC Measurement Section ...........................13
Changes to Figure 23 ......................................................................13
Changes to Figure 25 ......................................................................15
Added Additional Information Section .......................................16
Changes to Figure 27 to Figure 31 ................................................17
Change to Figure 38 ........................................................................20
6/12—Rev. H to Rev. I
Removed CERDIP Package............................................... Universal
Changes to Features, General Description, Product Highlights
Sections and Figure 1 ........................................................................ 1
Changes to Table 1 ............................................................................ 3
Changes to Table 2 ............................................................................ 6
Deleted Figure 3, Renumbered Sequentially ................................. 7
Changes to Figure 5, Figure 7, and Figure 8 Captions ................. 8
Changes to Figure 12 Caption ......................................................... 9
Changes to Figure 19 Caption .......................................................10
Changes to Figure 23 ......................................................................12
Changes to Figure 26 ......................................................................14
Changes to Scaling the Output Voltage Section ..........................15
Changes to Figure 27 ......................................................................16
Deleted Table 7 ................................................................................19
Updated Outline Dimensions ........................................................20
Changes to Ordering Guide ...........................................................21
10/08—Rev. G to Rev. H
Added Selectable Average or RMS Conversion Section and
Figure 27 ...........................................................................................14
Updated Outline Dimensions ........................................................20
Changes to Ordering Guide ...........................................................22
12/06—Rev. F to Rev. G
Changes to Specifications ................................................................. 3
Reorganized Typical Performance Characteristics ....................... 8
Changes to Figure 21 ......................................................................11
Reorganized Theory of Operation Section ..................................12
Reorganized Applications Section ................................................ 14
Added Scaling Input and Output Voltages Section .................... 14
Deleted Application Circuits Heading ......................................... 16
Changes to Figure 28 ...................................................................... 16
Added AD737 Evaluation Board Section .................................... 18
Updated Outline Dimensions........................................................ 20
hanges to Ordering Guide ............................................................. 21
1/05—Rev. E to Rev. F
Updated Format ................................................................. Universal
Added Functional Block Diagram .................................................. 1
Changes to General Description Section ....................................... 1
Changes to Pin Configurations and Function
Descriptions Section ......................................................................... 6
Changes to Typical Performance Characteristics Section ........... 7
Changes to Table 4 .......................................................................... 11
Change to Figure 24 ........................................................................ 12
Change to Figure 27 ........................................................................ 15
Changes to Ordering Guide ........................................................... 18
6/03—Rev. D to Rev. E
Added AD737JR-5 ............................................................. Universal
Changes to Features .......................................................................... 1
Changes to General Description ..................................................... 1
Changes to Specifications ................................................................ 2
Changes to Absolute Maximum Ratings........................................ 4
Changes to Ordering Guide ............................................................. 4
Added TPCs 16 through 19 ............................................................. 6
Changes to Figures 1 and 2 .............................................................. 8
Changes to Figure 8 ........................................................................ 11
Updated Outline Dimensions........................................................ 12
12/02—Rev. C to Rev. D
Changes to Functional Block Diagram .......................................... 1
Changes to Pin Configuration......................................................... 4
Figure 1 Replaced .............................................................................. 8
Changes to Figure 2 .......................................................................... 8
Figure 5 Replaced ............................................................................ 10
Changes to Application Circuits Figures 4, 6–8 .......................... 10
Outline Dimensions Updated........................................................ 12
12/99—Rev. B to Rev. C
Rev. J | Page 3 of 24
AD737
Data Sheet
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 = ±2.5 V,
input to Pin 1
EIN = 200 mV to
1 V rms
Over Temperature
JN, JR, KR
AN and AR
AD737A, AD737J
Typ
Max
−1.2
EIN = 200 mV rms,
±VS = ±2.5 V
EIN = 200 mV rms,
±VS = ±2.5 V
Min
AD737K
Typ
Max
0.2/0.2
0.4/0.5
−1.2
±2.0
0.007
0.007
0.014
0.014
Min
AD737J-5
Typ
Max
Unit
±mV/±POR1
0.2/0.3
0.2/0.3
0.4/0.5
±mV/±POR1
0.2/0.3
0.4/0.5
±mV/±POR1
POR
±2.0
0.02
±POR/°C
±POR/°C
vs. Supply Voltage
DC Reversal Error
Nonlinearity2
Input to Pin 13
Total Error,
External Trim
ADDITIONAL CREST
FACTOR ERROR4
For Crest Factors
from 1 to 3
For Crest Factors
from 3 to 5
INPUT CHARACTERISTICS
High-Z Input (Pin 2)
Signal Range
Continuous RMS
Level
EIN = 200 mV rms,
±VS = ±2.5 V to ±5 V
EIN = 200 mV rms,
±VS = ±5 V to
±16.5 V
DC-coupled,
VIN = 600 mV dc
±VS = ±2.5 V
VIN = 200 mV dc
EIN = 0 mV to
200 mV rms,
at 100 mV rms
AC coupled,
EIN = 100 mV rms,
after correction,
±VS = ±2.5 V
EIN = 0 mV to
200 mV rms
CAV = CF = 100 μF
CAV = 22 μF, CF =
100 μF, ±VS = ±2.5 V,
input to Pin 1
CAV = CF = 100 μF
0
−0.18
−0.3
0
−0.18
−0.3
0
−0.18
−0.3
%/V
0
0.06
0.1
0
0.06
0.1
0
0.06
0.1
%/V
1.3
2.5
1.3
2.5
1.7
0
0.25
0.35
0
0.25
0.1/0.2
0.7
0.7
0.1
0.1/0.2
%
%
2.5
%
200
200
1
Rev. J | Page 4 of 24
POR
±mV/±POR
1.7
2.5
POR
POR
0.02
0.1/0.2
2.5
0.35
±VS = +2.5 V
±VS = +2.8 V/−3.2 V
±VS = ±5 V to
±16.5 V
POR
200
1
mV rms
mV rms
V rms
Data Sheet
Parameter
Peak Transient Input
Input Resistance
Input Bias Current
Low-Z Input (Pin 1)
Signal Range
Continuous RMS Level
Peak Transient Input
Input Resistance
Maximum Continuous
Nondestructive
Input
Input Offset Voltage5
Over the Rated
Operating
Temperature
Range
vs. Supply
OUTPUT
CHARACTERISTICS
Output Voltage
Range
Output Resistance
FREQUENCY RESPONSE
High-Z Input (Pin 2)
1% Additional Error
3 dB Bandwidth
Low-Z Input (Pin 1)
1% Additional Error
3 dB Bandwidth
AD737
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
AD737A, AD737J
Typ
Max
Min
±0.9
AD737K
Typ
Max
±2.7
±4.0
1012
1
±VS = +2.5 V
±VS = +2.8 V/−3.2 V
±VS = ±5 V to
±16.5 V
±VS = +2.5 V
±VS = +2.8 V/−3.2 V
±VS = ±5 V
±VS = ±16.5 V
AD737J-5
Typ
Max
±0.9
±2.7
±VS = ±5 V
Min
±0.6
±4.0
1012
1
25
1012
1
25
25
300
300
1
300
1
±1.7
6.4
±1.7
±3.8
±11
8
All supply voltages
AC-coupled
9.6
±12
6.4
±1.7
±3.8
±11
8
9.6
±12
8
±3
30
8
±3
30
80
50
150
80
50
150
6.4
8
8
Unit
V
V
V
V
Ω
pA
mV rms
mV rms
V rms
9.6
±12
V
V
V
V
kΩ
V p-p
±3
30
mV
μV/°C
VS = ±2.5 V to ±5 V
VS = ±5 V to ±16.5 V
No load, output is
negative with
respect to COM
±VS = +2.8 V/−3.2 V
−1.6
−1.7
−1.6
−1.7
V6
±VS = ±5 V
−3.3
−3.4
−3.3
−3.4
±VS = ±16.5 V
±VS = ±2.5 V, input
to Pin 1
DC
−4
−5
−4
−5
V6
V
6.4
8
9.6
6.4
8
80
9.6
−1.1
–0.9
6.4
8
μV/V
μV/V
V6
9.6
kΩ
VIN = 1 mV rms
VIN = 10 mV rms
VIN = 100 mV rms
VIN = 200 mV rms
VIN = 1 mV rms
VIN = 10 mV rms
VIN = 100 mV rms
VIN = 200 mV rms
1
6
37
33
5
55
170
190
1
6
37
33
5
55
170
190
1
6
37
33
5
55
170
190
kHz
kHz
kHz
kHz
kHz
kHz
kHz
kHz
VIN = 1 mV rms
VIN = 10 mV rms
VIN = 40 mV rms
VIN = 100 mV rms
VIN = 200 mV rms
VIN = 1 mV rms
VIN = 10 mV rms
VIN = 100 mV rms
VIN = 200 mV rms
1
6
1
6
90
90
5
55
350
460
90
90
5
55
350
460
1
6
25
90
90
5
55
350
460
kHz
kHz
kHz
kHz
kHz
kHz
kHz
kHz
kHz
Rev. J | Page 5 of 24
AD737
Parameter
POWER-DOWN MODE
Disable Voltage
Input Current,
PD Enabled
POWER SUPPLY
Operating Voltage
Range
Current
Data Sheet
Test Conditions/
Comments
Min
AD737A, AD737J
Typ
Max
0
11
VPD = VS
+2.8/
−3.2
No input
Rated input
Powered down
Min
AD737K
Typ
Max
Min
AD737J-5
Typ
Max
0
11
±5
±16.5
120
170
25
160
210
40
+2.8/
−3.2
1
Unit
V
μA
±5
±16.5
120
170
25
160
210
40
±2.5
±5
±16.5
V
120
170
25
160
210
40
μA
μA
μA
POR is % of reading.
Nonlinearity is defined as the maximum deviation (in percent error) from a straight line connecting the readings at 0 V and at 200 mV rms.
After fourth-order error correction using the equation
y = −0.31009x4 − 0.21692x3 − 0.06939x2 + 0.99756x + 11.1 × 10−6
where y is the corrected result and x is the device output between 0.01 V and 0.3 V.
4
Crest factor error is specified as the additional error resulting from the specific crest factor, using a 200 mV rms signal as a reference. The crest factor is defined as
VPEAK/V rms.
5
DC offset does not limit ac resolution.
6
Value is measured with respect to COM.
2
3
Rev. J | Page 6 of 24
Data Sheet
AD737
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
Rating
±16.5 V
200 mW
θJA is specified for the worst-case conditions, that is, a device
soldered in a circuit board for surface-mount packages.
±12 V
±VS
Indefinite
+VS and −VS
−65°C to +125°C
300°C
500 V
Package Type
8-Lead PDIP (N-8)
8-Lead SOIC_N (R-8)
Table 3. Thermal Resistance
ESD CAUTION
Stresses at or above those listed under Absolute Maximum
Ratings may cause permanent damage to the product. This is a
stress rating only; functional operation of the product at these
or any other conditions above those indicated in the operational
section of this specification is not implied. Operation beyond
the maximum operating conditions for extended periods may
affect product reliability.
Rev. J | Page 7 of 24
θJA
165
155
Unit
°C/W
°C/W
AD737
Data Sheet
AD737
8
COM
CC 1
7
+VS
VIN 2
POWER DOWN 3
6 OUTPUT
TOP VIEW
–VS 4 (Not to Scale) 5 CAV
POWER DOWN 3
00828-002
CC 1
VIN 2
–VS 4
8
COM
AD737
7
+VS
TOP VIEW
(Not to Scale)
6
OUTPUT
5
CAV
00828-004
PIN CONFIGURATIONS AND FUNCTION DESCRIPTIONS
Figure 3. PDIP Pin Configuration (N-8)
Figure 2. SOIC_N Pin Configuration (R-8)
Table 4. Pin Function Descriptions
Pin No.
1
Mnemonic
CC
2
3
VIN
POWER DOWN
4
5
6
7
8
–VS
CAV
OUTPUT
+VS
COM
Description
Coupling Capacitor Connection for Indirect DC Coupling to Pin 2. In addition, CC is an alternative low
impedance input access to an 8 kΩ voltage to current (V to I) resistor.
RMS Input to FET.
Disables the AD737. When Pin 3 is grounded or pulled low, the AD737 is enabled; when Pin 3 is pulled high, it
changes to power saving mode.
Negative Power Supply.
Averaging Capacitor Connection.
DC Output (Negative Going Polarity).
Positive Power Supply.
Common.
Rev. J | Page 8 of 24
Data Sheet
AD737
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
0.7
VIN = 200mV rms
CAV = 100µF
CF = 22µF
1V
100mV
1% ERROR
10mV
–3dB
1mV
100µV
0.1
1
10
FREQUENCY (kHz)
100
00828-008
10% ERROR
0.5
0.3
0.1
0
–0.1
–0.3
–0.5
1000
00828-005
ADDITIONAL ERROR (% of Reading)
INPUT/OUTPUT LEVEL (rms/|DC|)
CAV = 22µF, C F = 4.7µF, C C = 22µF
0
6
8
10
SUPPLY VOLTAGE (±V)
12
14
16
14
16
16
10V
DC COUPLED
PEAK INPUT BEFORE CLIPPING (V)
CAV = 22µF, C F = 4.7µF, C C = 22µF
1V
100mV
1% ERROR
10mV
10% ERROR
1mV
100µV
0.1
1
10
FREQUENCY (kHz)
100
12
10
PIN 1
8
PIN 2
6
4
2
00828-009
–3dB
14
0
1000
00828-006
INPUT/OUTPUT LEVEL (rms/|DC|)
4
Figure 7. Additional Error vs. Supply Voltage
Figure 4. Frequency Response Driving Pin 1, Low-Z Input
(DC Output Polarity is Negative Going)
0
4
2
6
8
10
SUPPLY VOLTAGE (±V)
12
Figure 8. Peak Input Level for 1% Saturation vs. Supply Voltage
Figure 5. Frequency Response Driving Pin 2, High-Z Input
(DC Output Polarity is Negative Going)
25
10V
VS = ±2.5V,
CAV = 22µF, C F = 4.7µF, C C = 22µF
1V
SUPPLY CURRENT (µA)
INPUT/OUTPUT LEVEL (rms/|DC|)
2
100mV
10mV
20
15
10
1
10
FREQUENCY (kHz)
100
5
1000
Figure 6. Frequency Response with ±2.5V Supplies when Driving Pin 1,
Low-Z Input (DC Output Polarity is Negative Going)
00828-007
100µV
0.1
00828-020
1mV
0
2
4
6
8
10
12
14
DUAL SUPPLY VOLTAGE (±V)
16
18
Figure 9. Supply Current (Power-Down Mode) vs. Dual Supply Voltage
Rev. J | Page 9 of 24
AD737
Data Sheet
10mV
CAV = 10µF
CAV = 33µF
4
3
2
1
CAV = 100µF
CAV = 250µF
0
1
2
3
4
CREST FACTOR (VPEAK /V rms)
100µV
10µV
100
5
1k
10k
–3dB FREQUENCY (Hz)
100k
Figure 13. RMS Input Level vs. –3 dB Frequency; Negative DC Output
Figure 10. Additional Error vs. Crest Factor
1.0
0.8
VIN = 200mV rms
CAV = 100µF
CF = 22µF
0.6
0.5
0.4
ERROR (% of Reading)
0.2
0
–0.2
0
–0.5
–1.0
–1.5
–0.4
–2.0
–0.8
–60
00828-011
–0.6
–40
–20
0
20
40
60
80
TEMPERATURE (°C)
100
120
CAV = 22µF, CC = 47µF,
CF = 4.7µF
–2.5
10mV
140
00828-014
ADDITIONAL ERROR (% of Reading)
1mV
00828-013
5
AC-COUPLED
INPUT LEVEL (rms)
3ms BURST OF 1kHz =
3 CYCLES
200mV rms SIGNAL
CC = 22µF
CF = 100µF
00828-010
ADDITIONAL ERROR (% of Reading)
6
100mV
INPUT LEVEL (rms)
1V
2V
Figure 14. Error vs. RMS Input Level Using Circuit in Figure 29
Figure 11. Additional Error vs. Temperature
100
500
AVERAGING CAPACITOR (µF)
VIN = 200mV rms
CC = 47µF
CF = 47µF
300
200
10
–0.5%
100
0
0
0.2
0.4
0.6
RMS INPUT LEVEL (V)
0.8
1.0
1
10
00828-015
–1%
00828-012
DC SUPPLY CURRENT (µA)
400
100
FREQUENCY (Hz)
Figure 15. Value of Averaging Capacitor vs. Frequency
for Specified Averaging Error
Figure 12. DC Supply Current vs. RMS Input Level
Rev. J | Page 10 of 24
1k
Data Sheet
AD737
10nA
1V
–0.5%
1nA
INPUT BIAS CURRENT
100mV
10mV
1
10
100
10pA
00828-016
100fA
–55
1k
00828-019
1pA
AC-COUPLED
CAV = 10µF, CC = 47µF,
CF = 47µF
1mV
100pA
–35
45
65
25
5
TEMPERATURE (°C)
–15
FREQUENCY (Hz)
10V
4.0
VS = ±2.5V,
CAV = 22µF, CF = 4.7µF, CC = 22µF
3.5
1V
INPUT LEVEL (rms)
INPUT BIAS CURRENT (pA)
125
Figure 19. Input Bias Current vs. Temperature
Figure 16. RMS Input Level vs. Frequency for Specified Averaging Error
3.0
2.5
2.0
100mV
0.5%
10mV
10%
1mV
1.5
0
2
4
6
8
10
SUPPLY VOLTAGE (±V)
12
14
100µV
0.1
16
10
FREQUENCY (kHz)
100
1000
5
ADDITIONAL ERROR (% of Reading)
CC = 22µF
CF = 0µF
100mV
CAV = 10µF
CAV = 100µF
CAV = 33µF
00828-018
1mV
10ms
100ms
1s
SETTLING TIME
10s
100s
3 CYCLES OF 1kHz
200mV rms
VS = ±2.5V
CC = 22µF
CF = 100µF
4
CAV =
10µF
CAV =
22µF
CAV =
33µF
3
CAV =
100µF
2
CAV =
220µF
1
0
00828-022
1V
100µV
1ms
1
Figure 20. Error Contours Driving Pin 1
Figure 17. Input Bias Current vs. Supply Voltage
10mV
–3dB
1%
00828-017
1.0
INPUT LEVEL (rms)
105
85
00828-021
INPUT LEVEL (rms)
–1%
1
2
3
CREST FACTOR
4
5
Figure 21. Additional Error vs. Crest Factor for Various Values of CAV
Figure 18. RMS Input Level vs. Settling Time for Three Values of CAV
Rev. J | Page 11 of 24
AD737
Data Sheet
1.0
0
–0.5
–1.0
–1.5
–2.0
00828-023
ERROR (% of Reading)
0.5
CAV = 22µF, VS = ±2.5V
CC = 47µF, CF = 4.7µF
–2.5
10mV
100mV
INPUT LEVEL (rms)
1V
2V
Figure 22. Error vs. RMS Input Level Driving Pin 1
Rev. J | Page 12 of 24
Data Sheet
AD737
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.
The AD737 has four functional subsections: an input amplifier,
a full-wave rectifier, an rms core, and a bias section (see Figure 23).
The FET input amplifier allows a high impedance, buffered
input at Pin 2 or a low impedance, wide dynamic range input at
Pin 1. The high impedance input, with its low input bias
current, is ideal for use with high impedance input attenuators.
The input signal can be either dc-coupled or ac-coupled to the
input amplifier. Unlike other rms converters, the AD737
permits both direct and indirect ac coupling of the inputs. AC
coupling is provided by placing a series capacitor between the
input signal and Pin 2 (or Pin 1) for direct coupling and
between Pin 1 and ground (while driving Pin 2) for indirect
coupling.
Finally, the bias subsection permits a power-down function.
This reduces the idle current of the AD737 from 160 µA to
30 µA. This feature is selected by connecting Pin 3 to Pin 7 (+VS).
TYPES OF AC MEASUREMENT
The AD737 measures ac signals either by operating as an
average responding converter or by operating as 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 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 average absolute value of a sine wave voltage is 0.636 times
that of VPEAK; the corresponding rms value is 0.707 times VPEAK.
Therefore, for sine wave voltages, the required scale factor is
1.11 (0.707 divided by 0.636).
AC
CC = 10µF
+
DC
OPTIONAL RETURN PATH
CURRENT
MODE
ABSOLUTE
VALUE
CC
8
1
8kΩ
COM
VIN
VIN
+
8kΩ
2
7
+VS
CF
10µF
(OPTIONAL
LPF)
FET
OP AMP
IB < 10pA
POWER 3
DOWN
BIAS
SECTION
Mathematically, the rms value of a voltage is defined (using a
simplified equation) as
6
OUTPUT
V rms = Avg (V 2 )
RMS
TRANSLINEAR
CORE
–VS
4
5
CAV
CA
33µF
+
+VS
POSITIVE SUPPLY
00828-024
0.1µF
COMMON
0.1µF
NEGATIVE SUPPLY
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.
–VS
Figure 23. AD737 True RMS Circuit (Test Circuit)
The output of the input amplifier drives a full-wave precision
rectifier, which, in turn, drives the rms core. It is the core that
provides the essential rms operations of squaring, averaging,
and square rooting, using an external averaging capacitor, CAV.
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 pre-calibrated waveform; the
magnitude of the error depends on 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 has a computational error 11% (of
reading) higher than the true rms value (see Table 5).
The transfer function for the AD737 is
Without CAV, the rectified input signal passes through the core
unprocessed, as is done with the average responding connection
(see Figure 25). In the average responding mode, averaging is
carried out by an RC post filter consisting of an 8 kΩ internal
scale factor resistor connected between Pin 6 and Pin 8 and an
Rev. J | Page 13 of 24
VOUT = − Avg (VIN 2 )
AD737
Data Sheet
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. Crest factor is
defined as the ratio of the peak signal amplitude to the rms
amplitude (crest factor = VPEAK/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. These
types of waveforms require a long averaging time constant to
average out the long time periods between pulses. Figure 10
shows the additional error vs. the crest factor of the AD737 for
various values of CAV.
EO
IDEAL
EO
DC ERROR = EO – EO (IDEAL)
TIME
00828-026
AVERAGE EO = EO
DOUBLE-FREQUENCY
RIPPLE
CALCULATING SETTLING TIME
Figure 24. Output Waveform for Sine Wave Input Voltage
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. The dc
error component is, therefore, set solely by the value of the
averaging capacitor used—no amount of post filtering (using a
very large postfiltering capacitor, CF) allows the output voltage
to equal its ideal value. The ac error component, an output
ripple, can be easily removed using a large enough CF.
In most cases, the combined magnitudes of the dc and ac error
components must be considered when selecting appropriate values
for CAV and CF capacitors. This 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 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.
Figure 18 can be used to closely approximate the time required
for the AD737 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, an initial
rms input level of 100 mV, and a final (reduced) input level of
1 mV. From Figure 18, 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 inherent smoothness of the decay
characteristic of a capacitor/diode combination, this is the total
settling time to the final value (not the settling time to 1%, 0.1%,
and so on, of the final value). Also, this graph provides the
worst-case settling time because the AD737 settles very quickly
with increasing input levels.
Table 5. Error Introduced by an Average Responding Circuit When Measuring Common Waveforms
Type of Waveform 1 V Peak Amplitude
Undistorted Sine Wave
Symmetrical Square Wave
Undistorted Triangle Wave
Gaussian Noise (98% of Peaks
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