Low Cost, Low Power,
True RMS-to-DC Converter
AD736
Rev. H
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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
attenuator)
High input impedance: 1012 Ω
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-to-
dc 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 pro-
viding 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.
FUNCTIONAL BLOCK DIAGRAM
COM
OUTPUT
CC
VIN
AD736
FULL
WAVE
RECTIFIER
BIAS
SECTION
rms CORE
INPUT
AMPLIFIER
OUTPUT
AMPLIFIER
8k
8k
CF
–VS
+VS
CAV
1
2
3
4
8
7
6
5
00834-001
Figure 1.
The AD736 allows the choice of two signal input terminals: a
high impedance FET input (1012 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.
4. A high input impedance of 1012 Ω eliminates the need for an
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.
AD736
Rev. H | Page 2 of 20
TABLE OF CONTENTS
Features .............................................................................................. 1
General Description ......................................................................... 1
Functional Block Diagram .............................................................. 1
Product Highlights ........................................................................... 1
Revision History ............................................................................... 2
Specifications..................................................................................... 3
Absolute Maximum Ratings............................................................ 5
ESD Caution.................................................................................. 5
Pin Configuration and Function Descriptions............................. 6
Typical Performance Characteristics ............................................. 7
Theory of Operation ...................................................................... 10
Types of AC Measurement ........................................................ 10
Calculating Settling Time Using Figure 16 ............................. 11
RMS Measurement—Choosing the Optimum Value for CAV
....................................................................................................... 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
(CAV), and Filtering (CF) Capacitors......................................... 14
Evaluation Board ............................................................................ 16
Outline Dimensions ....................................................................... 18
Ordering Guide .......................................................................... 19
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
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
AD736
Rev. H | Page 3 of 20
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 VOUT = √Avg (VIN2)
CONVERSION ACCURACY 1 kHz sine wave
Total Error, Internal Trim1Using CC
All Grades 0 mV rms to 200 mV rms 0.3/0.3 0.5/0.5 0.2/0.2
0.3/0.3 ±mV/±% of reading
200 mV to 1 V rms −1.2 ±2.0 −1.2
±2.0 % of reading
TMIN to TMAX
A and B Grades @ 200 mV rms 0.7/0.7 0.5/0.5 ±mV/±% of reading
J and K Grades @ 200 mV rms 0.007 0.007
±% of reading/°C
vs. Supply Voltage
@ 200 mV rms Input VS = ±5 V to ±16.5 V 0 +0.06 +0.1 0 +0.06 +0.1 %/V
V
S = ±5 V to ±3 V 0 −0.18 −0.3 0 −0.18 −0.3 %/V
DC Reversal Error, DC-Coupled @ 600 mV dc 1.3 2.5 1.3 2.5 % of reading
Nonlinearity2, 0 mV to 200 mV @ 100 mV rms 0 0.25 0.35 0 0.25 0.35 % of reading
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 FACTOR3
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 200 200 mV rms
V
S = ±5 V to ±16.5 V 1 1 V rms
Peak Transient Input VS = +2.8 V, −3.2 V ±0.9 ±0.9 V
V
S = ±5 V ±2.7 ±2.7 V
V
S = ±16.5 V ±4.0 ±4.0 V
Input Resistance 1012 1012 Ω
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
S = ±5 V to ±16.5 V 1 1 V rms
Peak Transient Input VS = +2.8 V, −3.2 V ±1.7 ±1.7 V
V
S = ±5 V ±3.8 ±3.8 V
V
S = ±16.5 V ±11 ±11 V
Input Resistance 6.4 8 9.6 6.4 8 9.6 kΩ
Maximum Continuous
Nondestructive Input
All supply voltages ±12 ±12 V p-p
Input Offset Voltage4
J and K Grades ±3 ±3 mV
A and B Grades ±3 ±3 mV
vs. Temperature 8 30 8 30 μV/°C
vs. Supply VS = ±5 V to ±16.5 V 50 150 50
150 μV/V
V
S = ±5 V to ±3 V 80 80 μV/V
AD736
Rev. H | Page 4 of 20
AD736J/AD736A AD736K/AD736B
Parameter Conditions Min Typ Max Min Typ Max Unit
OUTPUT CHARACTERISTICS
Output Offset Voltage
J and K Grades ±0.1 ±0.5 ±0.1
±0.3 mV
A and B Grades ±0.5 ±0.3 mV
vs. Temperature 1 20 1 20 μV/°C
vs. Supply VS = ±5 V to ±16.5 V 50 130 50
130 μV/V
V
S = ±5 V to ±3 V 50 50 μV/V
Output Voltage Swing
2 kΩ Load VS = +2.8 V, −3.2 V 0 to
1.6
1.7 0 to
1.6
1.7 V
V
S = ±5 V 0 to
3.6
3.8 0 to
3.6
3.8 V
V
S = ±16.5 V 0 to 4 5 0 to 4 5 V
No Load VS = ±16.5 V 0 to 4 12 0 to 4 12 V
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)
for 1% Additional Error
Sine wave input
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)
for 1% Additional Error
Sine wave input
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 160 200 160
200 μA
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 connecting 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 = VPEAK/V rms.
4 DC offset does not limit ac resolution.
AD736
Rev. H | Page 5 of 20
ABSOLUTE MAXIMUM RATINGS
Table 2.
Parameter Rating
Supply Voltage ±16.5 V
Internal Power Dissipation1200 mW
Input Voltage ±VS
Output Short-Circuit Duration Indefinite
Differential Input Voltage +VS and –VS
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.
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
AD736
Rev. H | Page 6 of 20
PIN CONFIGURATION AND FUNCTION DESCRIPTIONS
CC1
VIN 2
CF3
–V
S4
COM
8
+VS
7
OUTPUT6
CAV
5
AD736
TOP VIEW
(Not to Scale)
00834-025
Figure 2. Pin Configuration
Table 3. Pin Function Descriptions
Pin No. Mnemonic Description
1 CCCoupling 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 VIN High Input Impedance Pin.
3 CFConnect an Auxiliary Low-Pass Filter Capacitor from the Output.
4 −VSNegative Supply Voltage if Dual Supplies Are Used, or Ground if Connected to a Single-Supply Source.
5 CAV Connect the Averaging Capacitor Here.
6 OUTPUT DC Output Voltage.
7 +VSPositive Supply Voltage.
8 COM Common.
AD736
Rev. H | Page 7 of 20
TYPICAL PERFORMANCE CHARACTERISTICS
–0.5
–0.3
–0.1
0
0.3
0.1
0.5
0.7
04286121410 16
SUPPLY VOLTAGE (±V)
ADDITIONAL ERROR (% of Reading)
V
IN
= 200mV rms
1kHz SINE WAVE
C
AV
= 100µF
C
F
= 22µF
00834-002
Figure 3. Additional Error vs. Supply Voltage
0
2
4
6
8
12
10
14
16
04286121410 16
SUPPLY VOLTAGE (±V)
PEAK INPUT BEFORE CLIPPING (V)
PIN 1
PIN 2
DC-COUPLED
00834-003
Figure 4. Maximum Input Level vs. Supply Voltage
0
4
2
10
16
14
12
8
6
0246 1081214
SUPPLY VOLTAGE (±V)
PEAK BUFFER OUTPUT (V)
16
1kHz SINE WAVE INPUT
00834-004
Figure 5. Peak Buffer Output vs. Supply Voltage
100µV
1mV
10mV
1V
100mV
10
V
0.1 1 10010 1000
–3dB FREQUENCY (kHz)
INPUT LEVEL (rms)
SINE WAVE INPUT, V
S
=±5V,
C
AV
= 22µF, C
F
= 4.7µF, C
C
= 22µF
1% ERROR
–3dB
10% ERROR
00834-005
Figure 6. Frequency Response Driving Pin 1
100µV
1mV
10mV
1V
100mV
10
V
0.1 1 10010 1000
–3dB FREQUENCY (kHz)
INPUT LEVEL (rms)
SINE WAVE INPUT, V
S
=±5V,
C
AV
= 22µF, C
F
= 4.7µF, C
C
= 22µF
1% ERROR
10% ERROR
–3dB
00834-006
Figure 7. Frequency Response Driving Pin 2
C
AV
= 100µF
C
AV
= 250µF
0
1
2
3
4
5
6
12345
CREST FACTOR (V
PEAK
/V rms)
ADDITIONAL ERROR (% of Reading)
C
AV
= 10µF
C
AV
= 33µF
3ms BURST OF 1kHz =
3 CYCLES
200mV rms SIGNAL
V
S
= ±5V
C
C
= 22µF
C
F
= 100µF
00834-007
Figure 8. Additional Error vs. Crest Factor with Various Values of CAV
AD736
Rev. H | Page 8 of 20
–0.8
–0.6
–0.2
–0.4
0
0.4
0.2
0.6
0.8
–60 –20–40 200 60 80 100 12040 140
TEMPERATURE (°C)
ADDITIONAL ERROR (% of Reading)
V
IN
= 200mV rms
1kHz SINE WAVE
C
AV
= 100mF
C
F
= 22mF
V
S
= ±5V
00834-008
Figure 9. Additional Error vs. Temperature
100
200
300
400
500
600
0 0.2 0.4 0.6 0.8 1.0
rms INPUT LEVEL (V)
DC SUPPLY CURRENT (µA)
V
IN
= 200mV rms
1kHz SINE WAVE
C
AV
= 100µF
C
F
= 22µF
V
S
= ±5V
00834-009
Figure 10. DC Supply Current vs. rms Input Level
10µV
100µV
1mV
10m
V
100 1k 10k 100k
–3dB FREQUENCY (Hz)
INPUT LEVEL (rms)
V
IN
= 1kHz
SINE WAVE INPUT
AC-COUPLED
V
S
= ±5V
00834-010
Figure 11. RMS Input Level (Pin 2) vs. −3 dB Frequency
–2.5
–2.0
–1.5
–1.0
–0.5
0
0.5
1.0
10mV 100mV 1V 2V
INPUT LEVEL (rms)
ERROR (% of Reading)
V
IN
= SINE WAVE @ 1kHz
C
AV
= 22µF, C
C
= 47µF,
C
F
= 4.7µF, V
S
= ±5V
00834-011
Figure 12. Error vs. RMS Input Voltage (Pin 2),
Output Buffer Offset Is Adjusted to Zero
1
10
100
10 100 1k
FREQUENCY (Hz)
C
AV
(µF)
–1%
–0.5%
V
IN
= 200mV rms
C
C
= 47µF
C
F
= 47µF
V
S
= ±5V
00834-012
Figure 13. CAV vs. Frequency for Specified Averaging Error
1mV
10mV
100mV
1
V
1 10 100 1k
FREQUENCY (Hz)
INPUT LEVEL (rms)
–0.5%
–1%
V
IN
SINE WAVE
AC-COUPLED
C
AV
= 10µF, C
C
= 47µF,
C
F
= 47µF, V
S
= ±5V
00834-013
Figure 14. RMS Input Level vs. Frequency for Specified Averaging Error
AD736
Rev. H | Page 9 of 20
1.0
1.5
2.0
2.5
3.0
4.0
3.5
02468 121410 16
SUPPLY VOLTAGE (±V)
INPUT BIAS CURRENT (pA)
00834-014
Figure 15. Pin 2 Input Bias Current vs. Supply Voltage
100µV
1mV
10mV
100mV
1
V
1ms 10ms 100ms 1s 10s 100s
SETTLING TIME
INPUT LEVEL (rms)
CAV = 10µF
CAV = 33µF
CAV = 100µF
VS = 5V
CC = 22µF
CF = 0µF
00834-015
Figure 16. RMS Input Level for Various Values of CAV vs. Settling Time
100fA
10n
A
1nA
100pA
10pA
1pA
–55 –35 –15 5 25 65 85 10545 125
TEMPERATURE (°C)
INPUT BIAS CURRENT
00834-016
Figure 17. Pin 2 Input Bias Current vs. Temperature
AD736
Rev. H | Page 10 of 20
THEORY OF OPERATION
rms
TRANSLINEAR
CORE
8
COM
AD736
+V
S
7
6
5
C
AV
FWR
CURRENT
MODE
ABSOLUTE
VALUE
1
2
3
4
C
A
33µF
A
C
C
C =
10µF
0.1µF
0.1µF
V
IN
–V
S
V
IN
C
C
+
OPTIONAL RETURN PATH
8k
+
DC
INPUT
AMPLIFIER
I
B
<10pA
OUTPUT
AMPLIFIER
8k
rms
OUTPUT
TO
COM
PIN
C
F
10µF (OPTIONAL)
+
BIAS
SECTION
0
0834-017
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,
CAV. Without CAV, the rectified input signal travels through the
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, CF, which is connected across the
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, CAV.
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
VPEAK; the corresponding rms value is 0.707 × VPEAK. Therefore, for
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.
AD736
Rev. H | Page 11 of 20
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 CAV
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 CAV, a trade-off between computational
accuracy and settling time is required.
Table 4. Error Introduced by an Average Responding Circuit when Measuring Common Waveforms
Waveform Type 1 V Peak Amplitude
Crest Factor
(VPEAK/V rms)
True RMS
Value (V)
Average Responding Circuit
Calibrated to Read RMS Value of
Sine Waves (V)
% of Reading Error Using
Average Responding Circuit
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
AD736
Rev. H | Page 12 of 20
RAPID SETTLING TIMES VIA THE AVERAGE
RESPONDING CONNECTION
Because the average responding connection shown in Figure 19
does not use the CAV averaging capacitor, its settling time does
not vary with the input signal level. It is determined solely by
the RC time constant of CF and the internal 8 kΩ resistor in the
output amplifier’s feedback path.
+VS
+VS
CF
33µF
CC
10µF
COM
OUTPUT
(OPTIONAL)
POSITIVE SUPPLY +VS
0.1µF
–VS
0.1µF
COMMON
NEGATIVE SUPPLY
VOUT
8
7
6
5
1
2
3
4
AD736
+
rms
CORE
+
CC
VIN
VIN
FULL
WAVE
RECTIFIER
CF
–VS
–VSCAV
BIAS
SECTION
INPUT
AMPLIFIER
8k
OUTPUT
AMPLIFIER
8k
00834-018
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 VOUT = VIN is never achieved exactly. Instead,
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 CF) allows the output voltage to equal its
ideal value. The ac error component, an output ripple, can be
easily removed by using a large enough post filtering capacitor, CF.
In most cases, the combined magnitudes of both the dc and
ac error components need to be considered when selecting
appropriate values for Capacitor CAV and Capacitor CF. 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.
DC ERROR = E
O
– E
O
(IDEAL)
AVERAGE E
O
= E
O
E
O
IDEAL
E
O
DOUBLE-FREQUENCY
RIPPLE
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.
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 = 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
periods between pulses). Figure 8 shows the additional error vs.
the crest factor of the AD736 for various values of CAV.
AD736
Rev. H | Page 13 of 20
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, Figure 22, Figure 23, and Figure 24.
Figure 21 and Figure 22 show the high impedance configurations,
and Figure 23 and Figure 24 show the low impedance connections
used to extend the input voltage range.
00834-026
AD736
COM
+V
S
+V
S
OUTPUTC
F
1MVOUT
DC
C
AV
C
C
V
IN
–V
S
1
2
3
4
8
7
6
5
C
AV
–V
S
Figure 21. High-Z AC-Coupled Input Connection (Default)
00834-027
AD736
COM
+V
S
+V
S
OUTPUT VOUT
DC
C
AV
C
C
V
IN
–V
S
1
2
3
4
8
7
6
5
C
AV
C
F
–V
S
Figure 22. High-Z DC-Coupled Input Connection
00834-028
AD736
COM
+V
S
+V
S
OUTPUT VOUT
DC
C
AV
C
C
V
IN
–V
S
1
2
3
4
8
7
6
5
C
AV
C
F
–V
S
Figure 23. Low-Z AC-Coupled Input Connection
00834-029
AD736
COM
+VS+VS
OUTPUT VOUTDC
CAV
CC
VIN
–VS
1
2
3
4
8
7
6
5
CAV
CF
–VS
Figure 24. Low-Z DC-Coupled Input Connection
AD736
Rev. H | Page 14 of 20
SELECTING PRACTICAL VALUES FOR INPUT
COUPLING (CC), AVERAGING (CAV), AND FILTERING
(CF) CAPACITORS
Table 5 provides practical values of CAV and CF for several
common applications.
The input coupling capacitor, CC, in conjunction with the
8 kΩ internal input scaling resistor, determine the −3 dB
low frequency roll-off. This frequency, FL, is equal to
)(8000)(2π
1
FaradsinCofValue
F
C
L=
Note that at FL, the amplitude error is approximately −30%
(–3 dB) of the reading. To reduce this error to 0.5% of the
reading, choose a value of CC that sets FL at one-tenth of the
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 Figure 27 should
be used in addition to CC.
Table 5. Capacitor Selection Chart
Application RMS Input Level
Low Frequency
Cutoff (−3 dB)
Max Crest
Factor
CAV
(μF)
CF
(μF) Settling Time1 to 1%
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.
+V
S
+V
S
C
AV
33µF
47k
1W
C
C
10µF
COM
OUTPUT
(OPTIONAL)
OUTPUT
8
7
6
5
1
2
3
4
AD736
+
rms
CORE
+
C
C
V
IN
FULL
WAVE
RECTIFIER
C
F
–V
S
–V
S
+V
S
–V
S
C
AV
BIAS
SECTION
INPUT
AMPLIFIER
8k
OUTPUT
AMPLIFIER
8k
C
F
10µF
1µF
1µF
(OPTIONAL)
+
OPTIONAL
A
C COUPLIN
G
CAPACITOR
0.01µF
1kV
2V
20V
200V
9M
900k
90k
10k
V
IN
200mV
1N4148
1N4148
00834-020
Figure 25. AD736 with a High Impedance Input Attenuator
AD736
Rev. H | Page 15 of 20
+V
S
+V
S
C
AV
33µF
COM
OUTPUT
OUTPUT
8
7
6
5
1
2
3
4
AD736
+
rms
CORE
C
C
V
IN
FULL
WAVE
RECTIFIER
C
F
–V
S
–V
S
C
AV
BIAS
SECTION
INPUT
AMPLIFIER
8k
OUTPUT
AMPLIFIER
8k
C
F
10µF
C
C
10µF
1µF
1µF
(OPTIONAL)
+
+IN
INPUT IMPEDANCE: 10
12
||10pF
–IN
AD711
+
3
2
6
00834-021
Figure 26. Differential Input Connection
7+VS
+VS
CAV
33µF
CC
10µF
COM
OUTPUT
(OPTIONAL)
OUTPUT
8
6
5
1
2
3
4
AD736
+
rms
CORE
+
CC
VIN FULL
WAVE
RECTIFIER
CF
–VS
–VS
+VS
CAV
BIAS
SECTION
INPUT
AMPLIFIER
8k
OUTPUT
AMPLIFIER
8k
CF
10µF
1µF
1µF
(OPTIONAL)
+
VIN
OUTPUT
VOS
ADJUST
1M
39M
AC-COUPLED
DC-COUPLED
1M
0.1µF
00834-022
Figure 27. External Output VOS Adjustment
+V
S
C
C
10µF
COM
OUTPUT
8
7
6
5
1
2
3
4
AD736
rms
CORE
+
C
C
V
IN
V
IN
FULL
WAVE
RECTIFIER
C
F
–V
S
C
AV
BIAS
SECTION
INPUT
AMPLIFIER
8k
OUTPUT
AMPLIFIER
8k
C
F
10µF (OPTIONAL)
+
+
1M
0.1µF
33µF
9V
100k
100k
4.7µF
4.7µF
V
S
2
V
S
2
00834-023
Figure 28. Battery-Powered Option
AD736
Rev. H | Page 16 of 20
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 31, Figure 32,
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.
00834-030
Figure 29. AD736 Evaluation Board
00834-032
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 dotted-
outline 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.
Table 6 is the bill of materials for the AD736 evaluation board.
00834-033
Figure 31. Evaluation Board—Component-Side Copper
00834-034
Figure 32. Evaluation Board—Secondary-Side Copper
00834-035
Figure 33. Evaluation Board—Internal Power Plane
00834-036
Figure 34. Evaluation Board—Internal Ground Plane
AD736
Rev. H | Page 17 of 20
00834-032
AD736
COM
C
AV
+V
S
+V
S
OUT J2
C
C
V
IN
C
F
–V
S
–V
S
1
2
3
4
7
6
5CAV
C6
0.1µF
C4
0.1µF
R1
1M
W2
IN
J1
VIN
LO-Z
W1
DC
COUP
P2
HI-Z SEL HI-Z
GND
CAV
33µF
16V+
CIN
0.1µF
CF2
GND1 GND2 GND3 GND4
R4
0
8
R3
0
+
C1
10µF
25V
C2
10µF
25V
–V
S
–V
S
+V
S
+
S
+
+
VOUT
CF1
NORM
SEL
J3
PD
+V
S
FILT
W3
AC COUP
C
C
W4
LO-Z IN
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 +VSComponents Corp. TP-104-01-02
1 Test loop Green −VSComponents 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
AD736
Rev. H | Page 18 of 20
OUTLINE DIMENSIONS
COMPLIANT TO JEDEC STANDARDS MS-001
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.
CORNER LEADS MAY BE CONFIGURED AS WHOLE OR HALF LEADS.
070606-A
0.022 (0.56)
0.018 (0.46)
0.014 (0.36)
SEATING
PLANE
0.015
(0.38)
MIN
0.210 (5.33)
MAX
0.150 (3.81)
0.130 (3.30)
0.115 (2.92)
0.070 (1.78)
0.060 (1.52)
0.045 (1.14)
8
14
5
0.280 (7.11)
0.250 (6.35)
0.240 (6.10)
0.100 (2.54)
BSC
0.400 (10.16)
0.365 (9.27)
0.355 (9.02)
0.060 (1.52)
MAX
0.430 (10.92)
MAX
0.014 (0.36)
0.010 (0.25)
0.008 (0.20)
0.325 (8.26)
0.310 (7.87)
0.300 (7.62)
0.195 (4.95)
0.130 (3.30)
0.115 (2.92)
0.015 (0.38)
GAUGE
PLANE
0.005 (0.13)
MIN
Figure 36. 8-Lead Plastic Dual In-Line Package [PDIP]
Narrow Body (N-8)
Dimensions shown in inches and (millimeters)
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.
0.310 (7.87)
0.220 (5.59)
0.005 (0.13)
MIN 0.055 (1.40)
MAX
0.100 (2.54) BSC
15°
0°
0.320 (8.13)
0.290 (7.37)
0.015 (0.38)
0.008 (0.20)
SEATING
PLANE
0.200 (5.08)
MAX
0.405 (10.29) MAX
0.150 (3.81)
MIN
0.200 (5.08)
0.125 (3.18)
0.023 (0.58)
0.014 (0.36) 0.070 (1.78)
0.030 (0.76)
0.060 (1.52)
0.015 (0.38)
14
58
Figure 37. 8-Lead Ceramic Dual In-Line Package [CERDIP]
(Q-8)
Dimensions shown in inches and (millimeters)
CONTROLLING DIMENSIONS ARE IN MILLIMETERS; INCH DIMENSIONS
(IN PARENTHESES) ARE ROUNDED-OFF MILLIMETER EQUIVALENTS FOR
REFERENCE ONLY AND ARE NOT APPROPRIATE FOR USE IN DESIGN.
COMPLIANT TO JEDEC STANDARDS MS-012-A A
012407-A
0.25 (0.0098)
0.17 (0.0067)
1.27 (0.0500)
0.40 (0.0157)
0.50 (0.0196)
0.25 (0.0099) ⋅ 45°
8°
0°
1.75 (0.0688)
1.35 (0.0532)
SEATING
PLANE
0.25 (0.0098)
0.10 (0.0040)
4
1
85
5.00 (0.1968)
4.80 (0.1890)
4.00 (0.1574)
3.80 (0.1497)
1.27 (0.0500)
BSC
6.20 (0.2441)
5.80 (0.2284)
0.51 (0.0201)
0.31 (0.0122)
COPLANARITY
0.10
Figure 38. 8-Lead Standard Small Outline Package [SOIC_N]
Narrow Body (R-8)
Dimensions shown in millimeters and (inches)
AD736
Rev. H | Page 19 of 20
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
AD736ARZ1–40°C to +85°C 8-Lead SOIC_N R-8
AD736ARZ-R71–40°C to +85°C 8-Lead SOIC_N R-8
AD736ARZ-RL1–40°C to +85°C 8-Lead SOIC_N R-8
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
AD736BRZ1–40°C to +85°C 8-Lead SOIC_N R-8
AD736BRZ-R71–40°C to +85°C 8-Lead SOIC_N R-8
AD736BRZ-RL1–40°C to +85°C 8-Lead SOIC_N R-8
AD736JN 0°C to +70°C 8-Lead PDIP N-8
AD736JNZ10°C to +70°C 8-Lead PDIP N-8
AD736KN 0°C to +70°C 8-Lead PDIP N-8
AD736KNZ10°C to +70°C 8-Lead PDIP N-8
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
AD736JRZ10°C to +70°C 8-Lead SOIC_N R-8
AD736JRZ-RL10°C to +70°C 8-Lead SOIC_N R-8
AD736JRZ-R710°C to +70°C 8-Lead SOIC_N R-8
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
AD736KRZ10°C to +70°C 8-Lead SOIC_N R-8
AD736KRZ-RL10°C to +70°C 8-Lead SOIC_N R-8
AD736KRZ-R710°C to +70°C 8-Lead SOIC_N R-8
AD736-EVALZ1 Evaluation Board
1 Z = RoHS compliant part.
AD736
Rev. H | Page 20 of 20
NOTES
©2007 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
C00834-0-2/07(H)