REV. D
a
Information furnished by Analog Devices is believed to be accurate and
reliable. However, no responsibility is assumed by Analog Devices for its
use, nor for any infringements of patents or other rights of third parties that
may result from its use. No license is granted by implication or otherwise
under any patent or patent rights of Analog Devices.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781/329-4700 www.analog.com
Fax: 781/326-8703 © Analog Devices, Inc., 2002
AD737
*
Low Cost, Low Power,
True RMS-to-DC Converter
*Protected under U.S. Patent Number 5,495,245.
FEATURES
Computes:
True rms Value
Average Rectified Value
Absolute Value
Provides:
200 mV Full-Scale Input Range
(Larger Inputs with Input Attenuator)
Direct Interfacing with 3 1/2 Digit
CMOS A/D Converters
High Input Impedance of 1012
Low Input Bias Current: 25 pA Max
High Accuracy: 0.2 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: 160 A Max Supply Current
No External Trims Needed for Specified Accuracy
AD736—A General-Purpose, Buffered Voltage
Output Version also Available
FUNCTIONAL BLOCK DIAGRAM
COM
OUTPUT
CC
VIN
AD737
FULL
WAVE
RECTIFIER
BIAS
SECTION RMS CORE
INPUT
AMPLIFIER
8k
8k
POWER
DOWN
–VS
+VS
CAV
1
2
3
4
8
7
6
5
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 temperature
or supply voltage. High accuracy is also maintained for input
waveforms with crest factors of 1 to 3. In addition, crest factors
as high as 5 can be measured (while introducing only 2.5%
additional error) at the 200 mV full-scale input level.
The AD737 has no output buffer amplifier, thereby significantly
reducing dc offset errors occurring at the output. This allows
the device to be highly compatible with high input impedance
A/D converters.
Requiring only 160 µA of power supply current, the AD737 is
optimized for use in portable multimeters and other battery-
powered applications. This converter also provides a power-down
feature that reduces the power supply standby current to less
than 30 µA.
The AD737 allows the choice of two signal input terminals: a
high impedance (10
12
Ω) FET input that directly interfaces with
high Z input attenuators and a low impedance (8 kΩ) input 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 may be used either singly or differentially.
The AD737 achieves a 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 four performance grades. The AD737J
and AD737K grades are rated over the commercial temperature
range of 0°C to +70°C. The AD737A and AD737B grades are
rated over the industrial temperature range of –40°C to +85°C.
The AD737 is available in three low cost, 8-lead packages:
plastic DIP, plastic SOIC, and hermetic CERDIP.
PRODUCT HIGHLIGHTS
1. The AD737 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 AD737 to perform true rms measurement.
3. The low power consumption of 0.72 mW makes the AD737
suitable for many battery-powered applications.
REV. D–2–
AD737–SPECIFICATIONS
(@ 25C, 5 V supplies, ac-coupled with 1 kHz sine wave input applied, unless
otherwise noted.)
AD737J/AD737A AD737K/AD737B
Parameter Conditions Min Typ Max Min Typ Max Unit
TRANSFER FUNCTION
V Avg V
OUT IN
=
()
2
V Avg V
OUT IN
=
()
2
CONVERSION ACCURACY 1 kHz Sine Wave
Total Error, Internal Trim
1
AC-Coupled Using C
C
All Grades 0–200 mV rms 0.2/0.3 0.4/0.5 0.2/0.2 0.2/0.3 ±mV/±% of
Reading
200 mV–1 V rms –1.2 ±2.0 –1.2 ±2.0 % of Reading
T
MIN
–T
MAX
A and B Grades @ 200 mV rms 0.5/0.7 0.3/0.5 ±mV/±% of
Reading
J and K Grades @ 200 mV rms 0.007 0.007 ±% of
Reading/°C
vs. Supply Voltage
@ 200 mV rms Input V
S
= ±5 V to ±16.5 V 0+0.06 +0.1 0+0.06 +0.1 %/V
@ 200 mV rms Input 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
Nonlinearity
2
, 0–200 mV @ 100 mV rms 0+0.25 +0.35 0+0.25 +0.35 % of Reading
Total Error, External Trim 0–200 mV rms 0.1/0.2 0.1/0.2 ±mV/±% of
Reading
ERROR vs. CREST FACTOR
3
Crest Factor 1 to 3 C
AV
, C
F
= 100 µF0.7 0.7 % Additional
Error
Crest Factor = 5 C
AV
, C
F
= 100 µF2.5 2.5 % Additional
Error
INPUT CHARACTERISTICS
High Impedance Input (Pin 2)
Signal Range
Continuous rms Level V
S
= +2.8 V, –3.2 V 200 200 mV rms
V
S
= ±5 V to ±16.5 V 11V rms
Peak Transient Input V
S
= +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 10
12
10
12
Ω
Input Bias Current V
S
= ±5 V 1 25 1 25 pA
Low Impedance Input (Pin 1)
Signal Range
Continuous rms Level V
S
= +2.8 V, –3.2 V 300 300 mV rms
V
S
= ±5 V to ±16.5 V l l V rms
Peak Transient Input V
S
= +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 Voltage
4
AC-Coupled
J and K Grades ±3±3mV
A and B Grades ±3±3mV
vs. Temperature 8 30 8 30 µV/°C
vs. Supply V
S
= ±5 V to ±16.5 V 50 150 50 150 µV/V
vs. Supply V
S
= ±5 V to ±3 V 80 80 µV/V
REV. D
AD737
–3–
AD737J/AD737A AD737K/AD737B
Parameter Conditions Min Typ Max Min Typ Max Unit
OUTPUT CHARACTERISTICS
Output Voltage Swing
No Load V
S
= +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 –3.3 –3.4 0 to –3.3 –3.4 V
V
S
= ±16.5 V 0 to –4 –5 0 to –4 –5 V
Output Resistance @ dc 6.4 8 9.6 6.4 8 9.6 kΩ
FREQUENCY RESPONSE
High Impedance Input (Pin 2)
For 1% Additional Error Sine Wave Input
V
IN
= 1 mV rms 1 1 kHz
V
IN
= 10 mV rms 6 6 kHz
V
IN
= 100 mV rms 37 37 kHz
V
IN
= 200 mV rms 33 33 kHz
±3 dB Bandwidth Sine Wave Input
V
IN
= 1 mV rms 5 5 kHz
V
IN
= 10 mV rms 55 55 kHz
V
IN
= 100 mV rms 170 170 kHz
V
IN
= 200 mV rms 190 190 kHz
Low Impedance Input (Pin 1)
For 1% Additional Error Sine Wave Input
V
IN
= 1 mV rms 1 1 kHz
V
IN
= 10 mV rms 6 6 kHz
V
IN
= 100 mV rms 90 90 kHz
V
IN
= 200 mV rms 90 90 kHz
±3 dB Bandwidth Sine Wave Input
V
IN
= 1 mV rms 5 5 kHz
V
IN
= 10 mV rms 55 55 kHz
V
IN
= 100 mV rms 350 350 kHz
V
IN
= 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 120 160 120 160 µA
V
IN
= 200 mV rms, No Load Sine Wave Input 170 210 170 210 µA
Power-Down Mode Current Pin 3 Tied to +V
S
25 40 25 40 µA
TEMPERATURE RANGE
Operating, Rated Performance
Commercial (0°C to +70°C) AD737J AD737K
Industrial (–40°C to +85°C) AD737A AD737B
NOTES
l
Accuracy is specified with the AD737 connected as shown in Figure 1 with capacitor C
C
.
2
Nonlinearity is defined as the maximum deviation (in percent error) from a straight line connecting the readings at 0 and 200 mV rms.
3
Error versus crest factor is specified as additional error for a 200 mV rms signal. Crest factor = V
PEAK
/V rms.
4
DC offset does not limit ac resolution.
Specifications are subject to change without notice.
Specifications shown in boldface are tested on all production units at final electrical test. Results from those tests are used to calculate outgoing quality levels.
REV. D–4–
AD737
CAUTION
ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily
accumulate on the human body and test equipment and can discharge without detection. Although the
AD737 features proprietary ESD protection circuitry, permanent damage may occur on devices
subjected to high energy electrostatic discharges. Therefore, proper ESD precautions are recommended
to avoid performance degradation or loss of functionality.
ABSOLUTE MAXIMUM RATINGS
1
Supply Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ±16.5 V
Internal Power Dissipation
2
. . . . . . . . . . . . . . . . . . . . 200 mW
Input Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ±V
S
Output Short-Circuit Duration . . . . . . . . . . . . . . . . Indefinite
Differential Input Voltage . . . . . . . . . . . . . . . . . . +V
S
and –V
S
Storage Temperature Range (Q) . . . . . . . . . –65°C to +150°C
Storage Temperature Range (N, R) . . . . . . . –65°C to +125°C
Operating Temperature Range
AD737J/AD737K . . . . . . . . . . . . . . . . . . . . . . 0°C to +70°C
AD737A/AD737B . . . . . . . . . . . . . . . . . . . –40°C to +85°C
Lead Temperature Range (Soldering 60 sec) . . . . . . . . . 300°C
ESD Rating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 500 V
NOTES
1
Stresses above those listed under Absolute Maximum Ratings may cause perma-
nent 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.
2
8-Lead Plastic DIP Package:
JA
= 165°C/W
8-Lead CERDIP Package:
JA
= 110°C/W
8-Lead Small Outline Package:
JA
= 155°C/W
PIN CONFIGURATIONS
Plastic DIP (N-8), CERDIP (Q-8), SOIC (SOIC-8)
COM
OUTPUT
C
C
V
IN
AD737
FULL
WAVE
RECTIFIER
BIAS
SECTION RMS CORE
INPUT
AMPLIFIER
8k
8k
POWER
DOWN
–V
S
+V
S
C
AV
1
2
3
4
8
7
6
5
ORDERING GUIDE
Temperature Package Package
Model Range Description Option
AD737AQ –40°C to +85°CCERDIP Q-8
AD737BQ –40°C to +85°CCERDIP Q-8
AD737JN 0°C to +70°CPlastic DIP N-8
AD737JR 0°C to +70°CSOIC R-8
AD737JR-REEL 0°C to +70°C13" Tape and Reel R-8
AD737JR-REEL7 0°C to +70°C7" Tape and Reel R-8
AD737KN 0°C to +70°CPlastic DIP N-8
AD737KR 0°C to +70°CSOIC R-8
AD737KR-REEL 0°C to +70°C13" Tape and Reel R-8
AD737KR-REEL7 0°C to +70°C7" Tape and Reel R-8
REV. D
Typical Performance Characteristics–AD737
–5–
SUPPLY VOLTAGE – V
ADDITIONAL ERROR – % of Reading
–0.5 02 16
4681012 14
–0.3
–0.1
0.1
0.3
0.5
0.7
0
VIN = 200mV rms
SINE WAVE @ 1kHz
CAV = 100F
CF = 22F
TPC 1. Additional Error vs. Supply
Voltage
FREQUENCY – kHz
INPUT LEVEL – rms
100V
0.1 1 1000
10 100
1mV
10mV
100mV
1V
10V
SINE WAVE INPUT, V
S
= 5V,
C
AV
= 22F, C
F
= 4.7F, C
C
= 22F
–3dB
10% ERROR
1% ERROR
TPC 4. Frequency Response
Driving Pin 1
TEMPERATURE – C
ADDITIONAL ERROR – % of Reading
–0.8
–60
–0.6
–0.4
–0.2
0
0.2
0.4
V
IN
= 200mV rms
SINE WAVE @ 1kHz
C
C
= 22F
C
AV
= 100F
C
F
= 22F
V
S
= 5V
–40 –20 0 20 40 60 80 100 120 140
0.6
0.8
TPC 7. Additional Error vs.
Temperature
SUPPLY VOLTAGE – V
PEAK INPUT BEFORE CLIPPING – V
002 16
4681012 14
2
4
6
8
12
16
DC-COUPLED
14
10
PIN 1
PIN 2
TPC 2. Maximum Input Level vs.
Supply Voltage
FREQUENCY – kHz
INPUT LEVEL – rms
100V
0.1 1 1000
10 100
1mV
10mV
100mV
1V
10V SINE WAVE INPUT, V
S
= 5V,
C
AV
= 22F, C
F
= 4.7F, C
C
= 22F
–3dB
10% ERROR
1% ERROR
TPC 5. Frequency Response
Driving Pin 2
RMS INPUT LEVEL – V
DC SUPPLY CURRENT – A
00
100
200
300
400
0.2 0.4 0.6 0.8 1.0
500
TPC 8. DC Supply Current vs.
RMS Input Level
DUAL SUPPLY VOLTAGE – V
SUPPLY CURRENT – A
502 16
4681012 14
10
20
25
15
18
TPC 3. Power-Down Current vs.
Supply Voltage
CREST FACTOR (VPEAK/V rms)
ADDITIONAL ERROR – % of Reading
015
234
1
2
3
4
6
3ms BURST OF 1kHz =
3 CYCLES
200mV rms SIGNAL
VS = 5V
CC = 22F
CF = 100F
8
CAV = 33F
CAV = 100F
CAV = 250F
CAV = 10F
TPC 6. Additional Error vs. Crest
Factor vs. C
AV
–3dB FREQUENCY – Hz
INPUT LEVEL – rms
10V
100 1k 100k10k
100V
1mV
10mV
VIN = SINE WAVE
AC-COUPLED
VS = 5V
TPC 9. –3 dB Frequency vs. RMS
Input Level (Pin 2)
REV. D–6–
AD737
INPUT LEVEL – rms
ERROR – % of Reading
–2.5
10mV 100mV 1V 2V
–2.0
–1.5
–1.0
–0.5
0
0.5
1.0
V
IN
= SINE WAVE @ 1kHz
C
AV
= 22F, C
C
= 47F,
C
F
= 4.7F, V
S
= 5V
TPC 10. Error vs. RMS Input Voltage
(Pin 2) Using Circuit of Figure 6
SUPPLY VOLTAGE – V
INPUT BIAS CURRENT – pA
1.0 0
1.5
2.0
2.5
3.0
246810
3.5
12 14 16
4.0
TPC 13. Pin 2 Input Bias Current vs.
Supply Voltage
100
10
FREQUENCY – Hz
C
AV
– F
10
1
100 1k
V
IN
= 200mV rms
C
C
= 47F
C
F
= 47F
V
S
= 5V
–1%
–0.5%
TPC 11. C
AV
vs. Frequency for
Specified Averaging Error
100V
1ms
SETTLING TIME
INPUT LEVEL – rms
1mV
10mV
100mV
1V
10ms 100ms 1s 10s 100s
C
AV
= 100F
C
AV
= 33F
C
AV
= 10F
V
S
= 5V
C
C
= 22F
C
F
= 0F
TPC 14. Settling Time vs. RMS Input
Level for Various Values of C
AV
FREQUENCY – Hz
INPUT LEVEL – rms
1mV 110 1k
100
10mV
100mV
1V
V
IN
= SINE WAVE
AC-COUPLED
C
AV
= 10F, C
C
= 47F,
C
F
= 47F, V
S
= 5V
–1% –0.5%
TPC 12. RMS Input Level vs. Fre-
quency for Specified Averaging Error
TEMPERATURE – C
INPUT BIAS CURRENT
100fA
–55
1pA
10pA
100pA
1nA
10nA
–35 –15 5 25 45 65 85 105 125
TPC 15. Pin 2 Input Bias Current vs.
Temperature
REV. D
AD737
–7–
CALCULATING SETTLING TIME USING TPC 14
TPC 14 may 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 TPC 14, the initial settling time (where the 100 mV
line intersects the 33 µF line) is around 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 (i.e., not the settling time to 1%,
0.1%, and so on, of the final value). Also, this graph provides the
worst-case settling time, since the AD737 settles very quickly
with increasing input levels.
TYPES OF AC MEASUREMENT
The AD737 is capable of measuring ac signals by operating as
either an average responding or a true rms-to-dc converter. As
its name implies, an average responding converter computes the
average absolute value of an ac (or ac and dc) voltage or current
by full wave rectifying and low-pass filtering the input signal;
this 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 that of V
PEAK
; the
corresponding rms value is 0.707 times V
PEAK
. Therefore, for
sine wave voltages, the required scale factor is 1.11 (0.707
divided by 0.636).
In contrast to measuring the average value, true rms measure-
ment is a universal language among waveforms, allowing the
magnitudes of all types of voltage (or current) waveforms to be
compared to one another and to dc. RMS is a direct measure of
the power or heating value of an ac voltage compared to that of
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.
Mathematically, the rms value of a voltage is defined (using a
simplified equation) as:
V rms Avg V=
()
2
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 convert-
ers 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. As an example,
if an average responding converter is calibrated to measure the
rms value of sine wave voltages and then is used to measure either
symmetrical square waves or dc voltages, the converter will have
a computational error 11% (of reading) higher than the true rms
value (see Table I).
Table I. Error Introduced by an Average Responding Circuit When Measuring Common Waveforms
Average Responding
Waveform Type Circuit Calibrated to % of Reading Error
1 V Peak Crest Factor Read RMS Value of Using Average
Amplitude (V
PEAK
/V rms) True RMS Value Sine Waves Will Read Responding Circuit
Undistorted Sine Wave 1.414 0.707 V 0.707 V 0%
Symmetrical Square Wave 1.00 1.00 V 1.11 V 11.0%
Undistorted Triangle Wave 1.73 0.577 V 0.555 V –3.8%
Gaussian Noise
(98% of Peaks <1 V) 3 0.333 V 0.295 V –11.4%
Rectangular 2 0.5 V 0.278 V –44%
Pulse Train 10 0.1 V 0.011 V –89%
SCR Waveforms
50% Duty Cycle 2 0.495 V 0.354 V –28%
25% Duty Cycle 4.7 0.212 V 0.150 V –30%
REV. D–8–
AD737
AD737 THEORY OF OPERATION
As shown by Figure 1, the AD737 has four functional subsections:
input amplifier, full-wave rectifier, rms core, and bias sections.
The FET input amplifier allows both a high impedance, buffered
input (Pin 2) or 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 input signal may be either dc- 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.
8
COM
+V
S
7
6
OUTPUT
5
C
AV
CURRENT
MODE
ABSOLUTE
VALUE
1
C
C
2
V
IN
3
POWER
DOWN
4
–V
S
8k
C
AV
33F
C
F
10F
(OPTIONAL)
V
IN
C
C
10F (OPTIONAL)
8k
FET
OP AMP
I
B
<10pA
TRANSLINEAR
CORE
POSITIVE SUPPLY +V
S
0.1F
–V
S
0.1F
COMMON
NEGATIVE SUPPLY
BIAS
SECTION
+
+
+
Figure 1. AD737 True RMS Circuit
The output of the input amplifier drives a full-wave precision
rectifier, which in turn, drives the rms core. It is in the core that
the essential rms operations of squaring, averaging, and square
rooting are performed, using an external averaging capacitor,
C
AV
. Without C
AV
, the rectified input signal travels through the
core unprocessed, as is done with the average responding
connection (Figure 2).
A final subsection, the bias section, permits a power-down func-
tion. This reduces the idle current of the AD737 from 160 µA
down to a mere 30 µA. This feature is selected by tying Pin 3 to
the +V
S
terminal. In the average responding connection, all of
the averaging is carried out by an RC post filter consisting of an
8 kΩ internal scale-factor resistor connected between Pins 6 and
8 and an external averaging capacitor, C
F
. In the rms circuit,
this additional filtering stage helps reduce any output ripple that
was not removed by the averaging capacitor, C
AV
.
RMS MEASUREMENT—CHOOSING THE OPTIMUM
VALUE FOR C
AV
Since the external averaging capacitor, C
AV
, 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
while the time it takes 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
AV
, a trade-off between computational accuracy and settling
time is required.
RAPID SETTLING TIMES VIA THE AVERAGE
RESPONDING CONNECTION
Because the average responding connection shown in Figure 2
does not use an averaging capacitor, its settling time does not
vary with input signal level; it is determined solely by the RC
time constant of C
F
and the internal 8 kΩ output scaling resistor.
COM
OUTPUT
C
AV
C
C
POWER
DOWN
–V
S
C
F
33F
V
IN
C
C
10F
(OPTIONAL)
POSITIVE SUPPLY +V
S
0.1F
–V
S
0.1F
COMMON
NEGATIVE SUPPLY
V
OUT
8
7
6
5
1
2
3
4
AD737
FULL
WAVE
RECTIFIER
BIAS
SECTION
INPUT
AMPLIFIER 8k
8k
RMS
CORE
+
+V
S
+
Figure 2. AD737 Average Responding Circuit
DC ERROR, OUTPUT RIPPLE, AND AVERAGING
ERROR
Figure 3 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 V
OUT
= V
IN
is never exactly achieved; instead,
the output contains both a dc and an ac error component.
EO
IDEAL
EO
DC ERROR = EO – EO (IDEAL)
AVERAGE EO = EO
DOUBLE-FREQUENCY
RIPPLE
TIME
Figure 3. Output Waveform for Sine Wave Input Voltage
REV. D
AD737
–9–
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 aver-
aging capacitor used—no amount of post filtering (i.e., using a
very large C
F
) will allow the output voltage to equal its ideal
value. The ac error component, an output ripple, may be easily
removed by using a large enough post filtering capacitor, C
F
.
In most cases, the combined magnitudes of both the dc and
ac error components need to be considered when selecting
appropriate values for capacitors C
AV
and C
F
. 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.
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
PEAK
/V rms). Many common wave-
forms, 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). TPC 6 shows the
additional error versus the crest factor of the AD737 for various
values of C
AV
.
SELECTING PRACTICAL VALUES FOR INPUT
COUPLING (C
C
), AVERAGING (C
AV
), AND FILTERING
(C
F
) CAPACITORS
Table II provides practical values of C
AV
and C
F
for several
common applications.
The input coupling capacitor, C
C
, in conjunction with the 8 kΩ
internal input scaling resistor, determines the –3 dB low frequency
rolloff. This frequency, F
L
, is equal to:
FTheValue of C in Farads
L
C
=
()( )
1
28000π,
Note that at F
L
, the amplitude error is approximately –30% (–3 dB)
of reading. To reduce this error to 0.5% of reading, choose a
value of C
C
that sets F
L
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 at Pin 2 should be used in
addition to capacitor C
C
.
Table II. AD737 Capacitor Selection Chart
Low Frequency Max Settling
Application RMS Input Level Cutoff (–3 dB) Crest Factor C
AV
C
F
Time* to 1%
General-Purpose 0–1 V 20 Hz 5 150 µF10 µF360 ms
RMS Computation 200 Hz 5 15 µF1 µF36 ms
0–200 mV 20 Hz 5 33 µF10 µF360 ms
200 Hz 5 3.3 µF1 µF36 ms
General-Purpose 0–1 V 20 Hz None 33 µF1.2 sec
Average 200 Hz None 3.3 µF120 ms
Responding
0–200 mV 20 Hz None 33 µF1.2 sec
200 Hz None 3.3 µF120 ms
SCR Waveform 0–200 mV 50 Hz 5 100 µF33 µF1.2 sec
Measurement 60 Hz 5 82 µF27 µF1.0 sec
0–100 mV 50 Hz 5 50 µF33 µF1.2 sec
60 Hz 5 47 µF27 µF1.0 sec
Audio
Applications
Speech 0–200 mV 300 Hz 3 1.5 µF0.5 µF18 ms
Music 0–100 mV 20 Hz 10 100 µF68 µF2.4 sec
*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.
REV. D–10–
AD737
Applications Circuits
COM
+V
AD589
1.23V
C
AV
C
C
POWER
DOWN 0.1F
C
C
10F
SWITCH CLOSED
ACTIVATES
POWER DOWN
MODE: AD737
DRAWS JUST
40A IN THIS
MODE
2V
20V
200V
9M⍀
900k⍀
90k⍀
10k⍀
V
IN
200mV
V
IN
–V
S
8
7
6
5
4
1
2
3
AD737
FULL
WAVE
RECTIFIER
BIAS
SECTION
INPUT
AMPLIFIER
8k⍀
8k⍀
+
RMS
CORE
+
+V
S
+
1F
OUTPUT 1M⍀
–V
S
+V
S
1N4148
1N4148
47k⍀
1W
1F
+
COMMON
33F
REF LOW
REF HIGH
3 1/2 DIGIT AD7136
TYPE CONVERTER
LOW
HIGH
ANALOG
9V
200k⍀
20k⍀
50k⍀
+
1PRV
0.01F
Figure 4. 3 1/2 Digit DVM Circuit
COM
INPUT
CAV
CC
POWER
DOWN
C2
0.01F
INPUT SCALE FACTOR ADJ
VIN
–VS
OUTPUT
R5
80.6k⍀
+VS
CAV
33F
+
OUTPUT
CF
0.47F
AD737JR
C1
0.47F
+
5V
R2
5k⍀
R1
69.8k⍀
1%
R3
78.7k⍀
0.01F
C3
0.01F
R7
100k⍀
R8
100k⍀
5V
2.5V
AD8541AR
C4
2.2F
C5
1F
5V
R4
5k⍀
OUTPUT ZERO
ADJUST
Figure 5. Battery-Powered Operation for 200 mV Max RMS Full-Scale Input
REV. D
AD737
–11–
COM
+VS
CAV
CC
CC
10F
VOUT
VIN
–VS
8
7
6
5
SCALE FACTOR
ADJUST
1
2
3
4
AD737
FULL
WAVE
RECTIFIER
BIAS
SECTION
INPUT
AMPLIFIER
8k
8k
+
RMS
CORE
+
CAV
33F
+
OUTPUT
POWER
DOWN
CF
10F
100
200
Figure 6. External Scale Factor Trim
COM
+V
S
C
AV
C
C
V
IN
–V
S
8
7
6
5
NOT
CONNECTED
1
2
3
4
AD737
FULL
WAVE
RECTIFIER
BIAS
SECTION
INPUT
AMPLIFIER
8k
8k
RMS
CORE
+
C
AV
+
OUTPUT
POWER
DOWN
C
C
10F
1k
+3500
PPM/C
60.4
**R
CAL
**R1 I
REF
10 *11
9
Q2
*Q1, Q2 PART OF
RCA CA3046 OR
SIMILAR NPN
TRANSITOR ARRAY
**R1 + R
CAL
IN OHMS = 10,000 4.3V
0dB INPUT LEVEL IN VOLTS
AD711 dB OUTPUT
100mV/dB
PRECISION
RESISTOR
CORP
TYPE PT/ST
2k
31.6k
SCALE
FACTOR
TRIM
13 Q1
12
14
*
Figure 7. dB Output Connection
COM
C
C
V
IN
AD737
FULL
WAVE
RECTIFIER
INPUT
AMPLIFIER
8k
+V
S
1
2
3
8
7
6
V
OUT
499
–V
S
+V
S
500k
OFFSET ADJUST
1M
1k
1k
SCALE
FACTOR
ADJUST
Figure 8. DC-Coupled V
OS
and Scale Factor Trims
REV. D
C00828–0–12/02(D)
PRINTED IN U.S.A.
–12–
AD737
Revision History
Location Page
12/02—Data Sheet changed from 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
8-Lead Standard Small Outline Package [SOIC]
Narrow Body
(R-8)
Dimensions shown in millimeters and (inches)
0.25 (0.0098)
0.19 (0.0075)
1.27 (0.0500)
0.41 (0.0160)
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)
85
41
5.00 (0.1968)
4.80 (0.1890)
4.00 (0.1574)
3.80 (0.1497)
1.27 (0.0500)
BSC
6.20 (0.2440)
5.80 (0.2284)
0.51 (0.0201)
0.33 (0.0130)
COPLANARITY
0.10
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-012AA
OUTLINE DIMENSIONS
8-Lead Ceramic Dip-Glass Hermetic Seal [CERDIP]
(Q-8)
Dimensions shown in inches and (millimeters)
14
85
0.310 (7.87)
0.220 (5.59)
PIN 1
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)
CONTROLLING DIMENSIONS ARE IN INCHES; MILLIMETERS DIMENSIONS
(IN PARENTHESES) ARE ROUNDED-OFF INCH EQUIVALENTS FOR
REFERENCE ONLY AND ARE NOT APPROPRIATE FOR USE IN DESIGN
8-Lead Plastic Dual-in-Line Package [PDIP]
(N-8)
Dimensions shown in inches and (millimeters)
SEATING
PLANE
0.015
(0.38)
MIN
0.180
(4.57)
MAX
0.150 (3.81)
0.130 (3.30)
0.110 (2.79) 0.060 (1.52)
0.050 (1.27)
0.045 (1.14)
8
14
5
0.295 (7.49)
0.285 (7.24)
0.275 (6.98)
0.100 (2.54)
BSC
0.375 (9.53)
0.365 (9.27)
0.355 (9.02)
0.150 (3.81)
0.135 (3.43)
0.120 (3.05)
0.015 (0.38)
0.010 (0.25)
0.008 (0.20)
0.325 (8.26)
0.310 (7.87)
0.300 (7.62)
0.022 (0.56)
0.018 (0.46)
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
COMPLIANT TO JEDEC STANDARDS MO-095AA
