REV. D
a
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AD736
Low Cost, Low Power,
True RMS-to-DC Converter
FUNCTIONAL BLOCK DIAGRAM
COM
OUTPUT
OUTPUT
AMPLIFIER
CC
VIN
AD736
FULL
WAVE
RECTIFIER
BIAS
SECTION
rms CORE
INPUT
AMPLIFIER
8k⍀
8k⍀
CF
–VS
+VS
CAV
1
2
3
4
8
7
6
5
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 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 AD736 offers higher accuracy at equal or
lower cost.
The AD736 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 AD736 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 AD736 has its own output buffer amplifier, thereby providing
a great deal of design flexibility. Requiring only 200 µA of power
supply current, the AD736 is optimized for use in portable
multimeters and other battery-powered applications.
The AD736 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
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 of 1012 V
Low Input Bias Current: 25 pA Max
High Accuracy: ⴞ0.3 mV ⴞ0.3% of Reading
RMS Conversion with Signal Crest Factors Up to 5
Wide Power Supply Range: +2.8 V, –3.2 V to ⴞ16.5 V
Low Power: 200 mA Max Supply Current
Buffered Voltage Output
No External Trims Needed for Specified Accuracy
AD737—An Unbuffered Voltage Output Version with
Chip Power Down also Available
that allows the measurement of 300 mV input level while oper-
ating from the minimum power supply voltage of +2.8 V, –3.2 V.
The two inputs may be used either singly or differentially.
The AD736 achieves a 1% of reading error bandwidth exceeding
10 kHz for 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 commercial temperature
range of 0°C to +70°C. The AD736A and AD736B grades are
rated over the industrial temperature range of –40°C to +85°C.
The AD736 is available in three low cost, 8-lead packages:
plastic miniDIP, plastic SOIC, and hermetic 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 10
12
Ω eliminates the need for an
external buffer when interfacing with input attenuators.
5. A low impedance input is available for those applications
requiring up to 300 mV rms input signal operating from low
power supply voltages.
REV. D–2–
AD736–SPECIFICATIONS
(@ 25ⴗC ⴞ5 V supplies, ac-coupled with 1 kHz sine wave input applied, unless
otherwise noted.)
AD736J/AD736A AD736K/AD736B
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 0.3/0.3 0.5/0.5 0.2/0.2 0.3/0.3 ±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 0.7/0.7 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
@ 200 mV rms Input VS = ±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 0.1/0.5 0.1/0.3 ±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
Continuous rms Level 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
Peak Transient Input V
S
= ±5 V ±2.7 ±2.7 V
Peak Transient Input V
S
= ±16.5 V ±4.0 ±4.0 V
Input Resistance 10
12
10
12
Ω
Input Bias Current V
S
= ±3 V to ±16.5 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
Continuous rms Level 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
Peak Transient Input V
S
= ±5 V ±3.8 ±3.8 V
Peak Transient Input 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
AD736
–3–
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 V
S
= ±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 V
S
= +2.8 V, –3.2 V 0 to +1.6 +1.7 0 to +1.6 +1.7 V
2 kΩ Load V
S
= ±5 V 0 to +3.6 +3.8 0 to +3.6 +3.8 V
2 kΩ Load V
S
= ±16.5 V 0 to +4 +5 0 to +4 +5 V
No Load V
S
= ±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
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
FREQUENCY RESPONSE
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
= l 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 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) AD736J AD736K
Industrial (–40°C to +85°C) AD736A AD736B
NOTES
l
Accuracy is specified with the AD736 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. Output offset voltage is
adjusted to zero.
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–
AD736
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
AD736J/AD736K . . . . . . . . . . . . . . . . . . . . . . . 0°C to +70°C
AD736A/AD736B . . . . . . . . . . . . . . . . . . . . –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
permanent damage to the device. This is a stress rating only; functional opera-
tion 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 Package:
JA
= 165°C/W
8-Lead CERDIP Package:
JA
= 110°C/W
8-Lead Small Outline Package:
JA
= 155°C/W
PIN CONFIGURATION
8-Lead MiniDIP (N-8), 8-Lead SOIC (RN-8),
8-Lead CERDIP (Q-8)
COM
OUTPUT
OUTPUT
AMPLIFIER
CC
VIN
AD736
FULL
WAVE
RECTIFIER
BIAS
SECTION
rms CORE
INPUT
AMPLIFIER
8k⍀
8k⍀
CF
–VS
+VS
CAV
1
2
3
4
8
7
6
5
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
AD736 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.
ORDERING GUIDE
Temperature Package Package
Model Range Description Option
AD736JN 0°C to +70°CPlastic Mini-DIP N-8
AD736KN 0°C to +70°CPlastic Mini-DIP N-8
AD736JR 0°C to +70°CPlastic SOIC RN-8
AD736KR 0°C to +70°CPlastic SOIC RN-8
AD736AQ –40°C to +85°CCERDIP Q-8
AD736BQ –40°C to +85°CCERDIP Q-8
AD736JR-Reel 0°C to +70°CPlastic SOIC RN-8
AD736JR-Reel-7 0°C to +70°CPlastic SOIC RN-8
AD736KR-Reel 0°C to +70°CPlastic SOIC RN-8
AD736KR-Reel-7 0°C to +70°CPlastic SOIC RN-8
REV. D –5–
Typical Performance Characteristics–AD736
SUPPLY VOLTAGE – ⴞV
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
1kHz SINE WAVE
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
= ⴞ5V,
C
AV
= 22F, C
F
= 4.7F, C
C
= 22F
–3dB
10% ERROR
1% ERROR
TPC 4. Frequency
Response Driving Pin 1
TEMPERATURE – ⴗC
ADDITIONAL ERROR – % OF READING
–0.8
–60
–0.6
–0.4
–0.2
0
0.2
0.4
V
IN
= 200mV rms
1kHz SINE WAVE
C
AV
= 100F
C
F
= 22F
–40 –20 0 20 40 60 80 100 120 140
0.6
0.8
TPC 7. Additional Error vs.
Temperature
SUPPLY VOLTAGE – ⴞV
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, VS = ⴞ5V,
CAV = 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
100 0
200
300
400
500
VIN = 1kHz
SINE WAVE INPUT
VS = ⴞ5V
CAV = 22F
CC = 10F
0.2 0.4 0.6 0.8 1.0
600
TPC 8. DC Supply Current
vs. rms lnput Level
–3dB FREQUENCY – Hz
INPUT LEVEL – rms
10V
100 1k 100k10k
100V
1mV
10mV
VIN = 1kHz
SINE WAVE INPUT
AC-COUPLED
VS = ⴞ5V
TPC 3. Peak Buffer Output
vs. Supply Voltage
CREST FACTOR (V
PEAK
/V rms)
ADDITIONAL ERROR – % OF READING
015
234
1
2
3
4
6
3ms BURST OF 1kHz =
3 CYCLES
200mV rms SIGNAL
V
S
= ⴞ5V
C
C
= 22F
C
F
= 100F
8
C
AV
= 33F
C
AV
= 100F
C
AV
= 250F
C
AV
= 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 = 1kHz
SINE WAVE INPUT
AC-COUPLED
VS = ⴞ5V
TPC 9. –3 dB Frequency vs.
rms Input Level (Pin 2)
REV. D–6–
AD736
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
= ⴞ5V
TPC 10. Error vs. rms Input
Voltage (Pin 2), Output Buffer
Offset is Adjusted to Zero
SUPPLY VOLTAGE – ⴞV
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
= ⴞ5V
–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
= ⴞ5V
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
= ⴞ5V
–1% –0.5%
TPC 12. rms Input Level vs. Fre-
quency for Specified Averaging
Error
TEMPERATURE – ⴗC
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
AD736
–7–
CALCULATING SETTLING TIME USING TPC 14
TPC 14 may 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, 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 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 (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 AD736 settles
very quickly with increasing input levels.
TYPES OF AC MEASUREMENT
The AD736 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 aver-
age 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
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. 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–
AD736
AD736 THEORY OF OPERATION
As shown by Figure 1, the AD736 has five functional subsections:
input amplifier, full-wave rectifier, rms core, output amplifier,
and bias section. 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 imped-
ance input attenuators.
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 con-
nection (Figure 2).
A final subsection, an output amplifier, buffers the output from
the core and also allows optional low-pass filtering to be performed
via the external capacitor, C
F
, 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, C
AV
.
8
COM
+V
S
7
6
RMS
OUTPUT
5
C
AV
CURRENT
MODE
ABSOLUTE
VALUE
1
C
C
2
V
IN
3
C
F
4
–V
S
8k⍀
C
AV
33F
C
F
10F (OPTIONAL)
V
IN
C
C
10F (OPTIONAL)
8k⍀
FET
OP AMP
I
B
<10pA
rms
TRANSLINEAR
CORE
POSITIVE SUPPLY +V
S
0.1F
–V
S
0.1F
COMMON
NEGATIVE SUPPLY
+
Figure 1. AD736 True rms Circuit
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.
COM
+V
S
OUTPUT
C
AV
C
C
C
F
–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
V
IN
–V
S
8
7
6
5
OUTPUT
AMPLIFIER
1
2
3
4
AD736
FULL
WAVE
RECTIFIER
BIAS
SECTION
INPUT
AMPLIFIER
8k⍀
8k⍀
+
rms
CORE
+
+V
S
Figure 2. AD736 Average Responding Circuit
RAPID SETTLING TIMES VIA THE AVERAGE
RESPONDING CONNECTION
Because the average responding connection shown in Figure 2
does not use the C
AV
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 resistor in the output
amplifier’s feedback path.
REV. D
AD736
–9–
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.
DC ERROR, OUTPUT RIPPLE, AND AVERAGING
ERROR
Figure 3 shows the typical output waveform of the AD736 with
a sine wave input applied. As with all real-world devices, the
ideal output of V
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
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 averaging
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 appro-
priate 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 AD736 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 shown in Figure 6 should
be used in addition to capacitor C
C
.
REV. D–10–
AD736
Applications Circuits
COM
+V
S
OUTPUT
C
AV
C
C
C
F
–V
S
C
F
10F (OPTIONAL)
C
C
10F
(OPTIONAL)
OPTIONAL
AC-COUPLING
CAPACITOR
0.01F
1kV
2V
20V
200V
9M
900k
90k
10k
V
IN
200mV
V
IN
–V
S
8
7
6
5
OUTPUT
AMPLIFIER
1
2
3
4
AD736
FULL
WAVE
RECTIFIER
BIAS
SECTION
INPUT
AMPLIFIER
8k⍀
8k⍀
+
rms
CORE
+
+V
S
C
AV
33F
+
1F
OUTPUT
1F
–V
S
+V
S
1N4148
1N4148
47k
1W
Figure 4. AD736 with a High Impedance Input Attenuator
COM
+VS
OUTPUT
CAV
CC
CF
–VS
CF
10F (OPTIONAL)
VIN
–VS
8
7
6
5
OUTPUT
AMPLIFIER
1
2
3
4
AD736
FULL
WAVE
RECTIFIER
BIAS
SECTION
INPUT
AMPLIFIER
8k⍀
8k⍀
+
rms
CORE
+VS
CAV
33F
+
1F
OUTPUT
1F
+IN
INPUT IMPEDANCE: 1012⍀
INPUT IMPEDANCE: 10pF
–IN
AD711 CC
10F
+
3
2
6
Figure 5. Differential Input Connection
REV. D
AD736
–11–
COM
+V
S
C
AV
C
C
C
F
C
F
10F (OPTIONAL)
C
C
10F
(OPTIONAL)
V
IN
V
IN
–V
S
8
7
6
5
OUTPUT
AMPLIFIER
1
2
3
4
AD736
FULL
WAVE
RECTIFIER
BIAS
SECTION
INPUT
AMPLIFIER
8k⍀
8k⍀
+
rms
CORE
+
C
AV
33F
+
1F
OUTPUT
1F
–V
S
+V
S
OUTPUT
V
OS
ADJUST
AC-COUPLED
DC-COUPLED
1M⍀39M⍀
AC-COUPLED
DC-COUPLED
1M
0.1F
Figure 6. External Output V
OS
Adjustment
COM
OUTPUT
CAV
CC
CF
CF
10
F (OPTIONAL)
VIN
–VS
8
7
6
5
OUTPUT
AMPLIFIER
1
2
3
4
AD736
FULL
WAVE
RECTIFIER
BIAS
SECTION
INPUT
AMPLIFIER
8k⍀
8k⍀
+
rms
CORE
+VS
33F
0.1F
C
F
10F
+
1M
+
9V
100k⍀
100k⍀
4.7F
4.7F
VS
VIN
2
Figure 7. Battery-Powered Option
COM
CC
VIN
AD736
FULL
WAVE
RECTIFIER
INPUT
AMPLIFIER
8k⍀
+VS
1
2
3
8
7
6VOUT
VIN
CC
+
Figure 8. Low Z, AC-Coupled Input Connection
REV. D
C00834–0–11/02(D)
PRINTED IN U.S.A.
–12–
AD736
8-Lead Standard Small Outline Package [SOIC]
Narrow Body
(RN-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) ⴛ 45ⴗ
8ⴗ
0ⴗ
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
Revision History
Location Page
11/02—Data Sheet changed from 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
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
