AD736-EVALZ [ADI]
Low Cost, Low Power, True RMS-to-DC Converter; 低成本，低功耗，真RMS至DC转换器
| 型号： | AD736-EVALZ |
| 厂家： | 
ADI |
| 描述： | Low Cost, Low Power, True RMS-to-DC Converter |
| 文件： | 总20页 (文件大小：480K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
Low Cost, Low Power,
True RMS-to-DC Converter
Data Sheet
AD736
FEATURES
FUNCTIONAL BLOCK DIAGRAM
CC
+VS
8kΩ
Converts an ac voltage waveform to a dc voltage and then
converts to the true rms, average rectified, or absolute value
200 mV rms full-scale input range (larger inputs with input
attenuator)
OUT
FULL WAVE
RECTIFIER
RMS
CORE
VIN
CF
8kΩ
(OPT)
CF
High input impedance: 1012
Ω
Low input bias current: 25 pA maximum
High accuracy: 0.3 mV 0.3% of reading
CAV
BIAS
SECTION
COM
CAV
–VS
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 µA maximum supply current
Buffered voltage output
No external trims needed for specified accuracy
Related device: the AD737—features a power-down control
with standby current of only 25 μA; the dc output voltage
is negative and the output impedance is 8 kΩ
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.
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 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.
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 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 (introducing only 2.5%
additional error) at the 200 mV full-scale input level.
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.
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.
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.
Rev. I
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
rightsof third parties that may result fromits use. Specifications subject to change without notice. No
license is granted by implication or otherwise under any patent or patent rights of Analog Devices.
Trademarks andregisteredtrademarks are the property of their respective owners.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781.329.4700
www.analog.com
Fax: 781.461.3113 ©1988–2012 Analog Devices, Inc. All rights reserved.
AD736
Data Sheet
TABLE OF CONTENTS
Features .............................................................................................. 1
RMS Measurement—Choosing the Optimum Value for CAV .... 11
General Description ......................................................................... 1
Functional Block Diagram .............................................................. 1
Product Highlights ........................................................................... 1
Revision History ............................................................................... 2
Specifications..................................................................................... 3
Absolute Maximum Ratings............................................................ 5
Thermal Resistance ...................................................................... 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
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
Additional Application Concepts............................................. 15
Evaluation Board ............................................................................ 17
Outline Dimensions ....................................................................... 19
Ordering Guide .......................................................................... 20
REVISION HISTORY
12/12—Rev. H to Rev. I
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
Changes to Features and Figure 1.................................................. 1
Change to Error vs. Crest Factor Parameter, Table 1.................. 3
Changes to Operating Voltage Range Parameter, Table 1.......... 4
Changes to Table 2........................................................................... 5
Added Table 3; Renumbered Sequentially ................................... 5
Changes to Figure 9......................................................................... 8
Changes to Figure 16....................................................................... 9
Changes to Figure 18..................................................................... 10
Added Additional Application Concepts Section and
Changes to Figure 25..................................................................... 15
Changes to Figure 29..................................................................... 17
Deleted Table 6............................................................................... 17
Changes to Ordering Guide ......................................................... 20
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
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
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
2/06—Rev. F to Rev. G
Updated Format.................................................................Universal
Rev. I | Page 2 of 20
Data Sheet
AD736
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
Min Typ Max
Parameter
Conditions
Min
Typ
Max
Unit
2
TRANSFER FUNCTION
CONVERSION ACCURACY
Total Error, Internal Trim1
All Grades
VOUT = √Avg (VIN )
1 kHz sine wave
Using CC
0 mV rms to 200 mV rms
200 mV to 1 V rms
0.3/0.3
−1.2
0.5/0.5
2.0
0.2/0.2 0.3/0.3 ±mV/±± of reading
−1.2
2.0
± of reading
TMIN to TMAX
A and B Grades
J and K Grades
@ 200 mV rms
@ 200 mV rms
0.7/0.7
0.007
0.5/0.5 ±mV/±± of reading
0.007
±± of reading/°C
vs. Supply Voltage
@ 200 mV rms Input
VS = ±5 V to ±16.5 V
VS = ±5 V to ±3 V
0
0
+0.06
−0.18
1.3
0.25
0.1/0.5
+0.1
−0.3
2.5
0
0
+0.06
−0.18
1.3
0.25
0.1/0.3
+0.1
−0.3
2.5
±/V
±/V
DC Reversal Error, DC-Coupled @ 600 mV dc
± of reading
± of reading
±mV/±± of reading
Nonlinearity2, 0 mV to 200 mV
Total Error, External Trim
ERROR VS. CREST FACTOR3
Crest Factor = 1 to 3
@ 100 mV rms
0 mV rms to 200 mV rms
0
0.35
0
0.35
CAV, CF = 100 µF
CAV, CF = 100 µF
0.7
2.5
0.7
2.5
± additional error
± additional error
Crest Factor = 3 to 5
INPUT CHARACTERISTICS
High Impedance Input
Signal Range (Pin 2)
Continuous RMS Level
VS = +2.8 V, −3.2 V
VS = ±5 V to ±16.5 V
VS = +2.8 V, −3.2 V
VS = ±5 V
200
1
200
1
mV rms
V rms
V
V
Peak Transient Input
0.9
4.0
0.9
4.0
±2.7
±2.7
VS = ±16.5 V
V
Input Resistance
Input Bias Current
1012
1
1012
1
Ω
pA
VS = ±3 V to ±16.5 V
25
25
Low Impedance Input
Signal Range (Pin 1)
Continuous RMS Level
VS = +2.8 V, –3.2 V
VS = ±5 V to ±16.5 V
VS = +2.8 V, −3.2 V
VS = ±5 V
300
1
300
1
mV rms
V rms
V
V
V
Peak Transient Input
±1.7
±3.8
±11
8
±1.7
±3.8
±11
8
VS = ±16.5 V
Input Resistance
Maximum Continuous
Nondestructive Input
6.4
9.6
±12
6.4
9.6
±12
kΩ
V p-p
All supply voltages
Input Offset Voltage4
J and K Grades
A and B Grades
vs. Temperature
vs. Supply
3
3
30
150
3
3
30
150
mV
mV
µV/°C
µV/V
µV/V
8
50
80
8
50
80
VS = ±5 V to ±16.5 V
VS = ±5 V to ±3 V
Rev. I | Page 3 of 20
AD736
Data Sheet
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
0.5
20
±0.1
0.3
0.3
20
mV
mV
µV/°C
µV/V
µV/V
A and B Grades
vs. Temperature
vs. Supply
1
50
50
1
50
50
VS = ±5 V to ±16.5 V
VS = ±5 V to ±3 V
130
130
Output Voltage Swing
2 kΩ Load
VS = +2.8 V, −3.2 V
VS = ±5 V
0 to
1.6
0 to
3.6
0 to 4
0 to 4 12
1.7
3.8
5
0 to
1.6
0 to
3.6
0 to 4
0 to 4 12
1.7
3.8
5
V
V
VS = ±16.5 V
VS = ±16.5 V
V
V
No Load
Output Current
2
2
mA
mA
Ω
Short-Circuit Current
Output Resistance
FREQUENCY RESPONSE
3
0.2
3
0.2
@ dc
High Impedance Input (Pin 2)
for 1± Additional Error
Sine wave input
VIN = 1 mV rms
VIN = 10 mV rms
VIN = 100 mV rms
VIN = 200 mV rms
±3 dB Bandwidth
VIN = 1 mV rms
VIN = 10 mV rms
VIN = 100 mV rms
VIN = 200 mV rms
1
6
37
33
1
6
37
33
kHz
kHz
kHz
kHz
Sine wave input
5
55
170
190
5
55
170
190
kHz
kHz
kHz
kHz
Low Impedance Input (Pin 1) Sine wave input
for 1± Additional Error
VIN = 1 mV rms
1
6
90
90
1
6
90
90
kHz
kHz
kHz
kHz
VIN = 10 mV rms
VIN = 100 mV rms
VIN = 200 mV rms
±3 dB Bandwidth
VIN = 1 mV rms
VIN = 10 mV rms
VIN = 100 mV rms
VIN = 200 mV rms
POWER SUPPLY
Sine wave input
5
55
350
460
5
55
350
460
kHz
kHz
kHz
kHz
Operating Voltage Range
+2.8,
−3.2
±5
±16.5
+2.8,
−3.2
± 5
±16.5
V
Quiescent Current
200 mV rms, No Load
TEMPERATURE RANGE
Operating, Rated Performance
Commercial
Zero signal
Sine wave input
160
230
200
270
160
230
200
270
µA
µA
0°C to 70°C
−40°C to +85°C
AD736JN, AD736JR
AD736AQ, AD736AR
AD736KN, AD736KR
AD736BQ, AD736BR
Industrial
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.
Rev. I | Page 4 of 20
Data Sheet
AD736
ABSOLUTE MAXIMUM RATINGS
THERMAL RESISTANCE
Table 2.
θJA is specified for the worst-case conditions, that is, a device
soldered in a circuit board for surface-mount packages.
Parameter
Rating
±16.5 V
200 mW
Supply Voltage
Internal Power Dissipation
Input Voltage
Pin 2 through Pin 8
Pin 1
Output Short-Circuit Duration
Differential Input Voltage
Storage Temperature Range (Q)
Storage Temperature Range (N, R)
Lead Temperature (Soldering, 60 sec)
ESD Rating
Table 3. Thermal Resistance
Package Type
θJA
Unit
°C/W
°C/W
°C/W
±VS
±12 V
8-Lead PDIP
8-Lead CERDIP
8-Lead SOIC
165
110
155
Indefinite
+VS and –VS
–65°C to +150°C
–65°C to +125°C
300°C
ESD CAUTION
500 V
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.
Rev. I | Page 5 of 20
AD736
Data Sheet
PIN CONFIGURATION AND FUNCTION DESCRIPTIONS
C
1
2
3
4
8
7
6
5
COM
+V
C
AD736
V
IN
S
TOP VIEW
C
OUTPUT
(Not to Scale)
F
–V
C
AV
S
Figure 2. Pin Configuration
Table 4. Pin Function Descriptions
Pin No. Mnemonic Description
1
CC
Coupling 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
3
4
5
6
7
8
VIN
CF
−VS
CAV
OUTPUT
+VS
COM
High Input Impedance Pin.
Connect an Auxiliary Low-Pass Filter Capacitor from the Output.
Negative Supply Voltage if Dual Supplies Are Used, or Ground if Connected to a Single-Supply Source.
Connect the Averaging Capacitor Here.
DC Output Voltage.
Positive Supply Voltage.
Common.
Rev. I | Page 6 of 20
Data Sheet
AD736
TYPICAL PERFORMANCE CHARACTERISTICS
0.7
10V
1V
SINE WAVE INPUT, V =±5V,
V
= 200mV rms
S
IN
1kHz SINE WAVE
C
= 22µF, C = 4.7µF, C = 22µF
AV
F
C
0.5
0.3
0.1
C
C
= 100µF
AV
= 22µF
F
100mV
10mV
1% ERROR
0
–0.1
–3dB
1mV
–0.3
–0.5
10% ERROR
100µV
0
0
0
2
4
6
8
10
12
14
16
16
16
0.1
1
10
100
1000
SUPPLY VOLTAGE (±V)
–3dB FREQUENCY (kHz)
Figure 3. Additional Error vs. Supply Voltage
Figure 6. Frequency Response Driving Pin 1
16
10V
1V
DC-COUPLED
SINE WAVE INPUT, V =±5V,
S
C
= 22µF, C = 4.7µF, C = 22µF
AV
F
C
14
12
10
8
100mV
10mV
PIN 1
1% ERROR
PIN 2
6
10% ERROR
4
1mV
–3dB
2
0
100µV
2
4
6
8
10
12
14
0.1
1
10
–3dB FREQUENCY (kHz)
100
1000
SUPPLY VOLTAGE (±V)
Figure 4. Maximum Input Level vs. Supply Voltage
Figure 7. Frequency Response Driving Pin 2
16
14
12
10
8
6
1kHz SINE WAVE INPUT
3ms BURST OF 1kHz =
3 CYCLES
200mV rms SIGNAL
C
= 10µF
5
4
3
AV
V
C
C
= ±5V
S
= 22µF
= 100µF
C
F
C
= 33µF
AV
6
4
2
1
0
C
= 100µF
AV
2
0
C
= 250µF
AV
2
4
6
8
10
12
14
1
2
3
4
5
SUPPLY VOLTAGE (±V)
CREST FACTOR (V
/V rms)
PEAK
Figure 5. Peak Buffer Output vs. Supply Voltage
Figure 8. Additional Error vs. Crest Factor with Various Values of CAV
Rev. I | Page 7 of 20
AD736
Data Sheet
0.8
1.0
0.5
V
= 200mV rms
IN
1kHz SINE WAVE
0.6
0.4
0.2
C
C
= 100µF
AV
= 22µF
= ±5V
F
S
V
0
–0.5
–1.0
–1.5
–2.0
–2.5
0
–0.2
–0.4
–0.6
–0.8
V
C
C
= SINE WAVE @ 1kHz
IN
= 22µF, C = 47µF,
AV
= 4.7µF, V = ±5V
C
F
S
–60 –40 –20
0
20
40
60
80
100 120 140
10mV
100mV
1V
2V
TEMPERATURE (°C)
INPUT LEVEL (rms)
Figure 9. Additional Error vs. Temperature
Figure 12. Error vs. RMS Input Voltage (Pin 2),
Output Buffer Offset Is Adjusted to Zero
600
500
100
10
1
V
= 200mV rms
= 47µF
= 47µF
IN
V
= 200mV rms
IN
C
C
V
C
F
1kHz SINE WAVE
C
C
= 100µF
AV
= ±5V
S
= 22µF
= ±5V
F
S
V
400
300
–0.5%
200
100
–1%
0
0.2
0.4
0.6
0.8
1.0
10
100
FREQUENCY (Hz)
1k
rms INPUT LEVEL (V)
Figure 10. DC Supply Current vs. rms Input Level
Figure 13. CAV vs. Frequency for Specified Averaging Error
10mV
1mV
1V
V
= 1kHz
IN
SINE WAVE INPUT
AC-COUPLED
–0.5%
V
= ±5V
S
–1%
100mV
10mV
1mV
100µV
10µV
V
SINE WAVE
IN
AC-COUPLED
C
C
= 10µF, C = 47µF,
AV
= 47µF, V = ±5V
C
F
S
100
1k
10k
100k
1
10
100
1k
–3dB FREQUENCY (Hz)
FREQUENCY (Hz)
Figure 11. RMS Input Level (Pin 2) vs. −3 dB Frequency
Figure 14. RMS Input Level vs. Frequency for Specified Averaging Error
Rev. I | Page 8 of 20
Data Sheet
AD736
4.0
3.5
3.0
2.5
10nA
1nA
100pA
10pA
1pA
2.0
1.5
1.0
100fA
0
2
4
6
8
10
12
14
16
–55
–35
–15
5
25
45
65
85
105
125
SUPPLY VOLTAGE (±V)
TEMPERATURE (°C)
Figure 15. Pin 2 Input Bias Current vs. Supply Voltage
Figure 17. Pin 2 Input Bias Current vs. Temperature
1V
100mV
10mV
1mV
VS = 5V
CC = 22µF
CF = 0µF
CAV = 100µF
CAV = 10µF
CAV = 33µF
100µV
1ms
10ms
100ms
1s
10s
100s
SETTLING TIME
Figure 16. RMS Input Level for Various Values of CAV vs. Settling Time
Rev. I | Page 9 of 20
AD736
Data Sheet
THEORY OF OPERATION
AC COUPLED
C
10µF
C =
+
DC
COUPLED
FULL-WAVE
RECTIFIER
AD736
C
COM
8
C
1
8kΩ
0.1µF
OUTPUT
AMPLIFIER
VIN
2
+VS
7
INPUT
AMPLIFIER
IB<10pA
8kΩ
CF
3
OUTPUT
6
BIAS
SECTION
RMS
TRANSLINEAR
CORE
−VS
CAV
5
4
0.1µF
CAV
33
TO COM PIN
µ
F
+
CF
10
(OPTIONAL LPF)
µ
F
+
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.
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
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,
C
AV. Without CAV, the rectified input signal travels through the
V
PEAK; the corresponding rms value is 0.707 × VPEAK. Therefore, for
core unprocessed, as is done with the average responding
connection (see Figure 19).
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.
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.
Rev. I | Page 10 of 20
Data Sheet
AD736
Mathematically, the rms value of a voltage is defined (using a
simplified equation) as
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.
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. 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 5).
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.
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.
Table 5. Error Introduced by an Average Responding Circuit when Measuring Common Waveforms
Average Responding Circuit
Crest Factor True RMS Calibrated to Read RMS Value of
(VPEAK/V rms) Value (V) Sine Waves (V)
% of Reading Error Using
Average Responding Circuit
Waveform Type 1 V Peak Amplitude
Undistorted Sine Wave
Symmetrical Square Wave
Undistorted Triangle Wave
Gaussian Noise (98± of Peaks <1 V)
Rectangular
1.414
1.00
1.73
3
2
10
0.707
1.00
0.577
0.333
0.5
0.707
1.11
0.555
0.295
0.278
0.011
0
+11.0
−3.8
−11.4
−44
−89
Pulse Train
0.1
SCR Waveforms
50± Duty Cycle
25± Duty Cycle
2
4.7
0.495
0.212
0.354
0.150
−28
−30
Rev. I | Page 11 of 20
AD736
Data Sheet
In most cases, the combined magnitudes of both the dc and
RAPID SETTLING TIMES VIA THE AVERAGE
RESPONDING CONNECTION
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.
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.
E
O
IDEAL
C
C
E
O
10µF
+
DC ERROR = E – E (IDEAL)
O
O
(OPTIONAL)
8kΩ
C
1
2
8
C
COM
+V
AVERAGE E = E
AD736
O
O
DOUBLE-FREQUENCY
RIPPLE
FULL
WAVE
RECTIFIER
V
+V
S
IN
8kΩ
7
TIME
V
S
IN
INPUT
AMPLIFIER
Figure 20. Output Waveform for Sine Wave Input Voltage
C
OUTPUT
F
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.
3
6
V
OUT
BIAS
SECTION
OUTPUT
AMPLIFIER
–V
S
rms
CORE
–V
S
C
AV
4
5
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.
C
33µF
F
+V
S
POSITIVE SUPPLY
COMMON
0.1µF
0.1µF
–V
NEGATIVE SUPPLY
S
Figure 19. AD736 Average Responding Circuit
DC ERROR, OUTPUT RIPPLE, AND AVERAGING
ERROR
the crest factor of the AD736 for various values of CAV
.
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.
Rev. I | Page 12 of 20
Data Sheet
AD736
APPLICATIONS
CONNECTING THE INPUT
1
2
3
4
8
7
6
5
C
V
COM
C
AD736
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.
+V
IN
S
+V
S
C
OUTPUT
F
VOUT
DC
–V
C
AV
S
C
AV
–V
S
Figure 23. Low-Z AC-Coupled Input Connection
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.
1
2
3
4
8
7
6
5
C
V
COM
C
AD736
+V
IN
S
+V
S
C
OUTPUT
F
VOUT
DC
–V
C
AV
S
C
AV
1
2
3
4
8
7
6
5
C
V
COM
C
AD736
+V
+V
–V
S
IN
S
S
1MΩ
Figure 24. Low-Z DC-Coupled Input Connection
C
OUTPUT
VOUT
DC
F
–V
C
AV
S
C
AV
–V
S
Figure 21. High-Z AC-Coupled Input Connection (Default)
1
2
3
4
8
7
6
5
C
V
COM
C
AD736
+V
IN
S
+V
S
C
OUTPUT
F
VOUT
DC
–V
C
AV
S
C
AV
–V
S
Figure 22. High-Z DC-Coupled Input Connection
Rev. I | Page 13 of 20
AD736
Data Sheet
Note that at FL, the amplitude error is approximately −30%
SELECTING PRACTICAL VALUES FOR INPUT
COUPLING (CC), AVERAGING (CAV), AND FILTERING
(CF) CAPACITORS
(–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.
Table 6 provides practical values of CAV and CF for several
common applications.
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.
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
1
FL =
2π (8000)(Value of CC in Farads)
Table 6. Capacitor Selection Chart
Low Frequency
Cutoff (−3 dB)
Max Crest
Factor
CAV
(µF)
CF
Application
RMS Input Level
(µF) Settling Time1 to 1%
General-Purpose RMS Computation
0 V to 1 V
20 Hz
200 Hz
20 Hz
200 Hz
20 Hz
200 Hz
20 Hz
200 Hz
50 Hz
60 Hz
50 Hz
60 Hz
5
5
5
5
150
15
33
10
1
10
1
360 ms
36 ms
360 ms
36 ms
1.2 sec
120 ms
1.2 sec
120 ms
1.2 sec
1.0 sec
1.2 sec
1.0 sec
0 mV to 200 mV
0 V to 1 V
3.3
General Purpose
Average
Responding
None 33
None 3.3
None 33
None 3.3
100
82
0 mV to 200 mV
0 mV to 200 mV
0 mV to 100 mV
SCR Waveform Measurement
5
5
5
5
33
27
33
27
50
47
Audio Applications
Speech
Music
0 mV to 200 mV
0 mV to 100 mV
300 Hz
20 Hz
3
10
1.5
100
0.5
68
18 ms
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.
Rev. I | Page 14 of 20
Data Sheet
AD736
47 kΩ, 1 W resistor and diode pair are a practical input
protection scheme for ac line connection measurements.
ADDITIONAL APPLICATION CONCEPTS
Figure 25 through Figure 28 show four application concepts.
Figure 25 shows the high input impedance FET input connected to
a multitap attenuator network used in various types of instruments
requiring wide ranges of voltages. For a direct network connection,
the gate-charge bleeding resistor is not required. The impedance of
the FET input is high enough (1012 Ω) so that the loading error
is negligible. Manufacturers and distributors of the matched
precision resistor networks shown in these figures can easily be
found on the Web. The voltages shown in the diagrams are the
input levels corresponding to 200 mV at each tap. Finally, the
Figure 26 shows both inputs connected differentially. Figure 27
shows additional components used for offset correction of the
output amplifier, and Figure 28 shows connections for single-
supply operation such as is the case for handheld devices.
Further information can be found in the AN-268 Application
Note—RMS-to-DC Converters Ease Measurement Tasks—and
the RMS to DC Converter Application Guide, both of which
can be found on the Analog Devices, Inc., website.
OPTIONAL
AC COUPLING
CAPACITOR
VIN FOR FULL
SCALE OUTPUT
C
C
10µF
+
V
IN
0.01µF
1kV
(OPTIONAL)
8kΩ
+V
S
C
1
2
8
C
COM
200mV
BAV199
AD736
9MΩ
900kΩ
90kΩ
FULL
WAVE
V
+V
S
IN
2V
8kΩ
RECTIFIER
7
6
+V
S
INPUT
47kΩ
1W
1µF
20V
200V
AMPLIFIER
C
F
OUTPUT
3
BIAS
–V
S
SECTION
OUTPUT
AMPLIFIER
10kΩ
–V
C
5
S
AV
rms
CORE
–V
4
S
+
C
33µF
AV
1µF
+
C
F
10µF (OPTIONAL)
Figure 25. AD736 with a High Impedance Input Attenuator
C
10µF
AD711
C
3
2
C
C
–IN
8kΩ
6
+
1
2
8
COM
AD736
FULL
WAVE
RECTIFIER
V
+V
S
IN
8kΩ
+IN
7
+V
S
1µF
INPUT
AMPLIFIER
12
INPUT IMPEDANCE: 10 Ω||10pF
C
OUTPUT
F
3
6
OUTPUT
BIAS
SECTION
OUTPUT
AMPLIFIER
–V
C
5
S
AV
rms
CORE
–V
4
S
+
C
33µF
AV
1µF
+
C
F
10µF (OPTIONAL)
Figure 26. Differential Input Connection
Rev. I | Page 15 of 20
AD736
Data Sheet
C
C
10µF
+
(OPTIONAL)
8kΩ
C
1
2
8
C
COM
AD736
FULL
WAVE
RECTIFIER
V
+V
S
IN
DC-COUPLED
IN
8kΩ
V
7
+V
S
1µF
INPUT
AMPLIFIER
0.1µF
C
OUTPUT
F
3
6
OUTPUT
BIAS
SECTION
AC-COUPLED
1MΩ
OUTPUT
AMPLIFIER
–V
C
5
S
AV
rms
CORE
+V
4
S
39MΩ
OUTPUT
1MΩ
+
V
OS
C
33µF
AV
1µF
ADJUST
+
C
F
–V
S
10µF (OPTIONAL)
Figure 27. External Output VOS Adjustment
C
C
10µF
+
C
C
COM
8kΩ
1
8
AD736
V
S
2
FULL
WAVE
RECTIFIER
+V
V
S
IN
0.1µF
8kΩ
V
2
7
IN
INPUT
AMPLIFIER
1MΩ
100kΩ
C
OUTPUT
V
S
F
4.7µF
4.7µF
2
3
6
BIAS
SECTION
9V
OUTPUT
AMPLIFIER
–V
C
5
S
AV
rms
CORE
4
+
100kΩ
33µF
+
C
F
10µF (OPTIONAL)
Figure 28. Battery-Powered Option
Rev. I | Page 16 of 20
Data Sheet
AD736
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.
Figure 31. Evaluation Board—Component-Side Copper
Figure 29. AD736 Evaluation Board
Figure 32. Evaluation Board—Secondary-Side Copper
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.
Figure 33. Evaluation Board—Internal Power Plane
Figure 34. Evaluation Board—Internal Ground Plane
Rev. I | Page 17 of 20
AD736
Data Sheet
–V +V
S
S
S
GND1 GND2 GND3 GND4
+
C1
10µF
25V
C2
10µF
25V
+
–V +V
S
W3
AC COUP
W1
DC
LO-Z
W4
R3
0Ω
COUP
LO-Z IN
+
C
C
VIN
P2
HI-Z SEL
CIN
0.1µF
J1
R4
0Ω
HI-Z
1
8
7
6
5
IN
C
V
COM
C
C6
0.1µF
AD736
2
3
4
+V
+V
S
S
VOUT
IN
GND
W2
J2
C
OUT
F
CF1
R1
C4
CAV
1MΩ
0.1µF
–V
C
AV
S
SEL
J3
CAV
33µF
16V+
NORM
PD
FILT
+V
S
–V
S
CF2
Figure 35. Evaluation Board Schematic
Rev. I | Page 18 of 20
Data Sheet
AD736
OUTLINE DIMENSIONS
0.400 (10.16)
0.365 (9.27)
0.355 (9.02)
8
1
5
4
0.280 (7.11)
0.250 (6.35)
0.240 (6.10)
0.325 (8.26)
0.310 (7.87)
0.300 (7.62)
0.100 (2.54)
BSC
0.060 (1.52)
MAX
0.195 (4.95)
0.130 (3.30)
0.115 (2.92)
0.210 (5.33)
MAX
0.015
(0.38)
MIN
0.150 (3.81)
0.130 (3.30)
0.115 (2.92)
0.015 (0.38)
GAUGE
0.014 (0.36)
0.010 (0.25)
0.008 (0.20)
PLANE
SEATING
PLANE
0.022 (0.56)
0.018 (0.46)
0.014 (0.36)
0.430 (10.92)
MAX
0.005 (0.13)
MIN
0.070 (1.78)
0.060 (1.52)
0.045 (1.14)
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.
Figure 36. 8-Lead Plastic Dual In-Line Package [PDIP]
Narrow Body (N-8)
Dimensions shown in inches and (millimeters)
0.005 (0.13)
MIN
0.055 (1.40)
MAX
5.00 (0.1968)
4.80 (0.1890)
8
5
0.310 (7.87)
0.220 (5.59)
1
4
8
1
5
4
6.20 (0.2441)
5.80 (0.2284)
4.00 (0.1574)
3.80 (0.1497)
0.100 (2.54) BSC
0.405 (10.29) MAX
0.320 (8.13)
0.290 (7.37)
0.50 (0.0196)
0.25 (0.0099)
1.27 (0.0500)
BSC
⋅
45°
1.75 (0.0688)
1.35 (0.0532)
0.060 (1.52)
0.200 (5.08)
MAX
0.25 (0.0098)
0.10 (0.0040)
0.015 (0.38)
8°
0°
0.150 (3.81)
MIN
0.200 (5.08)
0.125 (3.18)
0.51 (0.0201)
0.31 (0.0122)
COPLANARITY
0.10
1.27 (0.0500)
0.40 (0.0157)
0.25 (0.0098)
0.17 (0.0067)
SEATING
PLANE
0.015 (0.38)
0.008 (0.20)
SEATING
PLANE
0.023 (0.58)
0.014 (0.36)
15°
0°
0.070 (1.78)
0.030 (0.76)
COMPLIANT TO JEDEC STANDARDS MS-012-AA
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.
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.
Figure 37. 8-Lead Ceramic Dual In-Line Package [CERDIP]
(Q-8)
Figure 38. 8-Lead Standard Small Outline Package [SOIC_N]
Narrow Body (R-8)
Dimensions shown in inches and (millimeters)
Dimensions shown in millimeters and (inches)
Rev. I | Page 19 of 20
AD736
Data Sheet
ORDERING GUIDE
Model1
Temperature Range
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
0°C to +70°C
0°C to +70°C
0°C to +70°C
0°C to +70°C
0°C to +70°C
0°C to +70°C
0°C to +70°C
0°C to +70°C
0°C to +70°C
Package Description
8-Lead CERDIP
8-Lead CERDIP
8-Lead SOIC_N
8-Lead SOIC_N
8-Lead SOIC_N
8-Lead SOIC_N
8-Lead SOIC_N
8-Lead SOIC_N
8-Lead SOIC_N
8-Lead SOIC_N
8-Lead SOIC_N
8-Lead SOIC_N
8-Lead PDIP
Package Option
AD736AQ
AD736BQ
Q-8
Q-8
R-8
R-8
R-8
R-8
R-8
R-8
R-8
R-8
R-8
R-8
N-8
N-8
N-8
R-8
R-8
R-8
R-8
R-8
R-8
R-8
R-8
R-8
AD736AR-REEL
AD736AR-REEL7
AD736ARZ
AD736ARZ-R7
AD736ARZ-RL
AD736BR-REEL
AD736BR-REEL7
AD736BRZ
AD736BRZ-R7
AD736BRZ-RL
AD736JN
AD736JNZ
AD736KNZ
8-Lead PDIP
8-Lead PDIP
AD736JR
8-Lead SOIC_N
8-Lead SOIC_N
8-Lead SOIC_N
8-Lead SOIC_N
8-Lead SOIC_N
8-Lead SOIC_N
8-Lead SOIC_N
8-Lead SOIC_N
8-Lead SOIC_N
Evaluation Board
AD736JR-REEL
AD736JR-REEL7
AD736JRZ
AD736JRZ-RL
AD736JRZ-R7
AD736KRZ
AD736KRZ-RL
AD736KRZ-R7
AD736-EVALZ
0°C to +70°C
0°C to +70°C
0°C to +70°C
1 Z = RoHS Compliant Part.
©1988–2012 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
D00834-0-12/12(I)
Rev. I | Page 20 of 20
相关型号：
©2020 ICPDF网 联系我们和版权申明