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PDF AD736 Data sheet ( Hoja de datos )
| Número de pieza | AD736 | |
| Descripción | True RMS-to-DC Converter | |
| Fabricantes | Analog Devices | |
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Data Sheet
FEATURES
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)
High input impedance: 1012 Ω
Low input bias current: 25 pA maximum
High accuracy: ±0.3 mV ± 0.3% of reading
RMS conversion with signal crest factors up to 5
Wide power supply range: +2.8 V, −3.2 V to ±16.5 V
Low power: 200 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
GENERAL DESCRIPTION
The AD736 is a low power, precision, monolithic true rms-to-
dc converter. It is laser trimmed to provide a maximum error of
±0.3 mV ± 0.3% of reading with sine wave inputs. Furthermore,
it maintains high accuracy while measuring a wide range of
input waveforms, including variable duty-cycle pulses and triac
(phase)-controlled sine waves. The low cost and small size of
this converter make it suitable for upgrading the performance
of non-rms precision rectifiers in many applications. Compared
to these circuits, the AD736 offers higher accuracy at an equal
or lower cost.
The AD736 can compute the rms value of both ac and dc input
voltages. It can also be operated as an ac-coupled device by
adding one external capacitor. In this mode, the AD736 can
resolve input signal levels of 100 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.
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.
Low Cost, Low Power,
True RMS-to-DC Converter
AD736
FUNCTIONAL BLOCK DIAGRAM
CC 8kΩ
+VS
OUT
VIN
FULL WAVE
RECTIFIER
RMS
CORE
8kΩ
CF
(OPT)
CF
COM
BIAS
SECTION
CAV
CAV
–VS
Figure 1.
The AD736 allows the choice of two signal input terminals: a
high impedance FET input (1012 Ω) that directly interfaces with
High-Z input attenuators and a low impedance input (8 kΩ) that
allows the measurement of 300 mV input levels while operating
from the minimum power supply voltage of +2.8 V, −3.2 V. The
two inputs can be used either single ended or differentially.
The AD736 has a 1% reading error bandwidth that exceeds
10 kHz for the input amplitudes from 20 mV rms to 200 mV rms
while consuming only 1 mW.
The AD736 is available in four performance grades. The
AD736J and AD736K grades are rated over the 0°C to +70°C
and −20°C to +85°C commercial temperature ranges. The
AD736A and AD736B grades are rated over the −40°C to +85°C
industrial temperature range. The AD736 is available in three
low cost, 8-lead packages: PDIP, SOIC, and CERDIP.
PRODUCT HIGHLIGHTS
1. The AD736 is capable of computing the average rectified
value, absolute value, or true rms value of various input signals.
2. Only one external component, an averaging capacitor, is
required for the AD736 to perform true rms measurement.
3. The low power consumption of 1 mW makes the AD736
suitable for many battery-powered applications.
4. A high input impedance of 1012 Ω eliminates the need for an
external buffer when interfacing with input attenuators.
5. A low impedance input is available for those applications that
require an input signal up to 300 mV rms operating from low
power supply voltages.
Rev. I
Information furnished by Analog Devices is believed to be accurate and reliable. However, no
responsibilityisassumedbyAnalogDevices for itsuse,nor foranyinfringementsofpatentsor other
rights of third parties that may result from its use. Specifications subject to change without notice. No
license is granted by implication or otherwise under any patent or patent rights of Analog Devices.
Trademarksandregisteredtrademarksarethepropertyoftheirrespectiveowners.
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.
1 page  
Data Sheet
ABSOLUTE MAXIMUM RATINGS
Table 2.
Parameter
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
Rating
±16.5 V
200 mW
±VS
±12 V
Indefinite
+VS and –VS
–65°C to +150°C
–65°C to +125°C
300°C
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.
AD736
THERMAL RESISTANCE
θJA is specified for the worst-case conditions, that is, a device
soldered in a circuit board for surface-mount packages.
Table 3. Thermal Resistance
Package Type
8-Lead PDIP
8-Lead CERDIP
8-Lead SOIC
θJA
165
110
155
Unit
°C/W
°C/W
°C/W
ESD CAUTION
Rev. I | Page 5 of 20
5 Page 
Data Sheet
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. 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).
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.
AD736
The settling time corresponding to the new or final input level
of 1 mV is approximately 8 seconds. Therefore, the net time for
the circuit to settle to its new value is 8 seconds minus 80 ms,
which is 7.92 seconds. Note that because of the smooth decay
characteristic inherent with a capacitor/diode combination, this
is the total settling time to the final value (that is, not the settling
time to 1%, 0.1%, and so on, of the final value). In addition, this
graph provides the worst-case settling time because the AD736
settles very quickly with increasing input levels.
RMS MEASUREMENT—CHOOSING THE OPTIMUM
VALUE FOR CAV
Because the external averaging capacitor, CAV, holds the
rectified input signal during rms computation, its value directly
affects the accuracy of the rms measurement, especially at low
frequencies. Furthermore, because the averaging capacitor
appears across a diode in the rms core, the averaging time
constant increases exponentially as the input signal is reduced.
This means that as the input level decreases, errors due to
nonideal averaging decrease, and the time required for the
circuit to settle to the new rms level increases. Therefore, lower
input levels allow the circuit to perform better (due to increased
averaging) but increase the waiting time between measurements.
Obviously, when selecting CAV, a trade-off between computational
accuracy and settling time is required.
Table 5. Error Introduced by an Average Responding Circuit when Measuring Common Waveforms
Waveform Type 1 V Peak Amplitude
Crest Factor
(VPEAK/V rms)
Average Responding Circuit
True RMS Calibrated to Read RMS Value of
Value (V) Sine Waves (V)
Undistorted Sine Wave
1.414
0.707
0.707
Symmetrical Square Wave
1.00 1.00 1.11
Undistorted Triangle Wave
1.73
0.577
0.555
Gaussian Noise (98% of Peaks <1 V)
3
0.333
0.295
Rectangular
2 0.5 0.278
Pulse Train
10 0.1 0.011
SCR Waveforms
50% Duty Cycle
2
0.495
0.354
25% Duty Cycle
4.7
0.212
0.150
% of Reading Error Using
Average Responding Circuit
0
+11.0
−3.8
−11.4
−44
−89
−28
−30
Rev. I | Page 11 of 20
11 Page | ||
| Páginas | Total 20 Páginas | |
| PDF Descargar | [ Datasheet AD736.PDF ] | |
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