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AD737JRZ-5-R7 Datasheet(PDF) 12 Page - Analog Devices

Part No. AD737JRZ-5-R7
Description  Low Cost, Low Power, True RMS-to-DC Converter
Download  24 Pages
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
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AD737JRZ-5-R7 Datasheet(HTML) 12 Page - Analog Devices

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AD737
Data Sheet
Rev. I | Page 12 of 24
THEORY OF OPERATION
As shown in Figure 23, the AD737 has four functional subsec-
tions: an input amplifier, a full-wave rectifier, an rms core, and a
bias section. The FET input amplifier allows a high impedance,
buffered input at Pin 2 or a low impedance, wide dynamic range
input at Pin 1. The high impedance input, with its low input bias
current, is ideal for use with high impedance input attenuators.
The input signal can be either dc-coupled 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.
RMS
TRANSLINEAR
CORE
8
COM
+VS
7
6
OUTPUT
5
CAV
CURRENT
MODE
ABSOLUTE
VALUE
1
2
3
POWER
DOWN
4
CA
33µF
AC
CC = 10µF
CF
10µF
(OPTIONAL
LPF)
VIN
–VS
–VS
+VS
VIN
CC
+
OPTIONAL RETURN PATH
8kΩ
+
+
DC
BIAS
SECTION
FET
OP AMP
IB < 10pA
8kΩ
0.1µF
0.1µF
COMMON
POSITIVE SUPPLY
NEGATIVE SUPPLY
Figure 23. AD737 True RMS Circuit (Test Circuit)
The output of the input amplifier drives a full-wave precision
rectifier, which, in turn, drives the rms core. It is the core that
provides the essential rms operations of squaring, averaging,
and square rooting, using an external averaging capacitor, CAV.
Without CAV, the rectified input signal passes through the core
unprocessed, as is done with the average responding connection
(see Figure 25). In the average responding mode, averaging is
carried out by an RC post filter consisting of an 8 kΩ internal
scale factor resistor connected between Pin 6 and Pin 8 and an
external averaging capacitor, CF. In the rms circuit, this addi-
tional filtering stage reduces any output ripple that was not
removed by the averaging capacitor.
Finally, the bias subsection permits a power-down function.
This reduces the idle current of the AD737 from 160 µA to
30 µA. This feature is selected by connecting Pin 3 to Pin 7 (+VS).
TYPES OF AC MEASUREMENT
The AD737 is capable of measuring ac signals by operating as
either an average responding converter or a true rms-to-dc con-
verter. 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 VPEAK; the corresponding rms value is 0.707 times VPEAK.
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
)
( 2
V
Avg
V rms =
This involves squaring the signal, taking the average, and then
obtaining the square root. True rms converters are smart recti-
fiers; 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 has a computational error 11% (of reading)
higher than the true rms value (see Table 5).
The transfer function for the AD737 is
)
(
2
IN
OUT
V
Avg
V
=


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