AD8436ARQZ Просмотр технического описания (PDF) - Analog Devices
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AD8436ARQZ Datasheet PDF : 21 Pages
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AD8436
Data Sheet
THEORY OF OPERATION
OVERVIEW
The AD8436 is an implicit function rms-to-dc converter that
renders a dc voltage dependent on the rms (heating value) of an
ac voltage. In addition to the basic converter, this highly integrated
functional circuit block includes two fully independent, optional
amplifiers, a standalone FET input buffer amplifier, and a precision
dc output buffer amplifier (see Figure 1). The rms core includes
a precision current responding full-wave rectifier and a log-
antilog transistor array for current squaring and square rooting
to implement the classic expression for rms (see Equation 1).
For basic applications, the converter requires only an external
capacitor, for averaging (see Figure 31). The optional on-board
amplifiers offer utility and flexibility in a variety of applications
without incurring additional circuit board footprint. For lowest
power, the amplifier supply pins are left unconnected.
Why RMS?
The rms value of an ac voltage waveform is equal to the dc
voltage providing the same heating power to a load. A common
measurement technique for ac waveforms is to rectify the signal
in a straightforward way using a diode array of some sort, resulting
in the average value. The average value of various waveforms (sine,
square, and triangular, for example) varies widely; true rms is the
only metric that achieves equivalency for all ac waveforms. See
Table 5 for non-rms-responding circuit errors.
The acronym rms means “root-mean-square” and reads as follows:
“the square root of the average of the sum of the squares” of the
peak values of any waveform. RMS is shown in the following
equation:
erms
1
T
T
V
0
t
2
dt
(1)
For additional information, select Section I of the second edition of
the Analog Devices RMS-to-DC Applications Guide.
Table 5. General AC Parameters
Waveform Type (1 V Peak)
Sine
Square
Triangle
Noise
Rectangular
Pulse
SCR
DC = 50%
DC = 25%
Crest Factor
1.414
1.00
1.73
3
2
10
2
4.7
RMS Value
0.707
1.00
0.577
0.333
0.5
0.1
0.495
0.212
RMS Core
The core consists of a voltage-to-current converter (precision
resistor), absolute value, and translinear sections. The translinear
section exploits the properties of the bipolar transistor junctions
for squaring and root extraction (see Figure 24). The external
capacitor (CAVG) provides for averaging the product. Figure 20
shows that there is no effect of signal input on the transition times,
as seen in the dc output. Although the rms core responds to input
voltages, the conversion process is current sensitive. If the rms
input is ac-coupled, as recommended, there is no output offset
voltage, as reflected in Table 1. If the rms input is dc-coupled, the
input offset voltage is reflected in the output and can be calibrated
as with any fixed error.
V+
AC IN
ABSOLUTE
VALUE
V-TO-I CIRCUIT
+
5kΩ CAVG
V+
OUT
16kΩ
V–
Figure 24. RMS Core Block Diagram
Reading of an Average Value Circuit
Calibrated to an RMS Sine Wave
0.707
1.11
0.555
0.295
0.278
0.011
0.354
0.150
Error (%)
0
11.0
−3.8
−11.4
−44
−89
−28
−30
Rev. E | Page 10 of 21
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