AD8436 AD [Analog Devices], AD8436 Datasheet - Page 10

• Manufacturer Part Number: AD8436
• Description: Low Cost, Low Power
• Manufacturer: AD [Analog Devices]

AD8436
THEORY OF OPERATION
OVERVIEW
The
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 imple-
ment the classic expression for rms (see Equation 1). For basic
applications, the converter requires only an external capacitor, for
averaging (see Figure 30). 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 measure-
ment 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:
Table 5. General AC Parameters
Waveform Type (1 V p-p)
Sine
Square
Triangle
Noise
Rectangular
Pulse
SCR
DC = 50%
DC = 25%
AD8436
is an implicit function rms-to-dc converter that
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
Rev. 0 | Page 10 of 20
(1)
For additional information, select Section I of the 2
the
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 23). The external
capacitor (CAVG) provides for averaging the product. Figure 19
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.
AC IN
V-TO-I
Analog Devices RMS-to-DC Applications
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
ABSOLUTE
CIRCUIT
VALUE
Figure 23. RMS Core Block Diagram
V+
V–
5kΩ
CAVG
+
Guide.
V+
Error (%)
−89
−28
−30
0
11.0
−3.8
−11.4
−44
nd
edition of
16kΩ
OUT