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AD737JRZ-5 Datasheet(PDF) 14 Page - Analog Devices

Part No. AD737JRZ-5
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 Datasheet(HTML) 14 Page - Analog Devices

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AD737
Data Sheet
Rev. I | Page 14 of 24
APPLICATIONS INFORMATION
RMS MEASUREMENT—CHOOSING AN OPTIMUM
VALUE FOR CAV
Because the external averaging capacitor, CAV, holds the rec-
tified input signal during rms computation, its value directly
affects the accuracy of the rms measurement, especially at low
frequencies. Furthermore, because the averaging capacitor is
connected across a diode in the rms core, the averaging time
constant (τAV) increases exponentially as the input signal
decreases. It follows that decreasing the input signal decreases
errors due to nonideal averaging but increases the settling time
approaching the decreased rms-computed dc value. Thus,
diminishing input values allow the circuit to perform better
(due to increased averaging) while increasing the waiting time
between measurements. A trade-off must be made between
computational accuracy and settling time when selecting CAV.
RAPID SETTLING TIMES VIA THE AVERAGE
RESPONDING CONNECTION
Because the average responding connection shown in Figure 25
does not use an averaging capacitor, its settling time does not vary
with input signal level; it is determined solely by the RC time
constant of CF and the internal 8 kΩ output scaling resistor.
POSITIVE SUPPLY
+VS
0.1µF
–VS
0.1µF
COMMON
NEGATIVE SUPPLY
VOUT
CC
VIN
CF
33µF
COM
OUTPUT
AD737
BIAS
SECTION
INPUT
AMPLIFIER
8kΩ
8kΩ
POWER
DOWN
–VS
+VS
+
CAV
1
2
3
4
8
7
6
5
FULL-WAVE
RECTIFIER
RMS
CORE
Figure 25. AD737 Average Responding Circuit
Selectable Average or RMS Conversion
For some applications, it is desirable to be able to select between
rms-value-to-dc conversion and average-value-to-dc conversion.
If CAV is disconnected from the root-mean core, the AD737 full-
wave rectifier is a highly accurate absolute value circuit. A CMOS
switch whose gate is controlled by a logic level selects between
average and rms values.
VINRMS
–2.5V
1
8
7
6
5
4
3
2
CC
VIN
COM
+VS
OUT
CAV
–VS
33µF
33µF
AD737
VOUTDC
+2.5V
1MΩ
rms
AVG
NTR4501NT1
ASSUMED TO
BE A LOGIC
SOURCE
Figure 26. CMOS Switch Is Used to Select RMS or Average Responding Modes
SELECTING PRACTICAL VALUES FOR CAPACITORS
Table 6 provides practical values of CAV and CF for several
common applications.
The input coupling capacitor, CC, in conjunction with the 8 kΩ
internal input scaling resistor, determines the −3 dB low frequency
roll-off. This frequency, FL, is equal to
(
)
Farads
in
C
F
C
L
×
×
π
=
8000
2
1
(1)
Note that, at FL, the amplitude error is approximately −30%
(−3 dB) of reading. To reduce this error to 0.5% of reading,
choose a value of CC that sets FL at one-tenth of the lowest
frequency to be measured.
In addition, if the input voltage has more than 100 mV of dc
offset, the ac coupling network at Pin 2 is required in addition
to Capacitor CC.
SCALING INPUT AND OUTPUT VOLTAGES
The AD737 is an extremely flexible device. With minimal
external circuitry, it can be powered with single- or dual-
polarity power supplies, and input and output voltages are
independently scalable to accommodate nonmatching I/O
devices. This section describes a few such applications.
Extending or Scaling the Input Range
For low supply voltage applications, the maximum peak voltage
to the device is extended by simply applying the input voltage to
Pin 1 across the internal 8 kΩ input resistor. The AD737 input
circuit functions quasi-differentially, with a high impedance
FET input at Pin 2 (noninverting) and a low impedance input at
Pin 1 (inverting, see Figure 25). The internal 8 kΩ resistor behaves
as a voltage-to-current converter connected to the summing
node of a feedback loop around the input amplifier. Because the
feedback loop acts to servo the summing node voltage to match
the voltage at Pin 2, the maximum peak input voltage increases
until the internal circuit runs out of headroom, approximately
double for a symmetrical dual supply.


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