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AD736KR View Datasheet(PDF) - Analog Devices

Part NameDescriptionManufacturer
AD736KR Low Cost, Low Power, True RMS-to-DC Converter ADI
Analog Devices ADI
AD736KR Datasheet PDF : 8 Pages
1 2 3 4 5 6 7 8
RAPID SETTLING TIMES VIA THE AVERAGE
RESPONDING CONNECTION (FIGURE 17)
Because the average responding connection does not use an av-
eraging capacitor, its settling time does not vary with input sig-
nal level; it is determined solely by the RC time constant of CF
and the internal 8 koutput scaling resistor.
Figure 17. AD737 Average Responding Circuit
DC ERROR, OUTPUT RIPPLE, AND AVERAGING
ERROR
Figure 18 shows the typical output waveform of the AD737 with
a sine-wave input voltage applied. As with all real-world devices,
the ideal output of VOUT = VIN is never exactly achieved; in-
stead, the output contains both a dc and an ac error component.
Figure 18. Output Waveform for Sine-Wave Input Voltage
As shown, the dc error is the difference between the average of
the output signal (when all the ripple in the output has been
removed by external filtering) and the ideal dc output. The dc
error component is therefore set solely by the value of averag-
ing capacitor used–no amount of post filtering (i.e., using a
very large CF) will allow the output voltage to equal its ideal
value. The ac error component, an output ripple, may be easily
removed by using a large enough post filtering capacitor, CF.
In most cases, the combined magnitudes of both the dc and ac error
components need to be considered when selecting appropriate values
for capacitors CAV and CF. This combined error, representing the
maximum uncertainty of the measurement is termed the “averaging
error” and is equal to the peak value of the output ripple plus the dc
error. As the input frequency increases, both error components de-
crease rapidly: if the input frequency doubles, the dc error and ripple
reduce to 1/4 and 1/2 their original values, respectively, and rapidly
become insignificant.
AD737
AC MEASUREMENT ACCURACY AND CREST FACTOR
The crest factor of the input waveform is often overlooked when
determining the accuracy of an ac measurement. Crest factor is
defined as the ratio of the peak signal amplitude to the rms am-
plitude (C.F. = VPEAK/V rms). Many common waveforms, such
as sine and triangle waves, have relatively low crest factors (2).
Other waveforms, such as low duty cycle pulse trains and SCR
waveforms, have high crest factors. These types of waveforms
require a long averaging time constant (to average out the long
time periods between pulses). Figure 6 shows the additional er-
ror vs. crest factor of the AD737 for various values of CAV.
SELECTING PRACTICAL VALUES FOR INPUT
COUPLING (CC), AVERAGING (CAV) AND FILTERING
(CF) CAPACITORS
Table II provides practical values of CAV and CF for several
common applications.
Table II. AD737 Capacitor Selection Chart
Application
rms
Input
Level
Low
Max CAV CF Settling
Frequency Crest
Time*
Cutoff Factor
to 1%
(–3 dB)
General Purpose 0–1 V
rms Computation
20 Hz 5
200 Hz 5
150 µF 10 µF 360 ms
15 µF 1 µF 36 ms
0–200 mV 20 Hz 5
200 Hz 5
33 µF 10 µF 360 ms
3.3 µF 1 µF 36 ms
General Purpose
Average
Responding
0–1 V
20 Hz
200 Hz
0–200 mV 20 Hz
200 Hz
None 33 µF 1.2 sec
None 3.3 µF 120 ms
None 33 µF 1.2 sec
None 3.3 µF 120 ms
SCR Waveform 0–200 mV 50 Hz 5
Measurement
60 Hz 5
100 µF 33 µF 1.2 sec
82 µF 27 µF 1.0 sec
0–100 mV 50 Hz 5
60 Hz 5
50 µF 33 µF 1.2 sec
47 µF 27 µF 1.0 sec
Audio
Applications
Speech
0–200 mV 300 Hz 3
1.5 µF 0.5 µF 18 ms
Music
0–100 mV 20 Hz
10
100 µF 68 µF 2.4 sec
* Settling time is specified over the stated rms input level with the input signal increasing
from zero. Settling times will be greater for decreasing amplitude input signals.
The input coupling capacitor, CC, in conjunction with the 8 k
internal input scaling resistor, determine the –3 dB low fre-
quency rolloff. This frequency, FL, is equal to:
FL
=
1
2π (8, 000 )(The Value
of
CC
in
Farads )
Note that at FL, the amplitude error will be 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 the lowest fre-
quency to be measured.
In addition, if the input voltage has more than 100 mV of dc
offset, than the ac coupling network at Pin 2 should be used in
addition to capacitor CC.
REV. C
–7–
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