Datasheet

AD736 Data Sheet
Rev. I | Page 12 of 20
RAPID SETTLING TIMES VIA THE AVERAGE
RESPONDING CONNECTION
Because the average responding connection shown in Figure 19
does not use the C
AV
averaging capacitor, its settling time does
not vary with the input signal level. It is determined solely by
the RC time constant of C
F
and the internal 8 kΩ resistor in the
output amplifier’s feedback path.
+V
S
+V
S
C
F
33µF
C
C
10µF
COM
OUTPUT
(OPTIONAL)
POSITIVE SUPPLY
+V
S
0.1µF
–V
S
0.1µF
COMMON
NEGATIVE SUPPLY
V
OUT
8
7
6
5
1
2
3
4
AD736
+
rms
CORE
+
C
C
V
IN
V
IN
FULL
WAVE
RECTIFIER
C
F
–V
S
–V
S
C
AV
BIAS
SECTION
INPUT
AMPLIFIER
8kΩ
OUTPUT
AMPLIFIER
8kΩ
00834-018
Figure 19. AD736 Average Responding Circuit
DC ERROR, OUTPUT RIPPLE, AND AVERAGING
ERROR
Figure 20 shows the typical output waveform of the AD736
with a sine wave input applied. As with all real-world devices,
the ideal output of V
OUT
= V
IN
is never achieved exactly. Instead,
the output contains both a dc and an ac error component.
As shown in Figure 20, the dc error is the difference between
the average of the output signal (when all the ripple in the
output is removed by external filtering) and the ideal dc output.
The dc error component is therefore set solely by the value of
the averaging capacitor used. No amount of post filtering (that
is, using a very large C
F
) allows the output voltage to equal its
ideal value. The ac error component, an output ripple, can be
easily removed by using a large enough post filtering capacitor, C
F
.
In most cases, the combined magnitudes of both the dc and
ac error components need to be considered when selecting
appropriate values for Capacitor C
AV
and Capacitor C
F
. 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.
DC ERROR = E
O
– E
O
(IDEAL)
AVERAGE E
O
= E
O
E
O
IDEAL
E
O
DOUBLE-FREQUENCY
RIPPLE
TIME
00834-019
Figure 20. Output Waveform for Sine Wave Input Voltage
As the input frequency increases, both error components
decrease rapidly; if the input frequency doubles, the dc error
and ripple reduce to one quarter and one half of their original
values, respectively, and rapidly become insignificant.
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
amplitude (crest factor = V
PEAK
/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
periods between pulses). Figure 8 shows the additional error vs.
the crest factor of the AD736 for various values of C
AV
.