5962-8963701CA(2015) View Datasheet(PDF) - Analog Devices
Part Name
Description
Manufacturer
5962-8963701CA Datasheet PDF : 25 Pages
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Data Sheet
A preferable ripple reduction method is to use a post conversion
one or two-pole low-pass filter, as shown in Figure 11. Usually
a single-pole filter gives the best overall compromise between
ripple and settling time. Use the two-pole Sallen-Key for more
ripple attenuation.
1 BUFF IN
+
–
2 NIC
3 COMMON
OUTPUT
4 OFFSET BIAS
AD637
BUFF
OUT 14
ABSOLUTE
VALUE
VIN 13
VIN
NIC 12
+
C3
SQUARER/
DIVIDER
11
+VS
+VS
+VS 4.7kΩ 5 CS
DEN
6 INPUT 25kΩ –
dB
+
7 OUTPUT
10
–VS
–VS
RMS
–
OUT 9
+
25kΩ
+ CAV
CAV 8
+
C2
RX
24kΩ
NIC = NO INTERNAL CONNECTION
24kΩ
FOR A 1 POLE
FILTER SHORT RX
AND REMOVE C3
Figure 11. 2-Pole Sallen-Key Filter
Figure 12 shows values of CAV and the corresponding averaging
error as a function of sine wave frequency for the standard rms
connection. The 1% settling time is shown on the right side of
Figure 12.
Figure 13 shows the relationship between the averaging error,
signal frequency settling time, and averaging capacitor value.
Figure 13 is drawn for filter capacitor values of 3.3× the
averaging capacitor value. This ratio sets the magnitude of
the ac and dc errors equal at 50 Hz. As an example, by using a
1 µF averaging capacitor and a 3.3 µF filter capacitor, the ripple
for a 60 Hz input signal is reduced from 5.3% of the reading
using the averaging capacitor alone to 0.15% using the 1-pole
filter. This gives a factor of 30 reduction in ripple, and yet the
settling time only increases by a factor of 3. The values of
Filter Capacitor CAV and Filter Capacitor C2 can be calculated
for the desired value of averaging error and settling time by
using Figure 13.
The symmetry of the input signal also has an effect on the
magnitude of the averaging error. Table 7 gives the practical
component values for various types of 60 Hz input signals.
These capacitor values can be directly scaled for frequencies
other than 60 Hz—that is, for 30 Hz, these values are doubled,
and for 120 Hz they are halved.
Use Figure 14 to determine the required value of CAV, C2, and
C3 for the desired level of ripple and settling time.
AD637
100
100
0.01%
10
0.1%
ERROR
10
1.0
1%
10%
ERROR
ERROR
ERROR
1.0
0.1
0.1
0.01
1
10
100
1k
10k
INPUT FREQUENCY (Hz)
0.01
100k
Figure 12. Values for CAV and 1% Settling Time for Stated % of Reading; Averaging
Error (% DC Error + % Ripple (Peak)); Accuracy Includes ±20% Component
Tolerance
100
100
10
10
1
0.1
5% E1R%RE0OR.1RR%O0.ER0R1%ROERRROR
1
0.1
0.01
1
10
100
1k
10k
INPUT FREQUENCY (Hz)
0.01
100k
Figure 13. Values of CAV, C2, and 1% Settling Time for Stated % of Reading for
1-Pole Post Filter; Averaging Error (% DC Error + % Ripple (Peak) Accuracy
±20% Due to Component Tolerance)
100
100
10
10
1
0.1
5% E1R%RE0O.R1RR%O0E.R0R1R%OERRROR
1
0.1
0.01
1
10
100
1k
10k
INPUT FREQUENCY (Hz)
0.01
100k
Figure 14. Values of CAV, C2, and C3 and 1% Settling Time for Stated % of
Reading for 2-Pole Sallen-Key Filter; Averaging Error (% DC Error + %Ripple
(Peak) Accuracy ±20% Due to Component Tolerance)
Rev. L | Page 15 of 25
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