AD637 查看數據表（PDF） - Analog Devices
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AD637 Datasheet PDF : 10 Pages
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AD637
BUFFER INPUT
1
BUFFER
AD637
BUFFER
14 OUTPUT
RMS
OUTPUT
NC 2
ANALOG COM 3
OUTPUT
OFFSET
4
BIAS
SECTION
CHIP
SELECT
5
DENOMINATOR
25k⍀
INPUT
6
dB 7
ABSOLUTE
VALUE
SQUARER/DIVIDER
25k⍀
13
SIGNAL
INPUT
12 NC
+
C3
11
+VS
10
–VS
FILTER
9+
8
CAV
RX
24k⍀
+
C2
24k⍀
FOR 1 POLE
FILTER, SHORT
RX AND
REMOVE C3
Figure 8. Two Pole Sallen-Key Filter
Figure 9a 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
the graph.
Figure 9b shows the relationship between averaging error, signal
frequency settling time and averaging capacitor value. This
graph is drawn for filter capacitor values of 3.3 times the averag-
ing 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 averag-
ing capacitor and a 3.3 µF filter capacitor, the ripple for a 60 Hz
input signal will be reduced from 5.3% of reading using the
averaging capacitor alone to 0.15% using the single pole filter.
This gives a factor of thirty reduction in ripple and yet the set-
tling time would only increase by a factor of three. The values of
CAV and C2, the filter capacitor, can be calculated for the desired
value of averaging error and settling time by using Figure 9b.
The symmetry of the input signal also has an effect on the mag-
nitude of the averaging error. Table I gives practical component
values for various types of 60 Hz input signals. These capacitor
values can be directly scaled for frequencies other than 60 Hz,
i.e., for 30 Hz double these values, for 120 Hz they are halved.
For applications that are extremely sensitive to ripple, the two pole
configuration is suggested. This configuration will minimize
capacitor values and settling time while maximizing performance.
Figure 9c can be used to determine the required value of CAV,
C2 and C3 for the desired level of ripple and settling time.
100
0.01%
10
0.1%
ERROR
1.0
1%
10%
ERROR
ERROR
ERROR
VALUES FOR CAV AND
0.1
1% SETTLING TIME
FOR STATED % OF READING
AVERAGING ERROR*
ACCURACY ؎2% DUE TO
COMPONENT TOLERANCE
* %dc ERROR + %RIPPLE (Peak)
0.01
1
10
100
1k
10k
INPUT FREQUENCY – Hz
Figure 9a.
100
10
1.0
0.1
0.01
100k
100
100
VALUES OF CAV, C2 AND
1% SETTLING TIME FOR
STATED % OF READING
AVERAGING ERROR*
FOR 1 POLE POST FILTER
10
10
* %dc ERROR + % PEAK RIPPLE
ACCURACY ؎20% DUE TO
COMPONENT TOLERANCE
1.0
0.1
5% E1R%RE0OR.1RR%O0.ER0R1%ROERRROR
1.0
0.1
0.01
1
100
10
1.0
0.1
0.01
1
10
100
1k
10k
INPUT FREQUENCY – Hz
Figure 9b.
0.01
100k
100
VALUES OF CAV, C2 AND C3
AND 1% SETTLING TIME FOR
STATED % OF READING
AVERAGING ERROR*
2 POLL SALLEN-KEY FILTER
10
* %dc ERROR + % PEAK RIPPLE
ACCURACY ؎20% DUE TO
COMPONENT TOLERANCE
5% E1R%RE0O.R1RR%O0E.R0R1R%OERRROR
1.0
0.1
10
100
1k
10k
INPUT FREQUENCY – Hz
Figure 9c.
0.01
100k
–6–
REV. E
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