AD637
100
BUFFER
BUFFER INPUT
1
ABSOLUTE
VALUE
100
AD637
14
BUFFER
OUTPUT
RMS
OUTPUT
REQUIRED C
AV
– F
NC 2
ANALOG COM
OUTPUT
OFFSET
3
BIAS
SECTION
4
13
SIGNAL
INPUT
+
C3
+V
S
–V
S
10
10
12 NC
SQUARER/DIVIDER
25k
10
25k
9
FILTER
8
+
C
AV
11
1.0
1.0
CHIP
SELECT 5
DENOMINATOR
6
INPUT
dB
7
VALUES FOR C
AV
AND
1% SETTLING TIME
0.1 FOR STATED % OF READING
AVERAGING ERROR*
ACCURACY 2% DUE TO
COMPONENT TOLERANCE
* %dc ERROR + %RIPPLE (Peak)
1
10
100
1k
INPUT FREQUENCY – Hz
10k
0.1
0.01
0.01
100k
R
X
24k
+
C2
Figure 9a.
24k
100
FOR 1 POLE
FILTER, SHORT
R
X
AND
REMOVE C3
VALUES OF C
AV
, C2 AND
1% SETTLING TIME FOR
STATED % OF READING
AVERAGING ERROR*
FOR 1 POLE POST FILTER
* %dc ERROR + % PEAK RIPPLE
ACCURACY 20% DUE TO
COMPONENT TOLERANCE
100
Figure 8. Two Pole Sallen-Key Filter
Figure 9a shows values of C
AV
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
C
AV
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 C
AV
,
C2 and C3 for the desired level of ripple and settling time.
REQUIRED C
AV
(AND C2)
C2 = 3.3 C
AV
10
10
1.0
1.0
0.1
0.1
0.01
1
10
100
1k
INPUT FREQUENCY – Hz
10k
0.01
100k
Figure 9b.
100
VALUES OF C
AV
, C2 AND C3
AND 1% SETTLING TIME FOR
STATED % OF READING
AVERAGING ERROR*
2 POLL SALLEN-KEY FILTER
* %dc ERROR + % PEAK RIPPLE
ACCURACY 20% DUE TO
COMPONENT TOLERANCE
100
10
10
1.0
1.0
0.1
0.1
0.01
1
10
100
1k
INPUT FREQUENCY – Hz
10k
0.01
100k
Figure 9c.
–6–
REV. E
FOR 1% SETTLING TIME IN SECONDS
MULTIPLY READING BY 0.365
REQUIRED C
AV
(AND C2 + C3)
C2 = C3 = 2.2 C
AV
FOR 1% SETTLING TIME IN SECONDS
MULTIPLY READING BY 0.400
FOR 1% SETTLING TIME IN SECONDS
MULTIPLY READING BY 0.115
%
01
0.
R
O
R
ER
1%
0.
R
O
R
ER
1%
R
O
R
ER
%
10
5%
5%
1%
R
O
R
ER
R
O
R
R
ER
O
R
%
01
R
ER
0.
O
R
R
1%
0.
ER RO
1%
ER
1%
0.
R
O
R
R
ER
O
%
R
01
R
ER
0.
O
R
R
ER RO
ER
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