Datasheet
AD637
Rev. K | Page 10 of 20
FOR A SINGLE-POLE
FILTER SHORT RX
AND REMOVE C3
1
25k
DEN
INPUT
BUFF IN
BUFF
OUT
–V
S
CS
dB OUTPUT
3COMMON
BIAS
2NC
4
OUTPUT
OFFSET
+V
S
5
25k
6
7 8
9
10
+V
S
11
12NC
13
14 RMS OUT
V
IN
C
AV
+
AD637
ABSOLUTE
VALUE
00788-011
–V
S
+V
S
C
AV
V
IN
+
C2
4.7k
C3
24k
+
RX
24k
SQUARER/
DIVIDER
FOR 1% SETTLING TIME IN SECONDS
MULTIPLY READING BY 0.115
INPUT FREQUENCY (Hz)
100
0.01
1 100k10 100 1k 10k
10
1.0
0.1
10
%
ERR
OR
1
%
ERROR
0.1%
E
RROR
0.
0
1
%
ERROR
100
0.01
10
1.0
0.1
00788-012
*%dc ERROR + %RIPPLE (PEAK)
REQUIRED C
AV
(µF)
Figure 12. Values for C
AV
and 1% Settling Time for Stated % of Reading Averaging
Error* Accuracy Includes ±2% Component Tolerance (see * in Figure)
INPUT FREQUENCY (Hz)
100
0.01
1 100k10 100 1k 10k
10
1
0.1
5%
E
RRO
R
1%
ERRO
R
0.1%
E
RRO
R
0
.
01
%
ERR
O
R
100
0.01
10
1
0.1
00788-013
REQUIRED C
AV
(AND C2)
C2 = 3.3 × C
AV
FOR 1% SETTLING TIME IN SECONDS
MULTIPLY READING BY 0.400
*%dc ERROR + %RIPPLE (PEAK)
ACCURACY ±20% DUE TO
COMPONENT TOLERANCE
Figure 11. 2-Pole Sallen-Key Filter
Figure 12 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
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 C
AV
and Filter Capacitor C2 can be calculated for
the desired value of averaging error and settling time by using
Figure 13.
Figure 13. Values of C
AV
, C2, and 1% Settling Time for Stated % of Reading
Averaging Error* for 1-Pole Post Filter (see * in Figure)
INPUT FREQUENCY (Hz)
100
0.01
1110
100 1k 10k
10
1
0.1
00k
5
%
ERR
O
R
1%
ER
ROR
0
.1% ERROR
0
.
0
1%
E
RROR
100
0.01
10
1
0.1
00788-014
*%dc ERROR + %RIPPLE (PEAK)
ACCURACY ±20% DUE TO
COMPONENT TOLERANCE
REQUIRED C
AV
(AND C2 + C3)
C2 = C3 = 2.2 × C
AV
FOR 1% SETTLING TIME IN SECONDS
MULTIPLY READING BY 0.365
The symmetry of the input signal also has an effect on the
magnitude of the averaging error. Table 5 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.
For applications that are extremely sensitive to ripple, the 2-pole
configuration is suggested. This configuration minimizes capacitor
values and the settling time while maximizing performance.
Figure 14. Values of C
AV
, C2, and C3 and 1% Settling Time for Stated % of
Reading Averaging Error* for 2-Pole Sallen-Key Filter (see * in Figure)
Figure 14 can be used to determine the required value of C
AV
,
C2, and C3 for the desired level of ripple and settling time.