AD637B AD [Analog Devices], AD637B Datasheet - Page 6

• Manufacturer Part Number: AD637B
• Description: High Precision, Wide-Band RMS-to-DC Converter
• Manufacturer: AD [Analog Devices]

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