5962-8963701CA(2015) Просмотр технического описания (PDF) - Analog Devices
Номер в каталоге
Компоненты Описание
Список матч
5962-8963701CA Datasheet PDF : 25 Pages
| |||
AD637
Data Sheet
Referring to Figure 8 for optional external gain and offset trim
schematic. The following sections describe trimming for greater
accuracy in detail.
Offset Trim
Ground the input signal (VIN) and adjust R1 until the output
voltage at Pin 9 measures 0 V. Alternatively, apply the least
expected value of VIN.at the input VIN and adjust R1 until the dc
output voltage at Pin 9 measures the same value as the rms input.
Scale Factor Trim
Insert Resistor R4 in series with the input to decrease the range
of the scale factor. Connect a precision source to Pin 13 and
adjust the output for the desired full-scale input to VIN, using
either a calibrated dc or 1 kHz ac voltage, and adjust Resistor R3
to give the correct output at Pin 9 (that is, 1 V rms at the input
results in a dc output voltage of 1.000 V dc). A 2 V p-p sine
wave input yields 0.707 V dc at the output. Remaining errors
are due to the nonlinearity.
1 BUFF IN
+
–
2 NIC
OUTPUT
OFFSET
TRIM
3 COMMON
+VS
R2
OUTPUT
R1
1MΩ 4 OFFSET
50kΩ
–VS +VS 4.7kΩ 5 CS
BIAS
6
DEN
INPUT
25kΩ
–
dB
+
OUTPUT
7
AD637
BUFF
OUT 14
NC R4
ABSOLUTE VIN 13 147Ω
VALUE
NIC 12
SQUARER/
DIVIDER
+VS 11 +VS
–VS 10 –VS
RMS
–
OUT 9
+
25kΩ
+ CAV
CAV 8
VIN
RMSOUT
SCALE FACTOR TRIM
R3
1kΩ
NIC = NO INTERNAL CONNECTION
Figure 8. Optional External Gain and Offset Trims
CHOOSING THE AVERAGING TIME CONSTANT
The AD637 computes the true rms value of both dc and ac
input signals. For dc inputs, the output tracks the absolute
value of the input exactly. However, when the voltage is ac,
the converted dc output voltage asymptotically approaches the
theoretical rms value of the input. The deviation from the ideal
rms value is due to the implicit denominator inherent to averag-
ing over an infinite time span. Because the error diminishes as
the averaging period increases, it quickly becomes negligible.
The remaining error components are the ac ripple and dc offset
voltage, if any. The ac and averaging error components are both
functions of the input-frequency (f) and the averaging time
constant τ (τ: 25 ms/µF of averaging capacitance). Figure 9 shows
the output errors, which are enlarged for clarity. The frequency
of the ac component (ripple) is twice the frequency of the input,
the dc error is the RSS sum of the average rectified error and
any fixed value dc offset.
The value of CAV and the 25 kΩ feedback resistor establish the
averaging time constant, and solely determines the magnitude
of the rms-to-dc conversion error. Furthermore, any post-
conversion filtering does not improve the dc component
composite result.
Equation 1 defines the approximate peak value of the ac ripple
component of the composite output.
50 in % of reading where (τ > 1/ f )
6.3τf
(1)
EO
IDEAL
EO
DC ERROR = AVERAGE OF OUTPUT – IDEAL
AVERAGE ERROR
DOUBLE-FREQUENCY
RIPPLE
TIME
Figure 9. Enlarged Composite Conversion Result for a Sinusoidal Input
Increasing the value of the averaging capacitor or adding a post-
rms filter network reduces the ripple error.
The dc error appears as a frequency dependent offset at the
output of the AD637 and follows the relationship
0.16 +
1
6.4 τ 2
f
2
in
%
of
reading
100
10
PEAK RIPPLE
1.0
DC ERROR
0.1
10
100
1k
10k
SINE WAVE INPUT FREQUENCY (Hz)
Figure 10. Comparison of Percent DC Error to the Percent Peak Ripple over
Frequency Using the AD637 in the Standard RMS Connection with a 1 × µF CAV
The ac ripple component of averaging error is greatly reduced
by increasing the value of the averaging capacitor. However, the
value of the averaging capacitor increases exponentially while
the settling time increases directly proportion to the value of
the averaging capacitor (TS = 115 ms/µF of averaging capacitance).
Rev. L | Page 14 of 25
Share Link: 