5962-8963701CA(2015) Просмотр технического описания (PDF) - Analog Devices
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5962-8963701CA Datasheet PDF : 25 Pages
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Data Sheet
APPLICATIONS INFORMATION
STANDARD CONNECTION
The AD637 is simple to connect for a majority of rms
measurements. In the standard rms connection shown in Figure 5,
only a single external capacitor is required to set the averaging
time constant. In this configuration, the AD637 computes the
true rms of any input signal. An averaging error, the magnitude
of which is dependent on the value of the averaging capacitor,
is present at low frequencies. For example, if the filter capacitor,
CAV, is 4 µF, the error is 0.1% at 10 Hz and increases to 1% at
3 Hz. To measure ac signals, the AD637 can be ac-coupled by
adding a nonpolar capacitor in series with the input, as shown
in Figure 5.
+VS
4.7kΩ
1 BUFF IN +
–
AD637
BUFF OUT 14
NC
2 NIC
ABSOLUTE VIN 13
VALUE
VIN
3 COMMON
OUTPUT
4 OFFSET BIAS
SQUARER/
DIVIDER
NIC 12
(OPTIONAL)
+VS 11 +VS
5 CS
DEN
6 INPUT 25kΩ
–
dB
+
7 OUTPUT
10
–VS
–VS
–
RMS
OUT 9
VOUT = VIN2
+
25kΩ
+ CAV
CAV 8
NIC = NO INTERNAL CONNECTION
Figure 5. Standard RMS Connection
The performance of the AD637 is tolerant of minor variations
in the power supply voltages; however, if the supplies used
exhibit a considerable amount of high frequency ripple, it is
advisable to bypass both supplies to ground through a 0.1 µF
ceramic disc capacitor placed as close to the device as possible.
The output signal range of the AD637 is a function of the
supply voltages, as shown in Figure 6. Use the output voltage
buffered or nonbuffered, depending on the characteristics of the
load. Connect the buffer input (Pin 1) to common if buffering is
not required. The output of the AD637 is capable of driving
5 mA into a 2 kΩ load without degrading the accuracy of the
device.
AD637
20
15
10
5
0
0
±3 ±5
±10
±15
±18
SUPPLY VOLTAGE – DUAL SUPPLY (V)
Figure 6. Maximum VOUT vs. Supply Voltage
CHIP SELECT
The AD637 includes a chip select feature that allows the user
to decrease the quiescent current of the device from 2.2 mA to
350 µA. This is done by driving CS, Pin 5, to below 0.2 V dc.
Under these conditions, the output goes into a high impedance
state. In addition to reducing the power consumption, the
outputs of multiple devices can be connected in parallel to
form a wide bandwidth rms multiplexer. Tie Pin 5 high to
disable the chip select.
OPTIONAL TRIMS FOR HIGH ACCURACY
The AD637 includes provisions for trimming out output offset
and scale factor errors resulting in significant reduction in the
maximum total error, as shown in Figure 7. The residual error is
due to a fixed input offset in the absolute value circuit and the
residual nonlinearity of the device.
5.0
AD637K MAX
2.5
INTERNAL TRIM
0
AD637K
EXTERNAL TRIM
–2.5
AD637K: 0.5mV ± 0.2%
0.25mV ± 0.05%
EXTERNAL
–5.0
0
0.5
1.0
1.5
2.0
INPUT LEVEL (V)
Figure 7. Maximum Total Error vs.
Input Level AD637K Internal and External Trims
Rev. L | Page 13 of 25
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