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AD5934 Datasheet(PDF) 29 Page - Analog Devices
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AD5934 Datasheet(HTML) 29 Page - Analog Devices
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Rev. E | Page 29 of 31
SENSOR/COMPLEX IMPEDANCE MEASUREMENT
The operational principle of a capacitive proximity sensor is
based on the change of a capacitance in a RLC resonant circuit.
This leads to changes in the resonant frequency of the RLC
circuit, which can be evaluated as shown Figure 33.
It is first required to tune the RLC circuit to the area of resonance.
At the resonant frequency, the impedance of the RLC circuit is
at a maximum. Therefore, a programmable frequency sweep
and tuning capability is required, which is provided by the AD5934.
Figure 33. Detecting a Change in Resonant Frequency
An example of the use of this type of sensor is for a train proximity
measurement system. The magnetic fields of the train approaching
on the track change the resonant frequency to an extent that can
be characterized. This information can be sent back to a mainframe
system to show the train location on the network.
Another application for the AD5934 is in parked vehicle detection.
The AD5934 is placed in an embedded unit connected to a coil
of wire underneath the parking location. The AD5934 outputs a
single frequency within the 80 kHz to 100 kHz frequency range,
depending upon the wire composition. The wire can be modeled
as a resonant circuit. The coil is calibrated with a known impedance
value and at a known frequency. The impedance of the loop is
monitored constantly. If a car is parked over the coil, the impedance
of the coil changes and the AD5934 detects the presence of the car.
The AD5934 has found use in the area of corrosion monitoring.
Corrosion in a metal, such as aluminum, which is used in air
craft and ships, requires continuous assessment because the metal
is exposed to a wide variety of conditions, such as temperature and
moisture. The AD5934 offers an accurate and compact solution
for this type of measurement compared to the large and expensive
existing units on the market.
Mathematically the corrosion of a metal is modeled using a RC
network that consists of a resistance, R
, in series with a parallel
resistor and capacitor, R
. A system metal would typically
have values as follows: R
is 10 Ω to 10 kΩ, R
is 1 kΩ to 1 MΩ,
is 5 µF to 70 µF.
The frequency range of interest when monitoring corrosion is
0.1 Hz to 100 kHz.
To ensure that the measurement itself does not introduce a
corrosive effect, the metal needs to be excited with minimal
voltage, typically in the 200 mV region, which the AD5934 is
capable of outputting. A nearby processor or control unit, such
as the ADuc702x, would log a single impedance sweep from
0.1 kHz to 100 kHz every 10 minutes and download the results
back to a control unit. To achieve system accuracy from the
0.1 kHz to 1 kHz region, the system clock needs to be scaled
down from the 16.776 MHz nominal clock frequency to 500 kHz,
typically. The clock scaling can be achieved digitally using an
external direct digital synthesizer, such as the AD9834, as a
programmable divider that supplies a clock signal to MCLK
and that can be controlled digitally by the nearby microprocessor.
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