EVAL-AD5933EB Datasheet, PDF (32/40 Pages) Analog Devices – 1 MSPS, 12-Bit Impedance Converter, Network Analyzer

AD5933
BIOMEDICAL: NONINVASIVE BLOOD IMPEDANCE
MEASUREMENT
When a known strain of a virus is added to a blood sample
that already contains a virus, a chemical reaction takes place
whereby the impedance of the blood under certain conditions
changes. By characterizing this effect across different frequencies,
it is possible to detect a specific strain of virus. For example, a
strain of the disease exhibits a certain characteristic impedance
at one frequency but not at another; therefore, the requirement
is to sweep different frequencies to check for different viruses.
The AD5933, with its 27-bit phase accumulator, allows for
subhertz frequency tuning.
The AD5933 can be used to inject a stimulus signal through
the blood sample via a probe. The response signal is analyzed,
and the effective impedance of the blood is tabulated. The
AD5933 is ideal for this application because it allows the user
to tune to the specific frequency required for each test.
PROBE
1
16
2
15
AD5933
3 TOP VIEW 14
RFB ^{(Not to Scale)}
4
13
5
12
6
11
7
10
8
9
ADuC702x
TOP VIEW
(Not to Scale)
7V
0.1µF 10µF
ADR43x
2
6
4
Figure 36. Measuring a Blood Sample for a Strain of Virus
Data Sheet
SENSOR/COMPLEX IMPEDANCE MEASUREMENT
The operational principle of a capacitive proximity sensor is
based on the change of a capacitance in an RLC resonant
circuit. This leads to changes in the resonant frequency of the
RLC circuit, which can be evaluated as shown Figure 37.
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 AD5933.
RESONANT
FREQUENCY
CHANGE IN
RESONANCE DUE
TO APPROACHING
OBJECT
F_O
FREQUENCY (Hz)
Figure 37. 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 AD5933 is in parked vehicle detec-
tion. The AD5933 is placed in an embedded unit connected to
a coil of wire underneath the parking location. The AD5933
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
AD5933 detects the presence of the car.
Rev. E | Page 32 of 40