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AD5933

Data Sheet

Rev. F | Page 32 of 40

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

ADR43x

AD5933

TOP VIEW

(Not to Scale)

10µF

0.1µF

ADuC702x

TOP VIEW

(Not to Scale)

RFB

Figure 36. Measuring a Blood Sample for a Strain of Virus

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.

FREQUENCY (Hz)

RESONANT

FREQUENCY

CHANGE IN

RESONANCE DUE

TO APPROACHING

OBJECT

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.

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