AD5934_17 datasheet(28/31 Pages) AD | Impedance Converter, Network Analyzer

28 / 31 page

AD5934

Data Sheet

Rev. E | Page 28 of 31

For example, if the user measures ZUNKNOWN that is known to

have a small impedance value within the range of 90 Ω to

110 Ω over the frequency range of 30 kHz to 32 kHz, the

user may not be in a position to measure ROUT directly in

the factory/lab. Therefore, the user may choose to add on

an extra amplifier circuit like that shown in Figure 31 to the

signal path of the AD5934. The user must ensure that the

chosen external amplifier has a sufficiently low output series

resistance over the bandwidth of interest in comparison to the

impedance range under test (for an op amp selection guide, see

http://www.analog.com/opamps). Most amplifiers from Analog

Devices have a curve of closed-loop output impedance vs.

frequency at different amplifier gains to determine the output

series impedance at the frequency of interest.

The system settings are as follows:

VDD = 3.3 V

VOUT = 2 V p-p

R2 = 20 kΩ

R1 = 4 kΩ

Gain setting resistor = 500 Ω

ZUNKNOWN = 100 Ω

PGA setting = ×1

To attenuate the excitation voltage at VOUT, choose a ratio

of R1/R2. With the values of R1 = 4 kΩ and R2 = 20 kΩ,

attenuate the signal by 1/5th of 2 V p-p = 400 mV. The

maximum current flowing through the impedance is 400 mV/

90 Ω = 4.4 mA.

The system is subsequently calibrated using the usual method

with a midpoint impedance value of 100 Ω, a calibration

resistor, and a feedback resistor at a midfrequency point in the

sweep. The dynamic range of the input signal to the receive side

of the AD5934 can be improved by increasing the value of the

I-V gain resistor at the RFB pin. For example, increasing the I-V

gain setting resistor at the RFB pin increases the peak-to-peak

signal presented to the ADC input from 400 mV (RFB = 100 Ω)

to 2 V p-p (RFB = 500 Ω).

The gain factor calculated is for a 100 Ω resistor connected

between VOUT and VIN, assuming the output series resistance

of the external amplifier is small enough to be ignored.

When biasing the circuit shown in Figure 31, note that the

receive side of the AD5934 is hard-biased about VDD/2 by

design. Therefore, to prevent the output of the external

amplifier (attenuated AD5934 Range 1 excitation signal) from

saturating the receive side amplifiers of the AD5934, a voltage

equal to VDD/2 must be applied to the noninverting terminal

of the external amplifier.

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, resulting in the need to sweep

different frequencies to check for different viruses. The AD5934,

with its 27-bit phase accumulator, allows for subhertz frequency

tuning.

The AD5934 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 AD5934

is ideal for this application because it allows the user to tune to

the specific frequency required for each test.

PROBE

ADR43x

AD5934

TOP VIEW

(Not to Scale)

10µF

0.1µF

ADuC702x

TOP VIEW

(Not to Scale)

RFB

Figure 32. Measuring a Blood Sample for a Strain of Virus

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