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AD5934
For example, if the user measures Z
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 R
the factory/lab. Therefore, the user may choose to add on
an extra amplifier circuit like that shown in Figure 37 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
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
Z
= 100 Ω
UNKNOWN
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,
th
attenuate the signal by 1/5
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).
that is known to
UNKNOWN
directly in
OUT
Rev. A | Page 30 of 40
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 37, 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.
RFB
PROBE
7V
0.1µF
10µF
Figure 38. Measuring a Blood Sample for a Strain of Virus
1
16
ADuC702x
TOP VIEW
(Not to Scale)
2
15
AD5934
3
14
TOP VIEW
(Not to Scale)
4
13
5
12
6
11
7
10
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9
ADR43x
2
6
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