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Ez-USB microcontroller for the I
2
C to USB interface
to a personal computer for programming and data
acquisition (Cypress Semiconductor Corporation
2005).
A schematic of the measurement system is
shown in Figure 1. A key factor enabling the
measurement is close contact between the finger and
electrode. This can be achieved by incorporating
electrodes into a finger clip similar to a pulse
oximeter, or incorporating them into a cuff using a
fabric hook-and-loop closure. The cuff approach
can be less bulky and more secure than the clip,
although the cables may be inconvenient.
Figure 1: Schematic of finger pulse measurement.
Excitation and measurement electrodes were
constructed from copper tape strips 6 mm wide and
14 mm long covered with dielectric tape. Electrodes
were adhesive mounted on a fabric hook-and-loop
cuff for attachment to the finger. Electrodes were
connected to the AD7746 using coaxial cables
approximately 400 mm long. Measurements were
completed with the subject’s arm resting on a table
top approximately 20 cm below the level of the
heart. The capacitance measurement circuit was set
up using the evaluation board software (Analog
Devices, Inc. 2005). The IC was set to a typical
configuration with one excitation electrode driven
by excitation channel B with an amplitude of V
DD
/2,
and one input electrode connected to the positive
terminal of input channel 1. The capacitance
measurement was then single-ended with continuous
sampling at a rate of 16.1 Hz. No further signal
processing was used.
For wrist measurements, electrodes with
dimensions identical to those used in the finger
monitor were located on either side of the radial
artery. These were also fixed in place using a fabric
hook-and-loop band. The measurement procedure
was identical to the finger measurement.
This work was intended only as an initial
feasibility study for this technique and no efforts
were made to evaluate the effect of position and
motion artifacts, or the variation in results for
different subjects.
2.2 Electrical Model
An electrical model of the pulsatile flow is required
in order to enable discussion of the measured results
with respect to the physical system. At this stage a
simple model of the finger tissue can be used,
replacing it with a parallel capacitance and
resistance. However it is important to include series
capacitance between the finger and electrodes
representing the dielectric and any small air gap.
The model shown in Figure 2 can be easily
evaluated using a circuit simulator, numerical
calculation, or analytical methods.
C
f
R
f
C
e
C
e
Figure 2: Electrical model for finger impedance.
Permittivity of the finger tissue is high, and
depends on the exact composition of the tissue.
However if the relative permittivity is approximately
ε
r
=3000 and the finger diameter is 13 mm, the finger
capacitance using a parallel plate approximation
should be about 170 pF.
The signal to be measured is associated with the
pulsatile nature of the blood flow. The finger is not
a rigid structure, so the increase in blood pressure
during systole should correspond to an increase in
volume of the finger. In addition, if the electrodes
are attached to a rigid band, there will be a
corresponding decrease in the gap between the
finger and the electrodes.
A perceived drawback of a capacitive
measurement is that the signal will be loaded by
both the shunt resistance and the series capacitance
components. The finger resistance will influence the
measured value as it passes current, but provided the
resistance is fairly high and the measurement
frequency is high enough the effect is small. The
dielectric coating on the electrodes and any air gap
between electrodes and skin represent a low
permittivity and small capacitance. However due to
the series connection, these capacitances dominate
the overall impedance of the system regardless of
the measurement frequency. Fortunately, for a pulse
measurement the exact amplitude of the measured
impedance is much less important than the
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