Biomedical Engineering Reference
In-Depth Information
1 V). At the same time, a capacitor
C
a
2
(of the same value as
C
a
1
),
referred to as the
second active capacitor
, is charged through switch S2 to the negative of the
voltage level
V
src
(e.g.,
voltage level
V
src
(e.g.,
1V).
After
C
a
1
is fully charged to
V
src
and
C
a
2
is fully charged to
V
src
, a switch (S3) con-
nects
C
a
1
to the body, allowing it to discharge through the lead system and a dc-blocking
capacitor
C
dc block
(e.g., 1
s). Immediately thereafter,
C
a
2
is discharged across the lead system by closing a switch (S4) for the same amount of
time that
C
a
1
was discharged. With no other sources in the circuit, the voltage on each
active capacitor decays exponentially according to
µ
F) for a brief interval
t
CCD
(e.g., 10
µ
V
C
a
(
i
)
V
src
e
t
/
RC
a
(
i
)
where
V
C
a
(
i
)
is the voltage remaining on active capacitor
i
after a time
t
,
V
src
the initial volt-
age of the active capacitor,
R
the lumped resistance of the circuit, and
C
a
(
i
)
the capacitance
of the active capacitor
i
. The resistance of the circuit to the narrow pulse would then be
determined from
t
C
a
(
i
)
ln[
V
C
a
(
i
)
(
t
)/
V
src
]
R
As explained above, however, other sources in the circuit (e.g., the intracardiac electro-
gram, electrode polarization potentials) have a strong e
ff
ect on
V
C
a
(
i
)
(
t
) and make meas-
urement of
R
imprecise.
By using the discharge of both capacitors, which happens in reverse polarity through
the tissue, the e
ects of these sources of error are virtually canceled. This compensation
process is carried out by subtracting
V
C
a
1
from
V
C
a
2
before determining the resistive com-
ponent
R
of the impedance:
ff
t
C
a
ln{[
V
C
a
1
(
t
)
R
V
C
a
2
(
t
)]/2
V
src
}
Since the discharge polarity through the body is reversed for each phase, the subtrac-
tion of capacitor voltages results in twice the voltage signal while canceling interfering
signals:
V
C
a
1
(
t
)
V
C
a
2
(
t
)
voltage decay on a capacitor of size
C
a
due to discharge through the
resistive path
e
ff
ect of interference sources on a capacitor of size
C
a
voltage decay on a capacitor of size
C
a
due to discharge through
the resistive path
(
e
ff
ect of interference sources on a capacitor of
size
C
a
)
2(voltage decay on a capacitor of size
C
a
due to discharge through the
resistive path)
Active discharge is not needed at the end of the measurement cycle. The charge injected
by each phase is substantially similar but in the opposite direction. This results in the
desired net-zero charge
flow through the tissue for each measurement cycle.
Also at the end of the measurement cycle, the voltage di
fl
erence between the capacitors
is measured and sampled via a sample-and-hold circuit. Figure 8.24 is an actual dual
opposing capacitor discharge (DOCD) prototype circuit. The core circuit of the sensor is
presented in Figure 8.25. In it, capacitor
C
a
1
(C39) is charged to
ff
1.2 V through switch
IC9D and current-limiting resistor R11. At the same time,
C
a
2
(C47) is charged to
1.2 V
through IC9A. The ground path during this process is established through IC9C. All other
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