Biomedical Engineering Reference
In-Depth Information
At the end of the measurement cycle, a switch is closed for interval t ACTD to discharge the
dc-blocking capacitor and the capacitance at the electrode interface. After this switch is
opened, a fairly high value resistor, Rpas (e.g., 100 k
), completes the “passive” discharge
of these capacitances. The charge movement during both active and passive discharge
causes balancing of the injected charge, resulting in a net-zero charge
fl
flow through the tis-
sue for each measurement cycle.
Improved performance over the CCD technique is possible by modifying the impedance-
measurement circuit con
fi
guration as shown in Figure 8.23. Here, a
fi
first capacitor C a 1 (e.g.,
0.01
µ
F), referred to as the
fi
first active capacitor , is charged through switch S1 to a preselected
+Vsrc
S1
S3
+
Ca
Cdc_block
OUT
S/H
LEAD
Ca2
S4
-
HEART
S/H
S2
-Vsrc
S1
S2
S3
S4
S/H
Figure 8.23 Improved performance over the CCD technique can be achieved by using two opposing-polarity current injections per meas-
urement cycle: ( a ) simplified circuit diagram of the sensor; ( b ) simplified timing diagram. C a 1 is charged to V src and C a 2 to V src . These capac-
itors are then discharged in sequence. The discharge of both capacitors in reverse polarity through the tissue cancels sources of error and
yields twice the amount of signal as the circuit of Figure 8.22. The impedance is then given by
t CCD
C a ln{[ V C a 1 ( t CCD ) V C a 2 ( t CCD )]/2 V src }
R
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