DESIGN AND DEVELOPMENT OF MEDICAL ELECTRONIC ...

404 CARDIAC PACING AND DEFIBRILLATIONvoltage level V src (e.g., 1 V). At the same time, a capacitor C a2 (of the same value as C a1 ),referred to as the second active capacitor, is charged through switch S2 to the negative of thevoltage level V src (e.g., 1V).After C a1 is fully charged to V src and C a2 is fully charged to V src , a switch (S3) connectsC a1 to the body, allowing it to discharge through the lead system and a dc-blockingcapacitor C dc block (e.g., 1 µF) for a brief interval t CCD (e.g., 10 µs). Immediately thereafter,C a2 is discharged across the lead system by closing a switch (S4) for the same amount oftime that C a1 was discharged. With no other sources in the circuit, the voltage on eachactive capacitor decays exponentially according toV Ca(i) V src e t/RC a(i)where V Ca(i) is the voltage remaining on active capacitor i after a time t, V src the initial voltageof the active capacitor, R the lumped resistance of the circuit, and C a(i) the capacitanceof the active capacitor i. The resistance of the circuit to the narrow pulse would then bedetermined fromR As explained above, however, other sources in the circuit (e.g., the intracardiac electrogram,electrode polarization potentials) have a strong effect on V Ca(i) (t) and make measurementof R imprecise.By using the discharge of both capacitors, which happens in reverse polarity throughthe tissue, the effects of these sources of error are virtually canceled. This compensationprocess is carried out by subtracting V Ca1 from V Ca2 before determining the resistive componentR of the impedance:R tC a(i) ln[V Ca(i) (t)/V src ]tC a ln{[V Ca1 (t) V Ca2 (t)]/2V src }Since the discharge polarity through the body is reversed for each phase, the subtractionof capacitor voltages results in twice the voltage signal while canceling interferingsignals:V Ca1 (t) V Ca2 (t) voltage decay on a capacitor of size C a due to discharge through theresistive path effect of interference sources on a capacitor of size C a (voltage decay on a capacitor of size C a due to discharge throughthe resistive path effect of interference sources on a capacitor ofsize C a ) 2(voltage decay on a capacitor of size C a due to discharge through theresistive path)Active discharge is not needed at the end of the measurement cycle. The charge injectedby each phase is substantially similar but in the opposite direction. This results in thedesired net-zero charge flow through the tissue for each measurement cycle.Also at the end of the measurement cycle, the voltage difference between the capacitorsis measured and sampled via a sample-and-hold circuit. Figure 8.24 is an actual dualopposing capacitor discharge (DOCD) prototype circuit. The core circuit of the sensor ispresented in Figure 8.25. In it, capacitor C a1 (C39) is charged to 1.2 V through switchIC9D and current-limiting resistor R11. At the same time, C a2 (C47) is charged to 1.2 Vthrough IC9A. The ground path during this process is established through IC9C. All other