0195136047.pdf

438 DIGITAL CIRCUITS

• Advantages of PMOS and NMOS logic circuits.


• CMOS inverter switch and its characteristics.


• CMOS gates and CMOS technology.


• Notions of other logic families.

9.5 PRACTICAL APPLICATION: A CASE STUDY

Cardiac Pacemaker, a Biomedical Engineering Application

When blockage occurs to the biological signals that stimulate the heart to beat, the application

of electric pacemaker pulses forcing the heart to beat at a higher rate is very helpful. A demand

pacemaker is particularly helpful to the patient with partial blockage, in which case an electric

pulse to the heart muscle is only applied when a beat does not occur within a predetermined time

interval.

The pacemaker circuitry along with a lithium battery are enclosed in a metal case, which is

implanted under the skin on the patient’s chest. The mental case and the tip of thecatheter(a wire

enclosed in an insulating tube) form the electrical terminals of the pacemaker. A simplified block

diagram of a typical demand pacemaker is shown in Figure 9.5.1.

The input amplifier amplifies the natural heart signals, which have a small amplitude on the

order of 1 mV. Filtering eliminates certain frequency components so that heartbeats can be better

detected. A comparative circuit is then used to compare the amplified and filtered signal with

a threshold value and detect either a natural heartbeat or an output pacing pulse. This detection

decision is passed on to the counting and timing circuitry through an AND gate. The second input

to the AND gate comes from the counter circuit, such that input signals for 0.4 s after the start of

a natural or forced beat are ignored.

The timing functions are performed by counting the output cycles of a timing oscillator,

which generates a square wave with a precise period of 0.1 s. The digital signals produced by the

counter are passed on to a digital comparator, which compares them with signals from a reference

count generator and decides when the pulse generator is to deliver an output pulse of a specified

amplitude and duration.

Input

amplifier

and filter

Comparator AND

gate

Reset

terminal

Pacemaker case

Catheter

to heart

Digital

Counter comparator

Timing

oscillator

Output

pulse

generator

Reference

count

generator

+

−

Figure 9.5.1Simplified block diagram of demand cardiac pacemaker (power source connections not shown).