Biomedical Engineering Reference
In-Depth Information
Understand the basic concepts of analog
filter design and design basic filters.
Design low-pass, high-pass, and band-pass
filters.
Explain the different types of noise in a
biomedical instrument system.
9.1 INTRODUCTION
This chapter provides basic information about bioinstrumentation and electric circuit
theory used in other chapters. Many biomedical instruments use a transducer or sensor
to convert a signal created by the body into an electric signal. Our goal in this chapter
is to develop expertise in electric circuit theory applied to bioinstrumentation. We begin
with a description of variables used in circuit theory, charge, current, voltage, power, and
energy. Next, Kirchhoff's current and voltage laws are introduced, followed by resistance,
simplifications of resistive circuits, and voltage and current calculations. Circuit analysis
techniques are then presented, followed by inductance and capacitance, and solutions of
circuits using the differential equation method. Finally, the operational amplifier and time
varying signals are introduced.
Before 1900, medicine had little to offer the typical citizen because its resources were
mainly the education and little black bag of the physician. The origins of the changes that
occurred within medical science are found in several developments that took place in the
applied sciences. During the early nineteenth century, diagnosis was based on physical
examination, and treatment was designed to heal the structural abnormality. By the late
nineteenth century, diagnosis was based on laboratory tests, and treatment was designed
to remove the cause of the disorder. The trend toward the use of technology accelerated
throughout the twentieth century. During this period, hospitals became institutions of
research and technology. Professionals in the areas of chemistry, physics, mechanical
engineering, and electrical engineering began to work in conjunction with the medical field,
and biomedical engineering became a recognized profession. As a result, medical tech-
nology advanced more in the twentieth century than it had in the rest of history combined
(Figure 9.1).
During this period, the area of electronics had a significant impact on the development
of new medical technology. Men such as Richard Caton and Augustus Desire proved that
the human brain and heart depend on bioelectric events. In 1903, William Einthoven
expanded on these ideas after he created the first string galvanometer. Einthoven placed
two skin sensors on a man and attached them to the ends of a silvered wire that was
suspended through holes drilled in both ends of a large permanent magnet. The suspended
silvered wire moved rhythmically as the subject's heart beat. By projecting a tiny light beam
across the silvered wire, Einthoven was able to record the movement of the wire as waves
on a scroll of moving photographic paper. Thus, the invention of the string galvanometer
led to the creation of the electrocardiogram (ECG), which is routinely used today to mea-
sure and record the electrical activity of abnormal hearts and to compare those signals to
normal ones.
