Biomedical Engineering Reference
In-Depth Information
Electromagnetic energy radiating from noise sources is injected into the amplifier circuit or into
the patient by capacitive and/or inductive coupling. Even the action potentials fromnerve con-
duction in the patient generate noise at the sensor/amplifier interface. Filters are used to reduce
the noise and to maximize the signal-to-noise (S/N) ratio at the input of the A/D converter.
Low-frequency noise (amplifier d.c. offsets, sensor drift, temperature fluctuations, etc.) is
eliminated by a high-pass filter with the cutoff frequency set above the noise frequencies
and below the biological signal frequencies. High-frequency noise (nerve conduction, radio
broadcasts, computers, cellular phones, etc.) is reduced by a low-pass filter with the cutoff
set below the noise frequencies and above the frequencies of the biological signal that is
being monitored. Power line noise is a very difficult problem in biological monitoring, since
the 50- or 60-Hz frequency is usually within the frequency range of the biological signal that
is being measured. Band-stop filters are commonly used to reduce power line noise. The
notch frequency in these band-stop filters is set to the power line frequency of 50 or 60 Hz
with the cutoff frequencies located a few Hertz to either side.
The second type of corrupting signal is called inherent noise. Inherent noise arises from
random processes that are fundamental to the operation of the circuit's elements and thus is
reduced by good circuit design practice. While inherent noise can be reduced, it can never
be eliminated. Low-pass filters can be used to reduce high-frequency components. How-
ever, noise signals within the frequency range of the biosignal being amplified cannot be
eliminated by this filtering approach.
9.14.2 Computers
Computers consist of three basic units: the central processing unit (CPU), the arithmetic
and logic unit (ALU), and memory. The CPU directs the functioning of all other units and
controls the flow of information among the units during processing procedures. It is con-
trolled by program instructions. The ALU performs all arithmetic calculations (add, sub-
tract, multiply, and divide) as well as logical operations (AND, OR, NOT) that compare
one set of information to another.
Computer memory consists of read only memory (ROM) and random access memory
(RAM). ROM is permanently programmed into the integrated circuit that forms the basis
of the CPU and cannot be changed by the user. RAM stores information temporarily and
can be changed by the user. RAM is where user-generated programs, input data, and pro-
cessed data are stored.
Computers are binary devices that use the presence of an electrical signal to represent 1
and the absence of an electrical pulse to represent 0. The signals are combined in groups of
8 bits, a byte, to code information. A word is made up of 2 bytes. Most desktop computers
that are used today are 32-bit systems, which means they can address 4.295
10
9
locations
in memory. Most new computers today are 64-bit systems that can address 1.8447
10
19
locations in memory. The first microcomputers were 8-bit devices that could interact with
only 256 memory locations.
Programming languages relate instructions and data to a fixed array of binary bits so the
specific arrangement has only one meaning. Letters of the alphabet and other symbols
such as punctuation marks are represented by special codes. ASCII stands for American
Standard Code for Information Exchange. ASCII provides a common standard that allows
