PC-based Biomedical Instrument

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O utput

Signal
Sensor Com puter D isplay
Conditioning

M em ory
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Sensor: Measures the signal of interest

Signal Conditioning: Amplifiers

Filters

Computer: Has an analog to digital (A/D) board to acquire analog data

Has a serial port to acquire digital data
Processes the acquired data

Memory: To save the obtained data (RAM or hard drive)

Display: On the screen or as a printout

 Microcontroller Based Biomedical Instrument

1. A general purpose computer does a lot of things that are unnecessary

for some dedicated biomedical instruments

2. A computer is large in size and limits portability

3. Several applications require the use of a dedicated instrument:

portable (home measurement devices) or otherwise (ultrasound,
MRI, CT, etc.)

• Portable systems require the use of a microcontroller

• Larger systems necessitate the design of a dedicated computer
system (hard drive, display monitor, elaborate controls)

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Signal
Sensor M icrocontroller O utput
Conditioning

M em ory
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 Microcontroller Based Biomedical Instrument
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Signal
Sensor M icrocontroller O utput
Conditioning

M em ory
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Microcontroller
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M em ory

A/D Converter

M icroprocessor
Serial
Com m unication
Interface

Example: MC68HC11
M Motorola’s general mcu family
68HC Motorola HCMOS (high-density complementary
metal oxide semiconductor)
11 MC68HC11 family
 Microprocessors

Examples: 8085, 8086, 8088, 80286, 80386, 80486, Pentium

The inside of a microprocessor contains hardware that processes the

data based on commands from the software.

A microprocessor performs
• a small set of very simple tasks.
• a pre-determined series of steps according to a set of
instructions (assembly language)
• the operations very quickly

All that a microprocessor can do is to do simple standardized operations

over and over at a very fast rate, in proper order

Digital hardware consists of a few cheap and simple building blocks,

cleverly interconnected and repeated over and over again.

Digital data (represented by 0s and 1s) are processed by elements called

logic gates.
 A/D and D/A Converters

Analog to Digital: convert analog voltage to a binary representation

digital output

D0

analog input D1
A/D
Converter D2

D3

Timing
Signal

Digital to Analog: convert a digital word into an analog voltage

digital input

D0

D1 analog output
D /A
D2 Converter

D3

Timing
Signal

Parameters to be concerned about:

max. voltage level (analog) number of bits (digital)

when should it sample

 Logic Gates

NOT
A B
0 1
1 0

OR
A1 A2 B
0 0 0
0 1 1
1 0 1
1 1 1

AND
A1 A2 B
0 0 0
0 1 0
1 0 0
1 1 1

NOT: inverts
OR: 1 if any of its inputs is 1
0 if all inputs are 0
AND: 1 if all inputs are 1
0 if any of the input is 0
Semiconductors

Conductors Semiconductors Insulators

Resistors V=IR

linear characteristics

Passive circuits: contain resistors, capacitors and inductors

Active circuits: contain passive elements and active elements (diodes
and transistors)
Semiconductors

Silicon, Germanium, Gallium Arsenide, etc.

Si

Individual silicon atom

Si Si Si

Si Si Si

Si Si Si

Si Atoms in Lattice

• Very few charge carriers

• Poor electrical conductor
• called an intrinsic semiconductor

To make them carry current semiconductors are doped with impurities

called dopants.

Impurities may be donors or acceptors

Semiconductors

n-type p-type

Si Si Si Si Si Si

Si P Si Si B Si

Si Si Si Si Si Si

Phosphorus doped Si Boron doped Si

Electron Hole

Electrons and holes are the particles that conduct current in solid state
electronic devices

Electron movement

Current

Hole movement

Current
Semiconductors

If a donor-type impurity is added to the extent of 1 part in 108, the

conductivity of germanium at 30OC is multiplied by a factor of 12.

Adding n-type impurities decreases the number of holes

Adding p-type impurities decreases the number of electrons
majority carrier minority carrier
n-type electrons holes
p-type holes electrons

Mass Action Law: n p = n i2

for Si at 300OK, n i = 1.6x1010 electrons/cm3

Doping
i) increases conductivity
ii) produces a conductor in which the electric carriers are either
predominantly holes or electrons

ND + p = NA + n

for n-type material: n ≈ ND (donor atom density)

for p-type material: p ≈ NA (acceptor atom density)

Semiconductors

Current Conduction

Drift
V

V
E = volts / cm
L

Drift Velocity (electrons and holes)

vn = µ n E

Where µn is the electron mobility constant. The same equation

applies to holes.

Conductivity

σ = conductivity = q(nµ n + pµ p )

Resistance
 L L
R= =ρ
Aσ A

Where ρ is resistivity, the inverse of conductivity.

Current Density

J = σE
Semiconductors

Current Conduction

t=t1
Diffusion
t=t2

t=t3

Diffusion Current

 dn 
J n = qDn  
 dx 

The above expression applies to a current density of electrons; the

same expression applies to holes.