ENGR 401
Bioinstrumentation
LECTURE: MEASURANDS, SENSORS, SIGNALS,
AND ELECTRONICS I
DR. SABINE WEYAND
What is medical instrumentation?
▪ Instruments that measure or apply
a signal to/from the human body
▪ Used for
▪
▪
▪
▪
Screening
Diagnostics
Treatment
Monitoring
▪ Developed by biomedical
engineers
▪ Tough area: regulations, clinical
testing, reliability, ruggedness,
marketing..
▪ Most are evolutionary not
revolutionary products
▪ Add new features over time
2
Typical Medical Instrumentation System Diagram
Measurand
Sensor
Signal
Signal
Processing
(Filter and
Amplification)
Monitor
FIGURE 1 A simple block diagram of human-machine interface. Modified from (Chatterjee, 2010, Ch 1 and Webster Ch1)
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Measurand
Sensor
Measurands
Signal
Signal
Processing
(Filter and
Amplification)
Monitor
Measurand
▪ Physical quantity being
measured
▪ Main categories
1.
2.
3.
4.
5.
Biopotential (ions)
Pressure
Flow
Optical/imaging
Displacement (velocity,
acceleration, and force)
6. Impedance/conductivity
7. Temperature
8. Chemical concentrations
5
(Chatterjee 2010, Ch 1 and Webster 2020, Ch 1)
Human Body Signals: Barriers to Success
▪ What makes biomedical instruments so difficult to develop?
1.
2.
3.
4.
5.
Safety is of utmost importance
Measurement ranges are quite low compared with nonmedical parameters
Cannot take human apart or turn human off to measure signals
Large variability between objects (people)
Must be careful not to cause damage: applying energy can cause tissue damage (X-ray,
ultrasound…)
6. Lots of noise from other human body signals
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Measurand
Parameter
Measurement
Parameter Range
Signal Freq Range
(Hz)
Sensor or Method
ECG (heart)
0.5-4 mV
0.01-250
Skin electrode
EEG (brain)
5-300 microV
150
Skin electrode
Blood Pressure
0-400 mmHg
50
Strain gauge
(Webster 2020, Ch 1)
Measurand
Sensor
Signal
Signal
Processing
(Filter and
Amplification)
Monitor
Sensors/Transducers/Electrodes
Biomedical Sensors
▪ Sensors/Transducers: convert physical measurands into signals (mostly electrical
signals, but not always)
▪ There are many different sensors used in medical applications, some examples:
▪
▪
▪
▪
▪
▪
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Electrodes used in ECG, EEG, EMG, defibrillators, and external pacemakers
Strain Gauges
Piezoelectric sensors
Thermistors (resistance thermometer)
Thermocouples
Diaphragm pressure gauge
(Webster 2020, Ch 1)
Sensor Classification
▪ Single vs Multi sensors
▪ One sensor/transducer converts body signal into desired output or many
▪ Sampling vs Continuous Mode
▪ Is the signal very slow moving or require frequent/constant monitoring
▪ Large grey zone in classification
▪ Generating vs Modulating Mode
▪ Modulating requires an auxiliary energy source
▪ Generating requires no auxiliary energy source
▪ Direct vs Indirect
▪ Desired measurand is interfacing with sensor or not
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(Webster, 2020, Ch 1)
Sensors
▪ Sensors
▪ Classification
▪ Single vs Multi sensors
▪ Sampling vs Continuous Mode
▪ Generating vs Modulating
Mode
▪ Note: We often have several
sensor options /modes to
collect a given measurand
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(Chatterjee 2010, Ch 1 and Webster 2020, Ch 1)
Quick Question: Sensors Classification
▪ Single vs Multi sensors
▪ Sampling vs Continuous Mode
▪ Generating vs Modulating Mode
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Mercury Thermometer
Quick Question: Sensors Classification
▪ Single vs Multi sensors
▪ Sampling vs Continuous Mode
▪ Generating vs Modulating Mode
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Electronic Thermometer
Quick Question: Sensors Classification
▪ Single vs Multi sensors
▪ Sampling vs Continuous Mode
▪ Generating vs Modulating Mode
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Stethoscope – Acoustic
Quick Question: Sensors Classification
▪ Single vs Multi sensors
▪ Sampling vs Continuous Mode
▪ Generating vs Modulating Mode
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Stethoscope – Electronic
Quick Question: Sensors Classification
▪ Single vs Multi sensors
▪ Sampling vs Continuous Mode
▪ Generating vs Modulating Mode
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https://aneskey.com/principles-of-ultrasound-guided-regional-anesthesia/
Measurand
Sensor
Signal
Signal
Processing
(Filter and
Amplification)
(Bio)Signals and Systems
Monitor
Signals
▪ A signal is a function representing a
physical quantity or variable
▪ x(t) is a continuous-time signal if t is a
continuous variable
▪ x[n] is a discrete-time signal
▪ may represent a phenomenon for which
the independent variable is inherently
discrete
▪ may represent sampling a continuoustime signal x(t)
17
Schaum's outline of signals and systems
Sinusoid
▪ Practical signals consist of the sum of
sinusoids having different amplitudes,
frequencies, and phases
▪ Sinusoid analysis:
▪ What is the peak to peak Amplitude
▪ 10 V
▪ What is the root mean square (RMS) Amplitude
▪ 7.07 V
▪ What is the time period for one complete cycle
▪ 20 µs
▪ What is the frequency?
▪ 50 kHz
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FIGURE 3-1 A sinusoidal signal shown on the scope
(Chatterjee, 2010, Ch 3)
Sine waves
▪ The equation of an AC signal is given as:
▪ where Vs(t) is the voltage AC signal with a peak amplitude of “A” volts and a frequency of “f” hertz. ⍵=2"f is called the
angular frequency (noted in radians)
▪ Summation same frequencies
FIGURE 3-2a Summation of two sine waves of the same frequency but different amplitudes (Chatterjee, 2010, Ch 3)
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Sine waves
▪ Summation different frequencies
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FIGURE 3-2b Summation of two sine waves of different frequencies and amplitudes
(Chatterjee, 2010, Ch 3)
BioSignal
▪ Biosignals
▪ Physiological events
▪ Electrical, chemical, or
mechanical activity
▪ Various acquisition
methods
▪ Signals can be noisy
▪ Reduced to meaningful
signal
▪ Evaluated
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Nature of Analog Signals and Analysis
▪ Sinusoids are the building blocks of nature
▪ Human body has sinusoidal receptors:
▪ Cochlea (inner ear)
▪ Air Pressure Sinusoids
▪ Fluid filled vibrations travel travel down cochlea
▪ Base hair cells are stimulated by high frequency
(stiff and short) and the apex hair cells are
stimulated by low frequency (long and floppy)
▪ Freq. 20 Hz to 20 kHz
https://www.britannica.com/science/ear/Transmission-of-sound-within-the-inner-ear
▪ Rods and cones (retina)
▪ Electromagnetic sinusoids
▪ Freq. 430 THz to 790 THz
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https://www.aao.org/eye-health/anatomy/cones
Collecting and Analyzing BioSignals
▪ Physiological signals are continuous in both time and amplitude (analog signals)
▪ Most sensors are analog
▪ In order to process (analyze, amplify and filter) human body signals we perform
sampling and convert the analog signal to a digital discretized signal in time and
quantized amplitude (analog to digital signal) A to D converters
▪ Filtering and Amplification can/is be done on both analog and/or digital signal
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Discretize in Time and Amplitude
▪ Nyquist Sampling Theorem
▪ Nyquist frequency corresponds to the highest frequency presented in the signal
▪ Sampling frequency has to be more than doubled the Nyquist frequency in order to avoid errors
▪ Sinc – building blocks at each point added back together
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Quick Question
▪ If the highest frequency in a given signal is
120 Hz, how fast must the signal be sampled
to prevent aliasing?
25
Time and Frequency Domain
▪ Time-domain and frequencydomain
▪ Related
▪ Fourier series - summation of
sinusoidal signals with their
amplitudes, frequencies, and
phases to generate a periodic
signal
▪ Fourier transforms - the
nonperiodic signals
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Time and Frequency Domain
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Basic Components of Medical
Instruments – Electronics and
Circuits
Basic Electronic Components Used in Circuits
▪ Power/Voltage Source [DC vs AC circuits]
▪ Resistors
▪ Capacitors
▪ Inductor
▪ Diodes
▪ You must understand the basic functions of electronic components to:
▪ Understand the medical instrument system diagrams
▪ To understand filtering and amplification
▪ To model human body systems and sensor machine interfaces
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Resistors
▪ Resist Current - use resistor to control the voltages and the currents in your circuit
▪ Passive device
▪ Ideal Voltage current relationship (Ohm’s Law)
▪ Resistors in parallel
▪ Resistors in series
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DC Circuits in Series and Parallel
▪ The simplest type of circuit is the direct current (DC) circuit
▪ Voltage does not vary in time
FIGURE 4-2 A simple series circuit
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FIGURE 4-4 A simple parallel circuit
with current divisions
(Chatterjee, 2010, Ch4)
DC Circuit Series Voltage Drop Across Resister
▪ Calculate using Ohm’s Law of the circuit
and of each resistor
▪ Calculate using Voltage divider rule
(applied voltage to the circuit) x (resistance
of the resistor) / (total resistance in the
circuit)
▪ Source Voltage is the sum of the voltage
drops over each resistor (Kirchhoff’s voltage
law)
FIGURE 4-2 A simple series circuit
32
(Chatterjee, 2010, Ch4)
Quick Question
What is the voltage drop across the 100 Ohm resistor?
Or
FIGURE 4-2 A simple series circuit
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(Chatterjee, 2010, Ch4)
Quick Question
▪ What is the voltage drop across each resistor?
34
(Chatterjee, 2010, Ch4)
DC Circuit in Parallel Voltage and Current
▪ The voltage drop across each resistor will be the
same as the applied voltage; the current will be
different
▪ The total current in the circuit will be the
summation of these two currents
FIGURE 4-4 A simple parallel circuit
with current divisions
35
Quick Question
▪ What is the current through each resistor and the total current?
FIGURE 4-4 A simple parallel circuit
with current divisions
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Series DC Capacitive and Inductive Circuits
▪ Capacitors and inductors are used
in medical circuits for:
▪ Slow build up of voltage or
current as delay components
▪ Suppressing certain signal
frequencies
FIGURE 4-7 A simple RC series circuit
37
(Chatterjee, 2010, Ch4)
Capacitor
▪ Stores Energy - reservoirs for electrical charge, which take time to fill up or empty
out
▪ As we apply a DC voltage to a capacitor, the charge slowly builds up across the
capacitor in approximately five time constant
where C is capacitance in Coulombs, Eo permittivity of a vacuum, Er
relative dielectric constant of material between plates, A area of the
plates, d distance between plates
38
(Chatterjee, 2010, Ch4)
Inductor
▪ Inductors, like capacitors, store energy
▪ Apply a current through the inductor, a magnetic field is created around it
▪ In series it will resist alternating currents (AC) and let direct currents (DC) flow free
▪ Inductance of a coil (L) can be calculated as:
▪
▪
▪
▪
39
µ0 is the permeability of the core
N is the number of turns of wire
A is the cross- sectional area of the coil
l is the length of the coil
(Chatterjee, 2010, Ch4)
Summary of DC Concepts
40
(Chatterjee, 2010, Ch4)
AC Circuit Current and Voltage Relationship
▪ In purely resistive circuits, AC current and voltage are in sync (meaning that they
start and end at the same time; but this does not happen in capacitive or inductive
circuits)
FIGURE 4-10a Voltage and current
waveforms in a resistive circuit
41
(Chatterjee, 2010, Ch4)
AC Circuits: Reactance and Impedances
▪ Capacitors and inductors
▪ Frequency-dependent
▪ Resistors
▪ Frequency-independent
▪ Capacitor reactance
AC resistances are called reactance
▪ Inversely proportional to applied signal frequency
▪ The capacitor is a short at very high frequencies
▪ Inductors
▪ The inductor acts as a short at low frequencies. An ideal short exhibits a zero-ohm
resistance
42
(Chatterjee, 2010, Ch4)
Semiconductor Diode Rectifier
▪ Diode
▪ Semiconductor device silicon wafer doped with
impurities such as antimony or indium
▪ Positive and negative polarities
▪ A diode has positive and negative polarities and is
ON when its positive polarity is connected to the
positive terminal of a power supply
▪ Drop of approximately 0.7 volts across the silicon
diode
FIGURE 4-17a A forward-biased
diode circuit
(Chatterjee, 2010, Ch4)
43
Forward Biased Diode
FIGURE 4-17c A bipolar square wave applied to a forward-biased diode circuit
44
(Chatterjee, 2010, Ch4)
Zener Diode Regulator
▪ Forward or reversed-bias
▪ Mainly used in reverse-bias mode
▪ Can handle large current flow without damage t
FIGURE 4-18a A Zener diode circuit
45
(Chatterjee, 2010, Ch4)
Zener Diode
▪ When VS ≤ 5 V, there is no current through the Zener diode
▪ When VS > 5 V, the Zener now acts as a DC regulator of 5 V and the load RL
maintains a current of 5 V/3 K Ω
FIGURE 4-18b Forward-bias to the
Zener circuit of Figure 4-18a
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FIGURE 4-18c Reverse-bias to the Zener
circuit of Figure 4-18a
(Chatterjee, 2010, Ch4)
Diodes Summary
▪ A nonlinear passive
device
▪ light-emitter (LED) or a
light-detector
(photodiode)
▪ Permit current flow in
forward direction once
a certain voltage is
achieved
▪ The diode will conduct
when the anode is
approximately 0.6 V
above the cathode
47
Diode i-v characteristics
▪ What does a typical i-v curve look like for a diode?
i-v curve resistors
48
https://learn.sparkfun.com/tutorials/diodes/real-diode-characteristics
References
▪ Chatterjee, Shakti, and Aubert Miller. Biomedical Instrumentation Systems.
Available from: VitalSource Bookshelf, Cengage Learning US, 2010.
▪ Chapters 1-5
▪ Webster, J. G., & Clark, J. W. Medical instrumentation: Application and design. Fifth
Edition. New York: Wiley, 2020.
▪ Chapter 1
49