Guidelines
Design two complete, novel bioinstrumentation systems using biosignals that are relevant to an
important health problem. Novel means not introduced in class and not able to be found on the
internet. Each of your solutions should contain the following elements:
1) a labeled block diagram of a full instrument to acquire the signal, process it, and convert
to a measurand.
2) block diagram must include a sensor, sensor circuit, processing circuitry (active and/or
passive), and output (digital, analog, LED, alarm, etc.).
3) state a reason for each choice of bioinstrumentation element.
4) define signal range, instrument resolution, sensor sensitivity.
5) state guidelines for detecting healthy vs. unhealthy levels of the biosignal. How should a
clinician interpret the measurand?
6) what modifications would be needed to make this a senior design project? What is the
ideal team (student expertise? Faculty expertise? Experts outside the university)?
Answers should be submitted in one clearly labeled word document, with your name and
student ID, a title for each instrument, and responses to questions 1) through 6) clearly
marked as such. The rubric below will be used to grade:
Points given
0
1
2
3
1)
No block
diagram.
Two or more
blocks/labels
are missing.
One block is
missing.
Block diagram
is complete.
2)
No labels or
only very
generic labels of
components.
Lack of detail.
Two errors or
omissions
prevent strong
and clear signal
from flowing
from biosignal
to measurand
to output.
One error or
omission in
specific
components
prevents strong
and clear signal
from flowing
from biosignal
to measurand
to output.
Instrument
components
work together,
signal flows
correctly.
3)
No rationale
provided or only
very general
and generic
information
provided.
Three or four
mistakes in the
justification of
use of
instrumentation
components
One or two
mistakes in the
justification of
use of
instrumentation
components.
Each
instrumentation
component is
well explained.
Element #
4)
No equations
are provided,
and no
characteristics
are given
justifiable
numeric values.
Two
characteristics
and/or two
equations to
describe them
are missing.
One of the
three
characteristics
is missing. One
equation is
missing.
Range,
resolution, and
sensor
sensitivity are
defined and
correct.
Equations are
given.
5)
No ranges of
values
discussed, and
false readings
not discussed.
Healthy and
unhealthy
ranges are
provided but
with no
explanation.
False readings
mentioned but
with limited
explanation.
Healthy and
unhealthy
ranges of
output are
defined. No
discussion of
errors (false
readings), or
only very
general
discussion
without details.
Healthy and
unhealthy
ranges of
output are
defined. The
rates of false
positives and
false negatives
are estimated,
justified, and
ways to reduce
them are
discussed.
6)
Omitted.
No discussion of
resources and
timeline, only
general features
of team
discussed.
Some efforts
made to
translate to
senior design,
but team
members are
not discussed in
detail.
Translation to
senior design
project (limited
time and funds)
is discussed.
Essential
features of the
best team of
students and
faculty +
outsiders is
discussed.
Guidelines
Design two complete, novel bioinstrumentation systems using biosignals that are relevant to an
important health problem. Novel means not introduced in class and not able to be found on the
internet. Each of your solutions should contain the following elements:
1) a labeled block diagram of a full instrument to acquire the signal, process it, and convert
to a measurand.
2) block diagram must include a sensor, sensor circuit, processing circuitry (active and/or
passive), and output (digital, analog, LED, alarm, etc.).
3) state a reason for each choice of bioinstrumentation element.
4) define signal range, instrument resolution, sensor sensitivity.
5) state guidelines for detecting healthy vs. unhealthy levels of the biosignal. How should a
clinician interpret the measurand?
6) what modifications would be needed to make this a senior design project? What is the
ideal team (student expertise? Faculty expertise? Experts outside the university)?
Answers should be submitted in one clearly labeled word document, with your name and
student ID, a title for each instrument, and responses to questions 1) through 6) clearly
marked as such. The rubric below will be used to grade:
Points given
0
1
2
3
1)
No block
diagram.
Two or more
blocks/labels
are missing.
One block is
missing.
Block diagram
is complete.
2)
No labels or
only very
generic labels of
components.
Lack of detail.
Two errors or
omissions
prevent strong
and clear signal
from flowing
from biosignal
to measurand
to output.
One error or
omission in
specific
components
prevents strong
and clear signal
from flowing
from biosignal
to measurand
to output.
Instrument
components
work together,
signal flows
correctly.
3)
No rationale
provided or only
very general
and generic
information
provided.
Three or four
mistakes in the
justification of
use of
instrumentation
components
One or two
mistakes in the
justification of
use of
instrumentation
components.
Each
instrumentation
component is
well explained.
Element #
4)
No equations
are provided,
and no
characteristics
are given
justifiable
numeric values.
Two
characteristics
and/or two
equations to
describe them
are missing.
One of the
three
characteristics
is missing. One
equation is
missing.
Range,
resolution, and
sensor
sensitivity are
defined and
correct.
Equations are
given.
5)
No ranges of
values
discussed, and
false readings
not discussed.
Healthy and
unhealthy
ranges are
provided but
with no
explanation.
False readings
mentioned but
with limited
explanation.
Healthy and
unhealthy
ranges of
output are
defined. No
discussion of
errors (false
readings), or
only very
general
discussion
without details.
Healthy and
unhealthy
ranges of
output are
defined. The
rates of false
positives and
false negatives
are estimated,
justified, and
ways to reduce
them are
discussed.
6)
Omitted.
No discussion of
resources and
timeline, only
general features
of team
discussed.
Some efforts
made to
translate to
senior design,
but team
members are
not discussed in
detail.
Translation to
senior design
project (limited
time and funds)
is discussed.
Essential
features of the
best team of
students and
faculty +
outsiders is
discussed.
Bioinstrumentation—
Sensors and Actuators
Definitions
Transducer: a device which accepts energy of one form as an input, converts it to
another form and transmits it as an output
Sensor: accepts physical parameter as input, and converts it to an electrical signal
-also called an input transducer
-resistive
-inductive
-capacitive
-piezoelectric
Actuator: accepts electrical signal as input, and converts it to a mechanical output
-output transducer
-examples: valves, solenoids, pumps, motors
Signal Conditioning—Conceptual
Sensor Output (S)
Mod
Desired Output (D)
D = (S-N)/Mod
Noise(N)
Measurand (M)
Measurand (M)
Measurand (M)
Measurand (M)
The Wheatstone Bridge
Vi - i1R4 - i1R3 = 0
(1)
KVL
Vi - i2R2 - i2R1 = 0
(2)
KVL
V0 + i1R4 - i2R2 = 0
(3)
KVL
V0 - i1R3 + i2R1 = 0
(4)
KVL
i2 = (V0 + i1R4)/R2
(5)
(3) rearranged
Vi/(R1 + R2) = (V0 + i1R4)/R2
(6)
(2) rearr. subst. into (5)
Vi/(R1 + R2) = V0 /R2+ Vi R4/R2(R3 + R4)
(7)
(1) rearr. subst. into (6)
V0 = Vi [R2/(R1 + R2) – R4/(R3 + R4)]
(8)
(R2 times (7), rearr.)
R1/R2 = R3/R4
(9)
(V0 = 0)
The Resistive Temperature Detector
•
•
•
•
Low sensitivity (dR/dT)
High thermal inertia
Susceptible to mechanical damage
Resistance in the leads creates error
WIRE RTD
For a Platinum RTD:
α1 = 3.96 x 10-3 (°C-1)
α2 = -5.85 x 10-7 (°C-2)
Range = 0 – 200 °C
Ro = 100 Ω
housing
leads
connectors
glass
Pt
ceramic
What is the sensitivity at 0 and 200°C?
What is the resistance at 100 and 200°C?
Use:
RT = Ro (1 + α1T + α2T2 + …)
Derive dR/dT
THIN FILM RTD
Resistive Temperature Detector: Solution
An RTD is nonlinear…
RT (Ω)
RT (Ω)
1000
…but approx. linear over usable range
500
200
100
0
0
0
2000 4000
0
dRT/dT
T (°C)
0.3961
0.396
0.3959
0.3958
0.3957
100
T (°C)
What is the sensitivity at 0 and 200°C?
What is the resistance at 100 and 200°C?
0
100 200
T (°C)
dR/dT(0) = 0.396 Ω/°C
dR/dT(200) = 0.3957 Ω/°C
R(100) = 139 Ω
R(200) = 177 Ω
200
RTD Instrumentation Circuit
THREE LEAD COMPENSATION
R
Vi
R
• Rt is low, so contact resistance from the wires can interfere with the measurement.
• Therefore, compensation with three leads introduced into the bridge circuit
• At balance, L3 current = 0, and voltage drops through L1 and L2 are equal.
• (R+L2)/(Rt+L1) = constant
• The Sensitivity is ~ 1 mV/°C
V0 = Vi [(R+L2)/(R+Rt+L1+L2) – R/(R + R)]
R
Vi
R
Assume L1 = L2 = L, L3 = 0
V0 = Vi [(R+L)/(2L+R+Rt) – R/(R + R)]
V0 = Vi [(R+L)/(2L+R+Rt) – 1/2]
(voltage dividing principle across the left side)
(rearrange terms)
Rt = R[(Vi - 2V0)/(Vi + 2V0)] - L [(4V0)/(Vi + 2V0)]
Error term due to lead resistance
-accounted for in the calibration
Thermistors
•
•
•
•
•
•
A thermally-sensitive semiconductor resistor
Small size
Low thermal inertia
Nonlinear
-30 to 200 °C
Sensitivity of ~4%/°C
RT = A exp(B/T)
Derive dR/dT
1) Raise T
2) Increases # of active charge carriers by
promotion into the conduction band
3) Conduction band ~ current
4) Higher current ~ lower resistance
N-type: Fe2O3 doped with Ti
P-type: NiO doped with Li
Thermistor R vs. T and Sensitivity
RT (Ω)
600
400
200
0
0
100
T (°C)
200
T (K)
300
-16030
-16040
dRT/dT
-100
-16050
-16060
-16070
0
200
400
600
Thermistor Instrumentation Circuit
Amplified voltage divider
Bridge circuit
See worked out example of a thermistor
used in a bridge circuit
The thermistor is nonlinear because the
resistance-temperature curve is a decaying
exponential
Thermocouple
•
•
•
•
•
•
Passive device that generates a voltage difference that varies with temperature
Very small, inexpensive, rugged
Low thermal inertia
High sensitivity (5-80 µV/°C)
Reference point is required
nonlinear
Contact Potential: When two dissimilar metals contact, a contact potential develops and
drives electric charge from one metal to the other
𝛻V = -S(T)𝛻T
S = Seebeck
coefficient
Seebeck or Thermoelectric Effect: Current flowing in a loop consisting of two metals,
caused by temperature differences between the two junctions.
Thermocouple Instrumentation Circuit
• The reference junction could be placed in an ice-bath
• This is known as a cold junction thermocouple
𝑇
• V= 𝑙𝑎𝑡𝑒𝑚𝑆(𝑒𝑠𝑛𝑒𝑠 𝑇1 (𝑇) − 𝑆𝑚𝑒𝑡𝑎𝑙2 (𝑇)) 𝑑𝑇hard to solve for
𝑟𝑒𝑓
each temperature
• V = E(Tsense) - E(Tref)evaluate characteristic functions E, at
two temperatures.
• E(Tsense) = V + E(Tref)V is measured, Tref is set, so E(Tref) is
known. Tsense is found from a look-up table of E(Tsense)
• The above circuit is for a reference junction compensated
thermocouple: E’s are voltages, Tref is reference temperature,
Tsense is the temperature to measure
Thermocouples are nonlinear, but can be
approximately linear over a restricted range
RTD vs. Thermistor vs. Thermocouple
RTD
Thermistor
Thermocouple
Material
Metal (Pt)
Ceramic metal oxide
Dissimilar metal (Ni, Chromium,
Rhodium, Constantan, Iron)
Useful range
0-200 °C
-50-130 °C
-270-1260 °C
Sensing principle
ρ(T), so
R(T)
Charge carriers (T)
-Ohm’s Law
Thermoelectric (Seebeck) effect
sensitivity
quasilinear
(quadratic)
Nonlinear
(exponential)
~linear
(polynomial)
thermistor
R
RTD
TC
T
V
Thermocouples are very versatile
Bioinstrumentation—
Sensors and Actuators
Sensors and Actuators—
Summary of Part 1
• Sensors behavior is governed by the ruling equations describing the physical principle of
measurement
• Sensors may be linear (y=Ax) or nonlinear (y = b0 + Ax + b1x2 + b2x3+…)
• Sensor sensitivity S = Δ electric parameter / Δ physical parameter
• This can be local at a specific input value x, and is the partial derivative of the electrical
parameter with respect to x
• DON’T confuse the sensitivity of a sensor with that of a sensor + circuit. They are two
different numbers.
• Sensor nonlinearity δ = nonlinear terms / linear terms
• This can be local at a specific x, or global (determine x that maximizes nonlinear factor)
Thermistor: RT = A exp(B/T)
-semiconductor
RTD: RT = Ro (1 + α1T + α2T2)
-Pt wire or film
-R = ρL/A
Thermocouple: V=α(T1-T2)
-dissimilar metals
-Seebeck effect
If the Wheatstone bridge resistor labels get confusing…
1) Draw the currents in the right directions. 2) KCL, 3) Ohm’s law, 4) use voltage divider equation
twice to go halfway across the bridge, 5) subtract terms to get V0, 6) normalize by Vi
Vi
Vi
Vi
V0
V0
Vi
V0
Temperature sensing during cardiac
ablation to treat ahrhythmias
Live Surgical Demonstration
of Cardiac Ablation
A surgeon’s testimonial
of a multi-thermistor device
Resistive Strain Gauges
•
•
•
•
•
A resistive material (fine wire or metal foil) is placed on a flexible backing
The backing is placed on the object from which strain is to be measured
Forces that stretch or contract the wire along the long axis change the resistance (Figure)
A bridge circuit converts ΔR into ΔV, and amplifiers make the signal larger
Force→strain→ ΔR→ ΔV (V versus force turns out to be approximately linear)
• Note: 1 strain gauge = 1 axis strain and pressure; 2 perpendicular gauges = 2 axes; 4 in a
square = 2-axis measurement with temperature compensation
• See Class Notes for Derivation of gauge factor and use of the strain gauge in a circuit.
Semiconductor Strain Gauges
When energy is input to a semiconductor, its resistance eventually decreases because the
added energy promotes charge carriers to a level that allows more charge to flow (current)
In a metal wire, input of energy causes free charges to collide more often (more kinetic
energy), thus eventually causing increased resistance and a barrier to increased current
conductor
semiconductor
I
I
V
V
-V = reverse
bias
Self-heating
+V =
forward bias
• High gauge factor K
• Piezoresistive effect
• Nonlinear S
Semiconductor Transducers—General
Semiconductor: has electrical properties in between conductors and insulators
-silicon
-gallium arsenide
-germanium
-cadmium sulphide
n-type semiconductors: doped with impurities to have an excess of electrons
p-type semiconductors: doped to have reduced electrons
I=I0[exp(qV/kT)-1]
I
V
I0 is the leak current
A Semiconductor Pressure Sensor—
Designed by Biomedical Engineers
Semiconductors are not flexible—hard to make them
Strain Gauges are Diverse!
Additional Uses of Strain Gauges
• Flow monitoring in medical pumps
• Insulin
• Medical weighing
• Biomechanics research
• Medical imaging instruments
Capacitive Transducers
C = ε0εairA/x
(1) definition of capacitance
dC = -(ε0εairA/x2 )dx
(2) d/dx of (1)
dC/C = -dx/x
(3) plug (1) into (2)
Capacitive Pressure Transducers
De Sauty Bridge: 2 resistors + 2 capacitors
Vi = V0[Ct/(Ct+C1) – R1/(R1+R2)]
Ct/C1 = R2/R1
(at balance, V0=0)
Piezoelectric Transducers
The Piezoelectric effect: certain crystals, when deformed, generate an electrical charge
-quartz
-lithium sulfate
- barium titanate
• Frequency response depends on the RC time constant
Pressure and Vibration Sensing
V = SV*P*D
SV is the voltage sensitivity (V*m/N)
P is pressure (N/m2)
D = transducer thickness
-
+
[charge/force]
Compare to V = SV*P*D
SV = k/ε0εr
Photodiodes
Photodiodes types are application-specific
Range: 1 pW/cm2 – 100 mW/cm2
Uses: spectroscopy, imaging
(near infrared, visible, near UV),
position sensing, laser
profilometry, pulse oximetry.
Photodiode Types:
P/N
PIN
avalanche
Photodiodes are semiconductor junctions
bandgap
Conduction band: +1.12 eV
Charge carriers~T
Charge carriers~ hν100
Bioinstrumentation—
Op-Amps and Integrated Circuits
A timer circuit is an
integrated circuit
that controls
square wave duty
cycle
+5V
555
1 Ground
Vcc
8
2 Trigger
Discharge
7
3 Output
Threshold
6
Control
5
4 Reset
RA
RB
8
Ra
7
3
Rb
(a)
Th = -ln(0.5)(R
+Rln
𝑇𝐻 = a−
b)C 0.5
𝑅𝐴 + 𝑅𝐵 𝐶
4
6
2
C
5
1
C
5V
0V
(b)
𝑇𝐿 =T−
ln 0.5 𝑅𝐵 𝐶
l = -ln(0.5)RbC
(c)
Duty cycle=DC=TH/(TH+TL)
Figure 2.41 The 555 timer (a) Pinout for the 555 timer IC. (b) A popular circuit that utilizes a
555 timer and four external components creates a square wave with duty cycle > 50%. (c)
The output from the 555 timer circuit shown in (b).
555 Timer Integrated Circuit
Uses
• Timing
• Pulse Generation
• Oscillator
• Pulse width modulation
• Frequency divider
• Pulse generator
• Logic clock
• Tone generation
• Simple ADC
• Temperature sensing
(if connected to thermistor)
Composition
• 25 transistors
• 2 diodes
• 15 resistors
• Silicon chip
Pins
• 1 GND = ground
• 2 TRIG = OUT pin goes high and timing interval
starts when input falls below 1/3 Vcc
• 3 OUT = output voltage signal
• 4 RESET = resets the timing interval when
switched transiently to ground
• 5 CTRL = control access to the internal voltage
divider (2/3 Vcc by default)
• 6 THR = the timing interval ends when the
voltage at THR is greater than 2/3 Vcc/CTRL
• 7 DIS = open collector output from a transistor,
may discharge a capacitor between intervals
• 8 VCC = positive supply voltage
Digital to Analog Converter: (3-bit) Input versus (voltage) Output
7/8 Vref
Analog output
6/8 Vref
5/8 Vref
4/8 Vref
3/8 Vref
resolution =
2/8 Vref
1
2
1/8 Vref
n
Vref
0V
000
001
010
011
100
101
110
111
Figure 2.42 The ideal static behavior of a 3-bit DAC. For each digital string, there is
a unique analog output.
Digital to Analog
Converter—
Circuit
1. The digital input is b2, b1,
b0
2. Digital input controls
switches through the 3-8
decoder
3. Switches control a voltage
divider such that voltage
is divided by:
4. Resolution = Vref/2n
n is the bit-depth (3 in this
example—the 3 bits are b2,
b1, b0
b2
Vref
R
R
R
b1
b0
3-to 8 decoder
7
6
5
4
3
2
1
0
7/8 Vref
6/8 Vref
Vo
R
R
R
1/8 Vref
0
R
R
Figure 2.43 A 3-bit voltage scaling DAC converter.
“Switch” closure vs. Vo of a DAC
0
X
X
X
X
O
O
O
O
1
O
O
O
O
X
O
O
O
2
O
O
O
O
O
X
O
O
3
O
O
O
O
O
O
X
O
4
O
O
O
X
X
X
X
O
5
O
O
X
O
O
O
O
O
6
O
X
O
O
O
O
O
O
7
X
O
O
O
O
O
O
O
Vo
7/8
6/8
5/8
4/8
3/8
2/8
1/8
0
-0 is closest to ground; 7 is closest to Vref, as in slide 6
-Vo is a fraction of Vref
Analog to Digital Converter: (Voltage) Input versus (3-bit) Output
111
110
Digital output
101
100
011
010
001
000
0/8
1/8
2/8
3/8
4/8
5/8
6/8
7/8
Vref
Figure 2.44 Converting characteristic of 3-bit ADC converter.
Analog to Digital Converter: Circuit Example
Comparator:
Compares two
voltages, produces
positive or negative
square wave
Vin
2. 0.5Vref
Either < or >
Than Vin
Clock
Clock
+
_
Digital
control
logic
3.
“1” decrement
“0” increment
1. V=0.5Vref
Digital
output
DAC converter
Figure 2.45 Block diagram of a typical successive approximation ADC.
1.
2.
3.
4.
The loop starts with V = 0.5Vref (this is binary 100)
V is compared to Vin
If >, then the first bit = 1. If
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