Biomedical Instrumentation. Chapter 6 in Introduction to Biomedical Equipment Technology By Joseph Carr and John Brown. Signal Acquisition.
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Chapter 6 in
Introduction to Biomedical Equipment Technology
By Joseph Carr and John Brown
Output
Input
Input
Which is more sensitive? The left side one because you’ll have a larger change in y for a given change in x
Output
Input
Sensitivity Error
Sensor TerminologyOutput
Output
Input
Input
Offset Error
Zero offset error
Ideal
Measure
Output
Din(Max)
Input
LinearityP
F2
Input = x
F1
B
Q
HysteresisTolerance Band
Tresponse
100%
70%
Rising Response Time
Time
Ton
Response TimeTolerance Band
Tresponse
100%
70%
Rising Response Time
Time
Ton
Response TimeTdecay
F(t)
Decaying Response Time
Toff
Time
F(x)* = ax + bx2+cx4+ . . . +K
F(x)* = ax + bx3+cx5+ . . . +K
Output
F(x)
Output
F(x)
F(x) = mx + K
F(x) = mx + K
K
K
Input X
Input X
Dynamic LinearityMeasure of a sensor’s ability to follow rapid changes in the input parameters. Difference between solid and dashed curves is the non linearity as depicted by the higher order x terms
F(x) = ax + bx2+cx4+ . . . +K offsetting for K or you could assume K = 0
Symmetrical = F(x) = F(x) where F(x) * is symmetric around linear curve F(x) then
F(x) = ax +bx3 + cx5 +. . . + K offsetting for K or you could assume K =0
Dynamic LinearityAv = Vo/Vi
1.0
Frequency (w) radians per second
Frequency Response of Ideal and Practical SystemAv = Vo/Vi
1.0
0.707
FL
FH
Frequency (w) radians per second
Frequency Response of Ideal and Practical SystemMetal
Electrode
Electrolytic Solution where Skin is electrolytic and can be modeled as saline
2 cells A and B, A has 2 positive ions
And B has 3 positive ions thus have a
Potential difference of 3 –2 = 1 where B
is more positive than A
A
++
B
+++
Metal A
Vbe
Metal B
Electrolytic Solution
ElectrodesVae
Metal A
Vbe
Metal B
Electrolytic Solution
Vd = Differential voltage Vd
Rsa and Rsb = skin resistance at electrode A and B
Electrode A
C1a
Vea
+

Rsa
Cellular
Resistance
R1a
Rc

Vo
Mass
Tissue
Resistance
Rt
Vd
Cellular
Potentials
+
Electrode B
C1b
Veb
+

Rsb
R1b
Ionic Conduction
Electronic Conduction
PinTip
Connector
Shielded Wire
Electrode Surface
Medical Surface ElectrodesR1
RS = Spreading Resistance of the electrode and is a function of tip diameter
R1 and C1 are result of the effects of electrode/cell interface
C2 = Electrode Capacitance
RS
C2
C1
Vo
V1
Rs in metallic microelectrodes without glass coating:
Capacitance of C2 has units pF/cm
Es
A
R1
R3
R1
R3
+

Eo
EC
ED
+
Eo
+

Es
ED
EC

R2
R4
R4
R2
B
Rest Condition
L  DL = length
Compression
Strain GaugesTension
Strain GaugesL = length
Rest Condition
L = length; ΔL = change in L; ρ = resistivity
A = Area; ΔA = change in A
L = length; ΔL = change in L; ρ = resistivity
A = Area; ΔA = change in A
Note: Textbook forgot the ρ in equations 628 and 629 on page 110
A
R1 = SG1
R3 = SG3
+
Vo
ES
C
D

R4 = SG4
R2 = SG2
B
Mechanical Configuration
Electrical Circuit
+
Derivation:
Circuit
A
R1 = R +h
R3= Rh
Es
+

Eo
+
C
D

R2 = R  h
R4 = R +h
B
Note: Text book has wrongly stated that tension decreases R and compression increases R on page 112
VCB
+
IB

VCE
+

VBE

IE
BJT = Bipolar Junction TransistorIC
B = Base
C = Collector
E = Emitter
IE = I B + I C
+
+
VCB
Ic1
Ic2
VCB

+
+
VBE
DVBE
VBE


ccs1
ccs2
VEE
Solid State PN Temperature TransducersSolid State PN Junction Diode: the base emitter voltage of a transistor is proportional to temperature. For a differential pair the output voltage is:
K = Boltzman’s Constant = 1.38 x1023J/K
T = Temperature in Kelvin
IC1 = Collector current of BJT 1 mA
IC2 = Collector current of BJT 2 mA
q = Coulomb’s charge = 1.6 x10 19 coulombs/electron