Instrumentation 2 Flashcards
What do ADCs do
Convert continuous analog signal to digital signal
Describe the process of sampling
Digitising analogue signals into a series of discrete scales/readings
What is sampling time
The time between each sample/reading when you are converting analogue to digital signal
When converting analogue to digital signal, what would be the change to the amount of information you have/accuracy of the conversion if the sampling time was small
If the sampling time is small this means that when converting to digital, you are taking lots of samples (you will have more information/curve will be more accurate).
What is the equation for sampling frequency
Fs = 1/sampling time (Ts)
Digital signals have a minimum voltage change they can detect - true or false
True - digital signals have finite resolution (unlike continuous signals which have an infinite range).
What number system does digital system use
Binary
What is the equation for how many voltage steps a ADC can detect
N = 2^M
N - number of voltage steps a ADC can detect
M = number of bits in the ADC
How do you know how many bits are in a digital wave
By looking at the numbers on the Y axis and seeing how many values there are (i.e 10 = 2, 100 = 3)
What is the equation for the minimum voltage change an ADC can convert
ΔV = Vref / N
V ref - maximum voltage change the ADC can measure
N = number of voltage steps the ADC can detect
What is V ref in regards to ADC
maximum voltage change the ADC can detect
How do you find the V ref when looking at a ADC wave graph
This will be the highest number on the Y axis
What two things is the accuracy of the ADC dependant on
Sample frequency and resolution (number of bits)
What must the sampling rate be of ADC in order to capture all the frequencies in the wave
The sampling rate must be 2 times larger then the largest frequency in the wave
Fs > 2B
What does it mean for a signal to be ‘completely determined’ in ADC
A signal can be classified ‘completely determined’ is there is no frequencies in the signal that are higher then the bandwidth of the signal (B)
This doesn’t mean that the samples alone are the full signal, it means that we can get the full signal back from the conversion if we need too (because we haven’t lost any frequencies).
What must the ADC sampling rate be is the analogue maximum frequency is 500Hz
Sampling rate must at least be 1000Hz
How can analogue filters be used to help in ADC
They can be used before digitisation to ensure the input analogue signal doesn’t contain frequencies above the Nyquist limit (if there were signals above the Nyquist limit then these wouldn’t be able to be digitalised by the ADC)
When are digital filters used in a circuit
These are used after an analogue signal is digitised to remove any irrelevant parts of the signal
What is the usual pass band of a filter used in ECG
150Hz
What is the ideal frequency response for a unity gain buffer
Gain of 1 or 0dB
What is the corner frequency
This is the point when the attenuation of a signal is 3dB
Explain how you would draw a high pass filter
Axis: Y axis Gain, X aixs Frequency (Hz)
The ‘stop band’ will begin at low numbers, have a transition period beginning with the ‘transition band’ then will plateau off creating the ‘pass band’ at higher numbers after the ‘corner frequency’
Explain how you would draw a low pass filter
Axis: Y axis Gain , X aixs Frequency (Hz)
The ‘pass band’ will begin at high numbers, have a transition period beginning with the ‘corner frequency’ then will plateau off creating the ‘stop band’ at lower numbers after the ‘stop band’
Explain how you would draw a band pass filter
This will look like a mountain
Axis: y axis = Gain. X axis = frequency (Hz)
The ‘pass band’ will be at the peak of the mountain.
Explain how you would draw a notch filter
This will look like a deep crypt
Axis: y axis = Gain. x axis = frequency (Hz)
The stop band will be at the opening of the crypt. Frequencies on either side of the stop band will be able to pass through the filter
What sort of filter is used in power supplies to remove the 50Hz AC frequency
Notch filter
What many coefficients/taps does the FIR filter have
n+1
e.g 5th order FIR has 6 taps/coefficents
How is the output calculated in FIR filters
Multiplying the latest and n previous values of the input signal
e.g for a 4th order FIR filter: you multiply the most previous input by the 4 most previous inputs (5 number multiplying in total)
How is the output calculated in a IIR filter
multiplying the latest and n previous values of the input signal (same as FIR) AND multiplying the n precious values by their corresponding coefficient
What filters, FIR or IIR contains feedback loops and what can this result in the stability of the filter
IIR contains feedback loops and this can result in it being unstable if it is not designed properly
How many inputs and outputs does a IIR filter have
n + 1 input coefficent
n output coefficents
If you put a single 1 surrounded by zeros into a FIR and IIR filter respectively, what would be the output
FIR - the output would be finite (usually 0)
IIR - output would be infinate due to feedback
What is phase response in a filter
The amount by which a signal component of different frequencies are shifted in phase at the output of the filter, compared to the input
Why is phase delay not ideal
Differing phase responses at different frequencies can distort the signal
What is the ideal shape of the phase delay in a filter
Linear - this will mean all frequency components are shifted in time by the same amount such that the relationship that existed across frequencies at the input of the filter remain the same at the output
What filter, FIR or IIR has a linear phase response
FIR
What is the group delay of a filter and where on the filter graph is this found
This is the slope of the phase response (steepness of the line) and this corresponds to the absolute time delay of the signal thorugh the filter
What will the phase delay be on FIR filters that have symmetric coefficients
Phase delay will be linear
What is pass band ripple
The measure of how flat the amplitude of the frequency response is in the pass band of the filter
FIR filters with symmetric coefficients have what type of group delay
Constant group delay
What is the equation for group delay in FIR filter
FIR group delay = (n taps - 1) / 2fs
Fs = sampling frequency
What Law/Equations covers the basics of fluid dynamics
Ohms law
P = QR
What two factors is blood flow in the arteries dependant on
The energy needed to drive the blood
The resistance to the flow present
A rise in blood flow is defined by what three energies
Pressure energy
Kinetic energy
Gravitational energy
What is the relative velocities of laminar and turbulent flow
Laminar flow - consistent
Turbulent flow - fluctuations in velocity
In regards to the Reynolds number, when does laminar flow occur
Laminar flow occurs when the velocity is below the Reynolds number
What is the Reynolds number and how is it calculated
Predicts the flow patterns under different conditions
Re = inertia/viscious
What is the equations for flow
F = velocity x cross-sectional area
What are Newtonian fluids
Newtonian fluids are a category of fluids that have a constant viscosity regardless of the shear rate or the force applied to them.
What law do Newtonian fluids follow
Newtons Law of viscocity
τ=η⋅ (dy/du)
T = shear stress applied to the fluid
n = viscosity of fluid
du/dy = shear rate
What is the result on the flow and viscosity of a Newtonian fluid when it is subjected to different forces/stresses
This means their flow behavior or viscosity does not change when subjected to different forces or stresses.
What does the Youngs Modulus explain
The ratio between stress and strain used to express the elasticity of a fluid
(how easily can a fluid stretch and compress)
What is the equation for Youngs Modulus
E= σ/ϵ
𝐸= Young’s modulus,
σ is stress,
ϵ is strain.
What is shear stress
Forces that act parallel to the surface of the material
What is shear strain
force acting on material when subjected to movement
In non-Newtonian fluids what occurs to the viscosity when it experiences shear stress and strain
Viscocity changes (unlike Newtonian fluids)
What type of fluid is blood
Pseudoplastic
What would be the relathionship of shear stress and shear rate of an ideal fluid
Shear stress and shear rate would be directly proportional (linear)
What is a pseudoplastic fluid
A pseudoplastic fluid is a type of non-Newtonian fluid characterized by a decrease in viscosity as the shear rate increases.
What is a dilatant fluid
A dilatant fluid is a type of non-Newtonian fluid characterized by an increase in viscosity as the shear rate increases.
What are the four characteristics of an ideal fluid
Zero viscosity
Incompressible
Infinite bulk modulus
No surface tension
What is the equation for velocity under pulsatile flow
c = √Eh/pd
What does LaPlace wall tension tell us
The pressure applied onto the wall is directly proportional to the tensional stress and inversely proportional to the radius of the wall
According to LaPlace wall tension, how much pressure can be applied to small vessels
A lot - the smaller the vessel, the larger the pressure that can be applied to it
What is critical radius of arteries
This is the point at which arteries have expanded so much that they become unstable
How would you estimate the critical radius of an artery
The critical radius is usually 2 times the original internal diameter of the artery
How foes flow spilt when it gets to a bifurcation
Q = Q1 + Q2
What two forces does flow experience when it flows along a bend
Viscous force
Centrifugal force
What does Bernoulli equation tell us
The equation states that the total mechanical energy of the fluid remains constant if the flow is steady and frictionless (non-viscous)
What 5 things must we assume for Bernoulli’s equation to be applicable
Flow must be stead
Fluid my be incompressible
Viscosity must be negligible
Flow must be uniform
Flow must be irrotational
What law explains the flow incompressible and Newtonian fluids (laminar flow)
Poiseuille’s equation
What are the two forces involved in the elastic recoil of the lung
Tissue forces from stretching (elastic elements)
Surface forces (surface tension)
What is transpulmonary pressure
The difference between the pressure inside the lung (alveolar pressure) and the pressure outside the lung (pleural pressure)
What is elastance
The resistance to change in shape when mechnical load is applied
What law describes movement of gas in the respiratory system
Ficks Law of diffusion
What is the equations for Ficks Law of diffusion
V = DAP/T
V = volume of gas
D = diffusion cooefficent
A - surface area
P = pressure gradient
T = thickness of barrier
What are some factors that affect diffusion of gas
Area of membrane
Thickness of membrane
Solubility of gas
Molecular weight of gas
Pressure difference
What are the two factors that make an ideal gas
Does not attract or repel each other
Takes up no space (has no volume)
How many gases are ideal gases
No gas is truly ideal
What equation can we use to provide a good approximation of real gas behaviour
PV = nRT
n = number of moles
R = universal gas constant
T = temperature in kelvins
Avogadro’s Law describes what
Equal volumes of gases at the same temperature and pressure contain the same number of molecules
What does Boyles Law state
The absolute pressure exerted by a given mass of an ideal gas is inversely proportional to its volume
P = 1/V
What does Charls Law state
At a constant pressure, the volume is directly proportional to temperature
What does Gay-Lussacs Law state
For a constant volume, the pressure is directly proportional to the temperature
What Law describes why respiratory volume values change in regard to room temperature
Charls Law
What does Daltons Law state
Total pressure of a gas is the sum of the individuals pressure of all the gas molecules in the container
What are the three ideal gas laws
Boyles Law
Charls Law
Gay-Lussacs Law
What two characteristics are the best to reveal the operational condition of the respiratory system
Flow rate and Pressure
What is the best way to measure the pressure difference in the respiratory system
Using a differential pressure sensor
How does the use of an oxygen mask relate to ohms law
By accelerating oxygen through a small nozzel, causing a drop in pressure that allows air to be draw in and mixed with oxygen in the body
When in a circuit are analogue filters used
These are used before ADC
What does a high pass analogue filter remove
These remove any offset and large slow drifting components of the signal (often referred to as a DC offset filter)
What do low pass analogue filters remove
Remove high frequencies and allow the analogue signal to meet the Nyquist sampling theorem (often referred to as an anti-aliasing filter)
What frequency, high or low, is able to pass through a capacitor like a wire
High frequency
What frequency, high or low, will not be able to pass through a capacitor and why
Low frequency - capaictors have high impendance/resistance to low frequency signals
What is the effect of low frequency signals when a capacitor is present
A capacitor will block low frequency signals
What is the corner frequency in regards to capacitors
This is where the resistance of the capacitor become relevant (the frequency of the wave is now at an appropriate value that is will begin to charge the capacitor)
Explain the basic layout of the low pass analogue filter using resistor and capacitor.
How does this differ in a high pass analogue filter
Resistor would be in-between Vin and Vout. Capacitor will be on a branch off this wire.
In a high pass filter, the C and R will be swapped
What is the role off of a single stage analogue filter
20dB per decade
What is a decade in the terms of analoug filters
A decade occurs when the frequency is multiplied by 10
What are higher order analoug filters
Filters that have more than one filter present
What type of filter has the flattest group delay
Bessel filter
What filter has the steepest role off and what is sacrificed in this filter to allow this
Chebyshev filter - these can have large levels of distortion
What filter attempts to have the best of both role off and group delay
Butterworth filter
Why can you not just put one filter after another in a circuit when making a higher order filter
Putting them one after another will affect the impedance of the components before it
How can we have multiple filters in one circuit without affecting the impedance
Unity gain buffers can be used
In higher order filters that contain unity gain buffers, what other electrical components do they contain
They will have a resistor and capacitor in between each unity gain buffer.
What is the difference between active and passive analoug filters
Active filters require op amps
Passive filters require inductors
What is the signal chain in machinery
Physical stimulus
Sensor/Tranducer
Analoug Filter/Amp
ACD
Digital Processing
Display
What equipment is especially prone to change in transfer fucntion over time
Mechanical sensors and actuators
What are the two types of calibration
Measurement of an artificial signal source or test load
Measurement of a biological control
What is the normal frequency of an ECG as seen on the rhythm strip
0.15-150Hz
What sort of amplifiers are used in EEG
Differential amps
Concentric EEG recording needle
- Where is the reference active electrode located in the needel
- What degree of field does it have
Reference electrode on cannula, active electrode inside needle
180 degree field
Monopolar EEG recording needle
- Where is the reference active electrode located in the needel
- What degree of field does it have
Active electrode inside needle, reference electrode external
360 degree field
Bipolar EEG recording needle
- Where is the reference active electrode located in the needel
Active and reference electrode in needle
What is Laplacian Montage
Measures potential of all the neighbouring electrodes around it and averages them
What is the filter bandwidth setting of an EEG usually
0.5-70Hz
What is the sampling rate in EEG usually
500
What is Amplitude Integrated EEG
This is a compressed display of a single pair of electrodes to monitor brain activity over long periods of time
What sort of electrical activity are we measuring in motor nerve conduction studies
Compound action potentials
Explain the basics of performing a motor nerve conduction study
Surface electrodes are placed on the muscle that we are stimulating
Stimulating TWO points (e.g elbow and wrist)
What direction is the cathode facing in motor nerve conduction studies (black)
Cathode (black) facing the surface electrodes (black to black)
Where are the recording electrodes put when conducting a sensory motor nerve conduction study
Recording electrodes put on two points of one finger (black one closest to palm)
Explain the flow of ion in cathodal stimulation during nerve conduction studies
In cathodal stimulation, anions (-) are discharged into the body as current flows from the cathode (-) through the tissue, and back to the anode (+)
Explain the flow of ion in anodal stimulation during nerve conduction studies
In anodal stimulation, cations (+) are discharged into the body as current flows from the anode (+), through the tissue, and back to the cathode (-)
What is the definition of latency in regards to nerve conduction studies
The duration between a stimulating point and where the response is detected by the recording electrode
Why must an evoked potential be extracted using signal averaging
Because evoked potentials are of very low amplitude
What are evoked potentials
Electrical manifestation of the brains response to an external source
What is signal averaging
A signal processing technique aimed to increase strength of a signal relative to noise
What three factors must there be for signal averaging to work
Signal must be repetitive
Noise must be random
Temporal position of signal must be known
What is the aim of signal averaging
To increase to signal to noise ratio (higher the better)
What are visual evoked potential studies used for
Used to investigate the integrity and function of the optic nerve
What are the two types of stimuli used in visual evoked potential studies
- Pattern reversal (elicits stimuli through rapid change in a given pattern)
- Flash stimulation
What method is used when placing electrodes for visual evoke potential studies
Queen Square Method
How many sweeps is used in visual potential studies
100-200
What does somatosensory evoked potential studies allow us to analyse
How the electrical potential travel through the somatosensory nerve via the spinal cord
How many sweeps is used in somatosensory evoked potential studies
700-1200
What are brain auditory evoked potential studies used to analyse
Acoustic neuromas
How many sweeps are used in brain auditory potential studies
1800-2000
What is quantitative sensory testing
Standardised non-invasive test designed to diagnose small fibre neuropathy
What three methods of quantitative sensory testing are used
Thermal threshold, pressure sensor, vibration
What do sudomotor function exams measure
The nerve that control sweating
What sort of pathophysiology can sudomotor function tests help us diagnose
Autonomic nervous system disorders, peripheral neuropathies, pain disorders
What is measured in sudomotor function exams
Electrochemical skin conductance (ECS)
What is magnetoencephalography
Maps the function of the brain via magnetic fields produced by the brain
The amplitude and dynamic behaviour of manometry recordings is dependant on what 3 things
Luminal contents
Degree of closure of distal sections of the lumen
Rate of contractions of the luminal wall
How does a air monometer measure changes in pressure
Changes in pressure inside the balloon once inflated allows a direct measure of luminal activity
What type of GI catheter uses MEMS system
Solid state catheter
What are two limitations of solid state manometers
Prone to baseline drift and sensor drop out
How does fibre optic catheters measure pressure
mechanical system that blocks the light as pressure decreases and can detect how much light has been blocked)
What GI catheters are best used to reach long and hard to get areas of the GI tract
Fibre-optic manometers
What is the role of impedance monitoring
Monitor luminal contents in the oesophogus
Where are the electrodes located on the catheter used for impedance monitoring
Located on the outer surface of the catheter so they can contact to the luminal wall/luminal contents
What is the relative difference in electrical activity of a bolus in relation to the mucosal wall of the GI tract
The bolus has more electrical activity compared to the mucosal wall
Impedance Monitoring GI can detect non-acidic reflux events, true or false
True
A pH of what in a pH GI test indicated reflux from the stomach
~4
What are barsostat studies used for in GI
Used to investigate sensory ano-rectal disorders
What are 3 emerging technologies in GI
Motility tracking systems - magnetic pill swallowed and makes 3D map of GI tract
Small bowel high resoolution monometry
Functional Luminal Imaging Probe (FLIP)
What is the Power Supply Rejection Ratio (PSRR)
How well a circuit is able to supress vaiations in the power supply so that it doesnt effect the circuits performance
What is Common Mode rejection (CMRR)
The amplifiyers abilityto reject amplyfying signals that are common in both signals and only amplyfying the desired differential signal
Signal to noise ratio
- What is it
- What is the ideal SNR
The level of a desired signal to the level of background noise
Ratio higher than 1:! indicates more signal than noise (bigger the better)
In what environment are differential amplifiers good in and how does it achieve this
Good in noisy environments - it filters out noise by working on the principle that the unwanted electrical noise couples equally on both input terminals of the amplifier so therefore be rejected
What are capacitors good a filtering
They remove low frequency’s and noise and block DC signals and allow only high freuqnecy signals such as AC signals to pass through
What is the difference between an inverting and non-inverting amplifier
Inverting - Ground is connected to the + input
Non-inverting - Ground is connected to the - input
What is offset voltage
The very small amount of difference between the 2 inputs in a op-amp that is present in real life situations
What terminal is the negative feedback connected to in an op-amp
negative
What is the ideal difference between the + and - input terminals in an op-amp
0
What are the 3 main components of an op amp
Infinite gain
Infinite input impedance (no current)
Infinite output voltage
When a capacitor and resistor are in parallel, what is their relationship to voltage
They must always have the same voltage
What is the effect of Vout is the capacitance of a capacitor increase
V out deceases because the capacitor is eating more of its charge
What happens to the decibel calculation when the circuit attenuates a voltage
It becomes negative
What is gain dependant on
the frequency at which it was measured
What is the SI unit for inductance
L - henreys
How does a capacitor filter
By using the stored charge it is able to release voltage during the negative deflection of the AC wave, thus filtering out the drops in voltage
Where in the clinical setting are capacitive sensors/transducers used
Intracranial pressure monitoring/intraocular pressure measurements
What 3 things can be used to measure the uterine fundus in 2nd and 3rd trimester
Tape measure
Obsetric Calipers
Palvimerter
At what age gestation is a heart beat developed and at what age gestation can you hear it through transabdominal fetoscope
Heart beat developed ~41 days
You can hear it ~18 weeks
What are 3 artefacts that make foetal ECG difficult
Maternal heart beat
Foetal movement
Maternal movement
What equipment is able to detect the foetal heartbeat at ~10-12 weeks
Ultrasonic doppler
What is foetal carrdiotocography (CTG)
Ultrasound real time recordings of the foetal heart rate and uterine activity
What are the indirect and direct methods of foetal cardiotocography (CTG)
Indirect - sensors places on the maternal abdomen
Direct - sensors places on the foetus and within the uterine cavity
What change in heart rate during active labour is a sign of foetal wellbeing
An increase in heart rate from baseline
What is a common change in foetal heart rate during a contraction
Abrupt decrease in heart rate from baseline
What changes in foetal heart rate may indicate foetal distress
When the heart rate returns to baseline after the contractions end
What is the direct method of foetal electrocardiogram
Biopotential electrodes are places through the cervix on the foetal presenting part during labour (usually the head)
What part of the foetal ECG are difficult to detect, and how do we use this to calculate foetal heart rate
Only the R waves can be detected as the rest of the ECG components are buried in noise
Heart rate is calculated by 6000/R-R interval (mm / min)
What is the indirect method of measuring foetal ECG
Using doppler ultrasound - these usually detect the pulsatile movement of RBCs and/or opening and closing of the valves
What are some major differences between adults and neonates that we must consider when treating them
Size
Mobility
Ability to communicate
Anxiety factors
Body proportions
Different physiological functions
What are 5 limitations/challenges of working in the NICU/with neonates
- Heat produced from equipment is more likley to damage their skin
- Sentivie skin
- Electrodes producing pressure sores
- Prone to infection - must be more vigilent with disinfecting devices
- Warm environment makes it difficult to maintain impedance levels
What are 4 limitations to working in the warm environment of the NICU in regards to maintaining device impedance
- Increase skin moisture can reduce impedance
- Fluctuating temperature in the NICU can effect skin temperature/blood flow which can effect impedance
- Adhesives on electrodes may not stick as well in warm/humid conditions
- Device sensitivity to noise - ICE contains lots of devices such as incubators, humidifiers that create a lot of electrical noise
What is used to measure growth (height and weight) of infants/neonates
Stadiometer
How is babies breathing pattern measured at home
Apnoea machine using thoracic impedance
How do electrodes differ for neonates compared to adults
- Electrodes are smaller
- Adhesive and electrolyte paste are less irritating (weaker adhesive and low Cl-)
- Signal processing takes into account the normal high heart rates of neonates
What type of apneoa does transthoracic impedance monitoring in infants measures
Central apneoa only
How is blood pressure usually measured in NICU
Direct catheter put into the umbilical arteries
What are the two methods that blood gas monitoring can happening in neonates
- Drawing arterial/venous blood samples and analysing them in a blood gas analyser
- Continuous blood gas monitoring - catheter with electrochemical or optical sensors continuously analyses blood gases
What is PO₂ amperometry
technique used for measuring the partial pressure of oxygen in a gas or liquid phase.
How is PO2 Amperometry measured
Blood diffuses through an O2 permeable membrane - the greater the O2, the greater partial pressure, the higher the requirement for electrons to reduce it. The current at the cathode is directly related to the partial pressure of oxygen in the solution
how is blood pH measured in neonates
Measured directly from the tip of a catheter. Electrochemical sensor makes a poteniometric measurements
How is PCO2 measured in neonates
Measured electrochemically sensing the change in H+ as this is proportionate to the concentration of PCO2
Why is continuous EEG monitoring of neonates particularly important
Risk of seizures is greatest in the first 3 days of life - seizures in neonates are very subtle and cannot always be seen without an EEG
What type of EEG is used in the NICU and why
Amplitude Integrated EEG
- Only requires 1 pair of electrodes
High amplitude (for the smaller brain waves)
What percentage of time during a sleep study is the oxygen saturation allowed to drop below 92% in:
1 month old
3-4 month old
>2 year old
1 month - <82%
3-4 month old - <25%
2 year old - <15%
During a sleep study, clusters of oxygen desaturation are frequent or not frequent in:
1 month old
3-4 month old
>2 year old
1 month old - frequent
3-4 month old - frequent
>2 year old - <2
What structure in foetal development further develops to form the placenta
Chorion endometrium
What type of blood flows thorugh the umbilical veins and arteries, respectfully
Umbilical veins - oxygenated blood from placenta goes to foetus
Umbilical arteries - deoxygenated blood from foetus flows to placenta for excretion
What structure located on the foetal placenta portion is vital to gas and nutrient exchange from foetus to mother
Villi - these are surrounded by the maternal portion of the placenta where maternal blood from uterine arteries supplies the foetus and the uterine veins bring the foetus waste products back
What mechanical process moves oxygen from the
