Term 1 Test Flashcards
Definition of: Transducer
Sensor
Signal Conditioning
T: Converts energy from one form to another
S: device (typically transducer) that is used to convert various physical quantities to electrical signals
SC: collection of circuit functions that process a signal in order to change one or more properties (amplitude, frequency etc)
Examples of signal conditioning functions-7 in total
A Bat Flew Down Into BUs
and then Vanished
Amplification/Attenuation Buffering/Impedance Filtering DC offset correction Isolation Balenced to Unbalenced Conversion Voltage/Current conversion
Amplification/Attenutation: What do each do What kind of circuit is needed to do this What is amplification often used for What is attenuation often used for
Increasing/decreasing the power of a signal
An active circuit is needed
Amping up signals from sensors that produce weak outputs
Reducing signals that are too large to be processed properly
Buffering/Impedance matching:
What are they
When are they used
Buffer: op amp where input impedance is very high, output impedance is very low and gain is unity.
Prevents one stage from loading the next
Impedance matching: using an op amp to present a specified input/output impedance
Needed when two stages in a circuit need different impedances
Filtering: just definition
Seperation of signals on the basis of their frequency content
DC offset correction: what does it do
Example of when it is useful
Adding or removing DC offset from an AC signal
When using an ADC converter as input voltage must be completely positive
Isolation: what is it
Why would you use it
Isolating a signal electrically
ALthough there is an output signal there is no electrical current path between input and output
For safety or to protect components from damaging voltage
Balanced to Unbalanced Conversion: how does it work
Converts two conductors to a differential system
Voltage/Current conversion:
whys this useful
How to do it for large currents
Many sensors produce a current as output, but its generally more convenient to process the signal as a voltage
Pass the current through a low resistance
Circuit diagram vs system diagram
CD: contains topology of circuit component connections, drawn schematically
SD: concerned with relationships between functional blocks within a system
System concept diagram vs system module diagram
SCD: not as detailed, tells you how a system operates
SMD: detailed, tells you how real sub-system parts are connected (included signal types, power levels etc)
Op-amps:
Vout equation
Ideal op amp characteristics
Vout = A(V+ - V-) A = gain V+/V- = non-inverting and inverting inputs
A is infinite
Input impedance is infinite
Output impedance is zero
Non-inverting op amp:
What does it look like
Whats the gain equation
Vin = V+
V- : potential dividor between Vout and ground
G = 1 + R2/R1
R2 is top resistor, R1 is bottom
Inverting op amp:
What does it look like
Whats the gain equation
V+ = ground
V- = Vin through a resistor (R1)
R2 goes between Vout and between R1 and V-
G = -R2/R1
Op-amp limitations: input and output limits
Input limits: there is a max voltage difference that can be applied between V+ and V-
Output limit: output voltage cannot exceed power supply voltages
Op-amp limitations: unwanted phase shift
Each RC network in an op amp can cause unwanted phase shift.
Each network has 6dB/octave or 20dB/decade roll-off and -90 phase shift
If total phase shift is 180, we get positive feedback
Op-amp limitations: how to solve unwanted phase shift
Theres a purpose built rc circuit that rolls off sooner to have a more controlled roll off and max of 90 phase shift
Op-amps: removing DC components
Use capacitor before first resistor and the formula F = 1/(2πCR)
Op-amps: impedance matching for non-inverting op amp (Vin = V+)
Input: resister in parallel with Vin, Sets impedance
Output: Resistor in series
Remember about potential dividor
Op-amps: impedance matching for inverting op amp (V+ = ground)
Input: R1 = input impedance
Output: Resistor in series
Remember about potential dividor
Op amps: creating a buffer with non-inverting and inverting op amps
Non-inverting: connect Vout to V- without any resistors
Inverting: set R2 = R1
Op amps: get attenuation
Use a potential dividor into a buffer to avoid high output impedance
What is slew rate?
Whats the value of slew rate needed for undistorted output?
How quickly an op-amp can change its output voltage
Aω (V/s)
A = peak amplitude
ω = frequency
Biasing input currents:
Why is this needed?
How do you do it?
What do you need to watch out for>
As op amp input impedance isn’t infinite so a small current will flow.
If you have a capacitor at the input, it will accumulate charge.
To solve this place a resistor in parallel going to ground.
Watch out-this forms a high pass filter
Input offset voltage:
Why does it happen?
How do you fix it
An ideal op amp shouldn’t output voltage if the difference between two inputs is the same.
However in practise there is a small internal offset voltage
Most op-amps include a potentiometer to compensate for it
Common-Mode Rejection Ratio equations-2 types and last one in μV/V and dB
CMRR = 20*log(Adiff/Acom) (db) Adiff = differential voltage gain Acom = common-mode voltage gain CMRR = ΔVdiff/ΔVcom CMRR = -20*log(ΔVdiff/ΔVcom) V = voltage signals required to obtain the same change in output voltage
Power Supply Rejection Raito:
What is it
Two equations for it, in μV/V and dB
The extent to which the op-amp rejects fluctuations on the power supply rails
PSRR = ΔVdiff/ΔVPSU (μV/V)
PSRR = -20*log(ΔVdiff/ΔVPSU)
5 things to consider when choosing op-amps
General purpose (price, easy to use) High speed (high frequency) Precision (low offsets, high CMRR) Low power (low standby power consumption) Low noise
How to create a Wheatstone Bridge circuit to get two differential voltages when using a sensor
Diamond shape-Vsupply at top and ground at bottom
Two resistors on top two sides
Balance resistor on one side and sensor on the other
Voltage points are on the points on the left and right
Filter characteristics for n-pole filters-4 types
BBCE
What do the graphs look like
Bessel: Flat start, shallow roll off
Butterworth: flat start, less shallow roll-off
Chebyshev: has wavy start, steep roll-off
Elliptic: has wavy start and finish, steepest roll off
Low pass and high pass circuits-both for RC and LC circuits
Low-pass: RC and LC
High-pass: CR and CL
What does a Sallen-Key filter look like?
Use low pass for example
How to do a high pass?
Combination of inverting and non-inverting op-amps.
Potential dividors into V- (RA/B)
Resistors in series into V+ (R1/2)
Capacitor 1 between Vout and between R1 and R2
C2 goes to ground from between R2 and V+
Swap R1 and C1 etc for high pass
Designing a Sallen-Key filter
What values to choose?
What formulas to choose for butterworth and Chebyshev (RC = …)
R1 = R2 C1 = C2 Use RA and RB in the non-inverting formula K = 1 + RA/RB And get K from the table B: RC = 1/ω0 C: RC = 1/ω0*Cn Cn = normalising factor
Using a op-amp for current to voltage conversion
Whats the formula for Vout for both types?
Inverting Op amp except instead of the first resistor you put a diode facing either away from ground or towards a voltage rail
Ground: Vout = RId
Rail: Vout = -RId
Schmitt trigger:
What does it do?
What does it look like?
How to solve?
Detects when the input signal has crossed a certain threshold
Vin = V-
Vout connects to a potential dividor through resistor 3.
V+ = the gap between R3 and the potential dividor
Ask Henry
Peak detector:
What does it do?
What does it look like?
Detects max amplitude or peak of a AC wave
Vin = V+
Vout goes through a diode before splitting into 3.
One goes to V-
One goes to ground through a capacitor
One goes to Vout
For an n-bit ADC how many levels does it have?
What is sampling?
What is quantisation?
2^n levels
S: Recording the amplitude of a signal at specifc
Q: asigning a sample to one of a fixed number of amplitude values
ADC definitions:
What is Full Scale Value
What is Full Scale Range
What is resolution
FSV: High quantisation level
FSR: difference between highest and lowest quantisation levels
Resolution: number of different quantisation levels we have
Using ADC’s with microcontrollers: 3 things to remember
- ADC range is always positive
- FSV determined by MC max voltage input
- Most microcontrollers have a maximum source resistance
