Digital/Analogue Signals & Data Acquisition Flashcards
What is a single ended signal?
A single ended signal is a digital signal that is referenced to the ground.
What can digital single ended signals be subjected to?
They can be subject to interference which can cause spikes.
Spikes can be wrongly interpreted as pulses which can cause a system to lose the correct count.
Describe Line Driver Digital Output.
A line driver is used to avoid spikes caused by interference.
Two signals are produced with complimentary lines.
One line goes up and the other goes down.
Both lines pick up the same spike due to common mode interference.
As a result the common mode interference is rejected.
What is Optical Isolation?
The sharing of information without physical contact.
For example between a plant and a computer via the use of an LED and a photo-transistor.
The photo-transistor acts as an open collector. When it conducts, it shorts output to the ground, otherwise it is pulled up to Vcc by the resistor.
What are the different types of signal source?
Grounded - signal referenced to a system ground.
Floating - signal not referenced to a system ground.
What is the main enemy to resolution?
Interference. For example mains hum.
What is the problem of using a differential signal to remove interference?
N channels of input can only be used for N/2 differential signals.
This can be overcome by using a non-referenced single ended input, which allows all inputs to be referenced to a single value of interference.
What is Aliasing?
The misrepresentation of a signal.
What is Nyquist Theorem?
One must sample a signal at greater than 2x the frequency of the signal to accurately represent the frequency of the signal.
One must sample a signal at greater than 5x the frequency of the signal to accurately represent the shape of the signal.
What is the Nyquist Frequency?
Half the sampling frequency.
One will only get a proper representation of signals that are <= Nyquist Frequency.
Signals above the Nyquist Frequency will Alias.
What is the equation for Aliasing?
Alias Freq = |(Closest integer multiple of sampling freq - signal freq)|
Explain how to remove Alias.
Filter out any frequency higher than the Nyquist Frequency.
Use a faster data capture system (a better ADC).
What is Multiplexing?
Using switches to connect a single ADC to multiple sources.
Allows for simultaneous signal sampling.
