Lecture 7: Analogue to Digital Converter Flashcards
What is an ADC?
It will take an analogue input and output a digital number that is a representation of the voltage of the input.
Why do we need an ADC?
Embedded systems only work in digital numbers but the environment is all about analogue.
What is sampling?
It is the process that allows us to convert from continuous time to discrete time.
What is Quantization?
It is the process of replacing real values with an approximation from the finite set of discrete values.
How many discrete values are there?
It is equal to the number of Quantization levels.
How many Quantization levels are there?
For a N bit ADC, there will be 2^N levels.
What is the resolution of our ADC?
Range / 2^N
What is the range of our ADC?
Normally it is the range of reference voltages that we are given.
If we get given Vin = Vref * ADC / 1024 then what is our resolution and what bit is the ADC?
Resolution will be Vref / 2^10
Therefore it is a 10 bit adc.
How much will our digital value be incremented on the ADC output>
Every increment will be the resolution size.
What is successive approximation?
It is the method used by the ATMega8. It will start at the MSB and go down to the LSB. It uses a DAC to generate a voltage that the input signal is compared against. Based on the compare result, the bit is decided to be either a 0 or a 1. It will have a constant conversion time.
What is integration as a method of ADC?
A reference signal and an input signal will be constantly integrated until both values match. The time taken for this to occur is then converted to the digital representation of the signal.
Varying conversion time depending on the size of the input
What is counter based conversion?
Uses counter inputs to a DAC. When the DAC is larger than the input signal then the counter will stop and the value is stored. This has a varying conversion time depending on the size of the input signal.
What is parallel conversion?
Have a high number of comparators to simultaneously compare the input voltage. It is the fastest method but has a high cost.
What is the size of the ATMega8 ADC?
10 bit
What is the size of the ADC multiplexer that the ADC is connected to ?
8 bit.
How do we enable the ADC?
Set the bit in the ADCSRA register.
How long does a normal conversion take?
13 clock cycles.
How long does the first ADC conversion take?
25 clock cycles. This is because it has to set up the analogue circuitry.
What is the formula for conversion time?
Clock cycles required / Clock frequency
