Digital Componnents - ADC And DAC Flashcards
(19 cards)
The ASCII letter S is 1010011, show the parity bit for the letter S with odd and even parity
Even: 01010011
Odd: 11010011
The parity bit is added at the front of the binary code
Explain how the ADC staircase converter works?
- A counter counts whilst the analog input is less than the input signal
- When the output of the DAC is equal to or greater than the input signal, the clock stops counting and the binary code is read
- The counter increases with each clock cycle
In the ADC staircase converter, does it use a DAC or ADC converter to provide a comparison voltage?
DAC, Digital to Analog
What is a disadvantage of the ADC staircase converter?
Method is slow, Tavares 2^N clock cycles for N bit.
E.g. for 4 bits it will take 16 clock cycles for the ADC to complete the task
What is the initial value of the DAC comparator in the ADC staircase converter?
0
Explain how the ADC successive approximation converter (SAC) works?
- The MSB is set to high, all other bits are low. Is the value greater than my input signal? If yes, MSB is kept high and we move onto the next bit. If not, the MSB is set to LOW and the next bit is checked
Does the SAC (Shift Approximation Converter) also use a DAC comparator?
Yes
What are the 3 components of a SAC ( Successive Approximation Converter)
- Comparator
- DAC (the comparator will look at the DAC value, we call this a DAC comparator)
- Shift approximation register, to actually shift to the next bit to set to HIGH or LOW
Explain how the ADC Flash converter works
- In a circuit, a bunch of comparators are set up at specific voltages so that when the input voltage is equal to or greater than that voltage level, the comparator lights up
E.g. if 5 comparators are used, each one has a voltage level for an input signal between 0-1V:
Comparitor 1: 0.2V
….0.4V, 0.6V, 0.8V, 0.9V (for example)
When the input voltage is 0.7V, the first 3 comparitors will LIGHT UP and say ‘Yes the input voltage is equal to or greater than my voltage level’
- A logic circuit gets this data, and understand that the input voltage lies between 0.6V and 0.8V and will produce a binary (digital) number accordingly
What are the 3 types of ADCs?
- Staircase converter
- Successive Approximation Converter
- Flash Converter
What is the pro and con of the ADC Flash converter?
- Works instantly since all comparitors work at the same time
- For N number of bits you need 2^N-1 comparitors!
SO for 16 bits, you would need 65,535 comparators
When would you use a Flash Converter?
When speed is more important than costs
Explain how the DAC Binary Weighted works?
- Each bit is assigned to a resistor, current flows in the resistors if the bit is 1, current won’t flow through resistor if bit is 0
- The MSB has the most control, as it is always connected to the resistor with the lowest resistance. The LSB has the least impact, as it is always connected to the resistor of highest resistance. See page 108
What is the formula for Vout in a Binary Weighted DAC?
Vout = -Vref * ( (Rf/R) + (Rf/2R) + (Rf/4R) + (Rf/8R)….etc)
Or
Vout = If(current) * Rf(resistance). See page 108
What is the equation for step size?
Step site = total input / (2^N -1)
N = number of bits
3 Cons of the Binary Weighted DAC?
- Voltage levels must be exactly the same for each input
- Resistors must be insanely accurate! (Very low tolerance)
- Difficult and Expensive
How would you represent 4.3V in a 7V system using 3 bits?
- Find step size = 7 / (2^3 - 1)
Step size = 1V
- 4.3V is roughly 4V so 010
Go to page 111 and solve the DAC question
Vout =
Pro of the SAC?
Fixed conversion time
