Chapter 5: ADC Types Flashcards
The types of ADC?
- Ramp
- Successive approximation
- Flash/Parrallel
What makes up a comparator?
A comparator consists of a differential amplifier and compares two voltages and outputs a digital signal, either 1 or 0, indicating which is larger.
Main differences & similarities between ADCs?
differences mainly being the speed of operation and cost.
All those considered here use comparators to test whether the analogue signal is higher or lower than a test voltage
How does a Ramp ADC function?
In a ramp ADC, the test voltage starts at zero and a timer is started. It is then increased over time in equal steps. At some point the comparator will change state when the test voltage becomes greater than the analogue input. When this happens the timer is stopped, and the value on the timer can be related to the value of the input voltage.
Limitations of ramp ADC?
The ramp generator and counter are then cleared and the process repeated to sample the next input voltage.
The fastest that this ADC can operate is one resolution step per clock cycle. Therefore, for an N-bit resolution, each measurement can take 2N clock cycles
Graph for ramp ADC?
What is a successive approximation ADC?
A successive approximation ADC is similar to a ramp ADC but uses a different sequence of test voltages to try. Remember that the purpose is to generate a binary number representing the value of the input voltage
How does a successive approximation ADC create a binary representation of an analog signal?
This type of ADC works out one bit at a time, starting with the most significant bit. So the first test voltage will be in the middle of the full voltage range.
If the analogue voltage is above this, then the MSB will be 1, otherwise it will be 0. Testing the next bit splits the available voltage space again into two, with the bit determined by whether it is above or below the dividing line.
Continuing with all the bits results in an increasingly accurate value for the actual voltage
The graph for Successive approximations ADC?
What is a flash ADC?
A flash ADC compares the input signal with all possible analogue levels for that converter at the same time.
How does the flash ADC circuit function?
it consists of a huge number of comparators and long chain of precision resistors to create all the different test voltages between positive and negative reference voltages. A set of combinatorial logic then combines the outputs of the comparators to create the digital output signal
Advantages and disadvantages of flash ADC converters
Since all the 2 N comparisons in performed in parallel, this type of ADC is very fast. However, it is big and expensive.
Things to remember when conducting ADC?
Converting to and from analogue signals requires careful consideration of the signal being converted and potential error sources. A large number of bits do not always give high precision. One bit is often enough using PWM or Delta Sigma and has significant power advantages
