Difference between ADC types-counter,flash,SAR,dual slope
This page covers difference between various ADC types including block diagram, equation etc. It covers counter type ADC,parallel comparator (Flash) ADC, SAR(Successive Approximation Register) type ADC, Sigma Delta ADC,dual slope integrating type ADC etc.It mentions advantages and disadvantages of them.
As we know ADC is the short form of A to D Converter.
ADC converts analog information into digital information.
There are total three steps required for A to D conversion viz.
sampling, quantization and coding.
Example: Cell phone converts analog voice into digital data, process it and re-converts the same into analog form for retransmission into the air.
counter type ADC
Figure-1 depicts block diagram of counter type ADC. It uses counter and DAC. It compares DAC output with analog voltage and does the same till both are equal in magnitude. At this moment counter will stop.
Maximum conversion time is equal to (2n -1 )* TCLK for n-bit of ADC. The conversion time depends on analog input voltage.
parallel comparator (Flash) ADC
Figure-2 depicts block diagram of parallel comparator ADC or 2 bit flash ADC. It is the fastest ADC among all the ADC types. In flash ADC of n-bit in size, (2n - 1 ) comparators and 2n registers are needed.
Each comparator compares Vin to a different reference voltage, starting with Vref = 1/2 (lsb). Op-amp is used as comparator here.
Advantages: It is very fast as mentioned.
It requires many parts for example, 255 comparators are required for 8-bit ADC. Lower resolution, Expensive, large power consumption etc.
SAR(Successive Approximation Register) type ADC
Figure-3 depicts block diagram of SAR type ADC. SAR is the short form of Successive Approximation Register. SAR type ADC is mostly used in digital circuit to provide interface with the microprocessor.
In SAR type of ADC, conversion time is uniform for any analog voltage and it is equal to n*TCLK.
Advantages: It is capable of high speed.
It has medium accuracy compare to other ADC types.
Disadvantages: High resolution type of SAR ADC will be slower. Speed is limited to 5 MBPS.
Sigma Delta ADC
Figure-4 depicts block diagram of Sigma Delta ADC. Serial Output is proportional to analog input voltage.
It has high resolution.
It does not require any external components.
Disadvantages: It is slow in conversion due to oversampling.
dual slope integrating type ADC
Figure-5 depicts block diagram of Dual Slope Integrating type ADC.
It is used in the design of digital voltmeter.
For n bit dual slop type of ADC,
Vr = ( 2n/N ) * Va
Total time for conversion of input Va is expressed as follows:
Total Time = (2n + N)* TCLK
Advantages: It is more accurate ADC type among all.
It has greater noise immunity compare to other ADC types.
Disadvantages: It is the slowest ADC among all. In order to achieve better accuracy, it requires high precision external components.
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