Question

The number of comparators required in an 8-bit flash-type A/D converter is

• A. 8
• B. 7
• C. 256
• D. 255

Hint:

A Flash ADC has $2^N - 1$ comparators and $2^N$ resistors.
For an 8-bit converter, this means 255 comparators and 256 resistors are required.

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
The question asks for the exact number of analog comparators needed to implement an 8-bit Flash (simultaneous) Analog-to-Digital Converter (ADC).

Step 2: Key Formula or Approach:
For an $N$-bit Flash-type ADC:
- Number of comparators required:
\[ N_{comp} = 2^N - 1 \]
- Number of resistors required in the reference ladder:
\[ N_{res} = 2^N \]

Step 3: Detailed Explanation:

• Given resolution: $N = 8\text{ bits}$.

• Calculate the number of comparators needed using the formula:
\[ N_{comp} = 2^8 - 1 \]
\[ N_{comp} = 256 - 1 = 255 \]

• A Flash ADC is extremely fast because all these 255 comparators work in parallel to sample the input voltage simultaneously.

• However, as the number of bits increases, the number of comparators grows exponentially, making high-resolution Flash ADCs bulky and expensive.

Step 4: Final Answer:
The number of comparators required is $255$, which corresponds to Option (D).