Question

An $8-$bit, unipolar Successive Approximation Register type $ADC$ is used to convert $3.5V$ to digital equivalent output. The reference voltage is $+5V$. The output of the $ADC$ at the end of $3rd$ clock pulse after the start of conversion, is

• A. $00000011$
• B. $10100000$
• C. $10000000$
• D. $00000001$

Answer 1

The correct option is B $10100000$

The reference voltage is $5V$
The number of bits in $ADC$ are $8$.
So, the resolution will be $=\frac{5}{2^8-1}=\frac{5}{255}$
The applied input is $3.5V$
The successive approximation $ADC$ start working from the MSB so,
After one clock:
$SAR$ will toggle it's $MSB$ from $0\to 1$ so output of $SAR$ will be $10000000$.
After second clock:
$SAR$ will toggle its $7th$ bit from $0\to 1$ but $11000000$ wil result in value greater than $3.5$ so output of (SAR\) after $2nd$ clock will be $10000000$
After third clock
$SAR$ will toggle it's $6th$ bit from $0\to 1$ and output will be $10100000$