An $8$ bit successive approximation analog to digital converter has full scale reading of $2.55 \mathrm{~V}$ and its conversion time for an analog input of $1 \mathrm{~V}$ is $20 \; \mu \mathrm{s}$. The conversion time for a $2 \mathrm{~V}$ input will be
- $10 \; \mu \mathrm{s}$
- $20 \; \mu \mathrm{s}$
- $40 \; \mu \mathrm{s}$
- $50 \; \mu \mathrm{S}$