In which of the following types of A/D converter does the conversion time almost double for every bit added to the device?

The maximum conversion time in counter type A/D converter is given as:

Counter type ADC: (2n – 1) Tclk≈ 2ⁿ T_clk

So, for every bit added conversion time doubles.

For n-bit conversion, the conversion time for different ADC are: 

Successive approx. time ADC: n Tclk

Flash type ADC: Tclk

Dual slope ADC: (2n+1 – 1) Tclk

The successive approximation A/D converter has a shorter conversion time compared to the counter ramp A/D converter, i.e the correct sequence will be: TD < TC< TA< TB

• Counter type ADC and successive approximate ADC uses DAC
• Counter type ADC uses linear search and successive approximation type ADC uses binary search
• A ring counter is used in successive approximation type ADC
• Flash type ADC is fastest ADC
• Flash type ADC requires no counter
• For an n-bit ADC, flash type ADC requires (2n – 1) comparators
• Dual slope ADC is most accurate