Question

An 8-bit, unipolar successive approximation Register type ADC is used to convert 3.5 V to digital equivalent output. The reference voltage is +5V. The output of the ADC at the end of third clock pulse after start of conversion, is
1. 1010 0000
2. 1000 0000
3. 0000 0001
4. 0000 0011

Answer 1

Correct Answer - Option 1 : 1010 0000

Concept:

A successive-approximation ADC is a type of analog-to-digital converter that converts a continuous analog waveform into a discrete digital representation using a binary search through all possible quantization levels before finally converging upon a digital output for each analog voltage conversion

If V_in  ≥ V_DAC ⇒ maintain a loaded bit

V_in ≤  V_DAC⇒ clear the loaded bit

Calculation:

Given,

Reference voltage V_R = 5 V, number of bits n is 8

Resolution \(= \frac{5}{{{2^8} - 1}} \approx 20\ mv\)

Clock pulse 1:

1 st clock = 10000000

Output voltage V₀ = 20 mV × 2⁷ = 2.5 V

As 3.5 V > 2.5 V it maintains the loaded bit

Clock pulse 2:

2 nd clock pulse = 11000000

Output voltage V₀ = 20 mV × (2⁷ + 2⁶) = 3.75 V

As 3.5V < 3.75 V it clears the loaded bit

Clock pulse 3:

So at the end of the 3^rd clock pulse output is = (10100000)₂