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GATE 2015 EE – Question 50

Analog and Digital Electronics · Schmitt triggers, Sample and hold circuits, A/D and D/A converters · 2 marks · Multiple choice

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

  1. 1010 0000
  2. 1000 0000
  3. 0000 0001
  4. 0000 0011

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Show answer and explanation

Correct answer: (A) 1010 0000

Explanation

One step is $\frac{5}{256}$ V. The register tries the top bit first: $10000000$ is 2.5 V, which is below 3.5 V, so the bit stays. Next $11000000$ is 3.75 V, which is above 3.5 V, so that bit is cleared. Next $10100000$ is 3.125 V, which is below 3.5 V, so it stays. After the third clock the output is $1010\ 0000$.