GATE 2025 IN – Question 43

Digital Electronics · ADC and DAC · 2 marks · Multiple choice

A 3-bit DAC uses an ideal opamp and switches, each closed when its corresponding digital input is 1, as shown. For input 110, the resistance $R_{in}$ seen from the reference source and current $I$ are

Diagram for GATE 2025 IN question 43

$R_{in}=2$ kΩ and $I=3$ mA
$R_{in}=12$ kΩ and $I=0.5$ mA
$R_{in}=\infty$ Ω and $I=1$ mA
$R_{in}=\infty$ Ω and $I=3$ mA

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Correct answer: (A) $R_{in}=2$ kΩ and $I=3$ mA

Explanation

D2 and D1 close while D0 remains open.

The active 3 kΩ and 6 kΩ branches terminate at virtual ground, so Rin=3k||6k=2 kΩ.

Their currents are 6/3000+6/6000=3 mA, which flows through feedback.

Thus **A**.