Binary-weighted DAC (inverting) output voltage A binary-weighted DAC has a feedback resistor Rf = 12 kΩ. If 50 µA flows through Rf into the op-amp summing junction, what is the output voltage?
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A0.6 V
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B–0.6 V
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C0.1 V
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D–0.1 V
Answer
Correct Answer: –0.6 V
Explanation
Introduction / Context:Many DACs use an inverting summing amplifier. The output voltage is set by the current entering the summing node and the feedback resistor value.
Given Data / Assumptions:
- Rf = 12 kΩ.
- Summing-node current I = 50 µA flowing through Rf.
- Ideal op-amp assumption with virtual ground at the summing node.
Concept / Approach:For an inverting configuration, Vout = −I * Rf. The sign is negative because current into the summing node produces an output that drives the inverting input to virtual ground.
Step-by-Step Solution:Convert units: 50 µA = 50 × 10^−6 A.Compute magnitude: V = I * R = 50 × 10^−6 * 12,000 = 0.6 V.Apply sign: Vout = −0.6 V.
Verification / Alternative check:Dimensional check: amperes × ohms = volts; 50e−6 × 1.2e4 = 0.6 confirms the value.
Why Other Options Are Wrong:0.6 V: wrong sign for inverting mode.±0.1 V: inconsistent with I * R calculation.
Common Pitfalls:Forgetting the negative sign of inverting amplifiers or mixing µA and mA.
Final Answer:–0.6 V