Op-amp applications – can a resistor-weighted summing operational amplifier be configured to implement a digital-to-analog converter (DAC) by converting digital input lines into a proportional analog voltage?

Introduction / Context: Digital-to-analog conversion can be realized using resistor networks and an operational amplifier. The widely taught approach is a binary-weighted or R–2R ladder network feeding a summing node at the inverting input of an op-amp. This question checks whether a summing op-amp can serve as the core of a DAC stage.

Given Data / Assumptions:

• Binary input lines are available (e.g., D0…Dn) with known logic levels.
• Resistors are chosen to produce binary-weighted currents at the summing node.
• The op-amp operates within its linear region, with proper reference and supply rails.

Concept / Approach: A summing amplifier converts input currents through its feedback network to an output voltage: Vout = -Rf * (V1/R1 + V2/R2 + ...). Using binary-weighted resistors (or an R–2R ladder that presents equivalent currents), digital “1” lines contribute fixed current portions while “0” contribute none. The op-amp sums these currents to produce a proportional analog voltage referenced to ground or a set reference level, thus implementing a DAC function.

Step-by-Step Solution:

Verification / Alternative check: Replace weighted resistors with an R–2R ladder feeding the op-amp; behavior and transfer function are equivalent but with improved matching and tolerance requirements.

Why Other Options Are Wrong: Inductors are unnecessary; rail-to-rail capability depends on voltage range, not the principle; MHz operation is not a requirement for DAC functionality.

Common Pitfalls: Ignoring op-amp headroom, output drive limits, and resistor tolerance which directly affect linearity (INL/DNL) and accuracy.

Final Answer: Correct