Recognizing a converter topology from a schematic: The circuit shown (ladder of alternating R and 2R branches terminating at a summing node and an op-amp) is best identified as which of the following?

Introduction / Context:
Many textbooks include images of two common DAC core topologies: the binary-weighted resistor DAC and the R–2R ladder DAC. Being able to distinguish them at a glance is a foundational skill for reading mixed-signal schematics and recognizing how switching networks generate accurate analog levels from digital codes.

Given Data / Assumptions:

• Figure shows repeated sections of resistors with only two values, R and 2R.
• Each bit controls a switch that connects a ladder node to either reference or ground.
• Ladder converges to a summing node, commonly at an op-amp input.
• No explicit resistor ratios of 1:2:4:8 visible at the summing node.

Concept / Approach:
An R–2R ladder uses just two resistor values and forms a repetitive ladder, making it layout-friendly and tolerant of matching errors compared to large binary-weighted ratios. In contrast, a binary-weighted DAC ties bit switches to resistor values with clear 1R, 2R, 4R, 8R scaling at the summing node. The presence of repeated “R” and “2R” segments identifies an R–2R ladder DAC.

Step-by-Step Solution:

Verification / Alternative check:
Compute the Thevenin equivalences of ladder segments; each bit contributes a binary-weighted current at the summing node despite uniform resistor values—characteristic of R–2R topology.

Why Other Options Are Wrong:

• Four-bit decoder: Purely digital; no summing of analog currents.
• Binary-weighted-input DAC: Would show explicit 1R:2R:4R:8R resistors tied directly to the op-amp node.
• Four-bit A/D converter: An ADC requires a comparator array or conversion logic, not just a resistor ladder to an op-amp.

Common Pitfalls:
Confusing a resistor string DAC (ladder of equal resistors to a tap network) with an R–2R ladder; overlooking the alternating R and 2R pattern.

Final Answer:
R/2R ladder D/A converter