How does an R-2R ladder DAC achieve binary-weighted output with a standardized resistor network?

• A. It uses a ladder with two resistor values to realize fine-grained weights.
• B. It uses a bank of capacitors with binary-weighted ratios.
• C. It derives weights by using varactor diodes.
• D. A repeating ladder of equal resistors creates binary-weighted voltage drops; switching bits connects resistors to the output, yielding monotonic steps.

Answer: (D)

The key idea is that a DAC can build the analog value from many bits by combining fixed, predictable contributions from each bit. In an R-2R ladder, the network uses only two resistor values, R and 2R, arranged in a repeating pattern. Each bit controls a switch that connects either the reference voltage or ground to the ladder. Because of the way the ladder is structured, the voltage (or current) contributed by each successive bit is a fixed fraction of the reference and each higher bit’s contribution is half of the previous one. Put together, the outputs from all bits sum to a value that matches the binary input, giving a monotonic, evenly stepped analog output. This binary-weighted sum comes from the uniform ladder and the way each tap adds a precise amount to the output when a bit is switched. The other options describe different devices or architectures (capacitors, varactors) that aren’t how a standard R-2R ladder DAC implements binary weighting.