In an R-2R ladder DAC, what is the reason for using only two resistor values?

• A. To simplify matching and manufacturing
• B. To increase speed
• C. To improve linearity beyond the ideal
• D. To increase noise immunity

Answer: (A)

The main reason for using only two resistor values in an R-2R ladder DAC is to simplify manufacturing and matching while preserving accurate binary weighting. By building every rung from the same two values—R and 2R—the ladder behaves predictably: all branches have the same ratio, so the currents flowing through each rung depend only on whether a bit is connected to the ladder or grounded. This uniformity makes it easy to source, assemble, and mate parts in large quantities, and it keeps the errors from resistor tolerances aligned across all bits, preserving the intended digital-to-analog relationship. Using more than two values would introduce many different tolerances across many parts, making matching harder and degrading accuracy. The other benefits (speed, beyond-ideal linearity, or noise immunity) aren’t the primary driver; the key advantage is straightforward, consistent production and reliable weighting.