In an R-2R ladder DAC, why are two resistor values used in the network?

• A. Because it allows each bit to have a unique and precise resistance.
• B. Because using two values minimizes area and simplifies routing.
• C. Because it uses only two resistor values (R and 2R), simplifying manufacturing and improving matching across bits.
• D. Because it eliminates the need for any feedback network.

Answer: (C)

Using only two resistor values in an R-2R ladder keeps the network simple and, more importantly, makes the loading and the bit contributions predictable and uniform. The R-2R structure is built so that the impedance seen looking into each node stays the same (typically 2R) regardless of which other bits are connected. This means every bit toggle adds a fixed, proportional current to the output, giving the correct binary-weighted effect without interactions between bits.

Having only two values, R and 2R, also makes manufacturing and matching much easier. Resistors of the same value can be made very consistent across the ladder, reducing mismatches that would otherwise distort the ideal linear transfer function as different weights are used. It also simplifies layout and routing because you don’t need many different resistor sizes.

So, the main idea is that two values ensure a constant impedance and uniform bit contributions, while also simplifying fabrication and matching. It’s not about giving each bit a unique resistance, nor primarily about area minimization or removing a feedback network—the core benefit is predictable, scalable behavior with simple, well-matched parts.