Here is a diagram of a classical R-2R ladder D/A design. Renesas uses these in most H8 series MCUs, and there operation is straightforward. The voltage at the noninverting terminal of the operational amplifier (and at its output) is a function of two things: the switches that are connected to the reference voltage (AVref), and the value of that reference voltage. Specifically, AVref determines the range of the D/A converter, since the resistor divider network can only provide a voltage level between AVref (maximum) and AVss (minimum) to the op amp. The number of “steps” in the output is 2n-1, where “n” is the number of bits. The circuit shown here has a 4-bit R-2R ladder, so its output has 15 steps. An 8-bit D/A, by contrast, produces 255 steps.