Q. WEIGHTED-RESISTOR CONVERTER?

Figure shows a 4-bit weighted-resistor D/A converter which includes a reference voltage source, a set of four electronically controlled switches, a set of four binary-weighted precision resistors, and an op amp. Each binary bit of digital input code controls its own switch. The switch closes with a bit value of 1, and the switch stays open with binary 0. The resistor connected to the most significant bit (MSB), b₀, has a value of R; b₁ is connected to 2R, b₂ to 4R, and b₃ to 8R.

Thus, each low-order bit is connected to a resistor that is higher by a factor of 2. For a 4-bit D/A converter, the binary input range is from 0000 to 1111.

An important design parameter of a D/A converter is the resolution, which is the smallest output voltage change, V , which for an n-bit D/A converter is given by

The range of resistor values becomes impractical for binary words longer than 4 bits. Also, the dynamic range of the op amp limits the selection of resistance values. To overcome these limitations, the R-2R ladder D/A converter is developed.