In this chapter, a variety of methods are discussed for realizing integrated Nyquist-rate digital-to-analog converters (DAC). Nyquist-rate D/A converters can be roughly categorized into four main types: decoder-based, binary-weighted, thermometer-code, and hybrid. Oversampling D/A converters are discussed separately in Chapter 18 due to their importance and because they are best described using many signal processing concepts.
Perhaps the most straightforward approach for realizing an
N-bit D/A converter is to create 2
N reference signals and pass the appropriate signal to the output, depending on the digital input word. We refer to such D/A converters as decoder-based converters.
Key Point: Decoder-based D/A converters operate by creating a set of analog reference levels and selecting one by turning on appropriate switches selected by an input digital code.
One of the first integrated MOS 8-bit D/A converters was based on selecting one tap of a segmented resistor string by a switch network [Hamadé, 1978].1 The switch network was connected in a tree-like decoder, as in the 3-bit D/A converter shown in Fig. 16.1. Notice that there will be one, and only one, low-impedance path between the resistor string and the input of the buffer, and that path is determined by the digital input word,
Bin. In a CMOS implementation, ...