Part 2 looks at interpolating DACs and sigma-delta DACs. For background reading, check out Basics of ADCs and DACs.
DAC Structures
The most commonly used DAC structures (other than a simple 1-bit DAC based on a single switch used with a reference voltage) are binary weighted DACs or ladder networks, but these, though relatively simple in structure, require quite complex analysis. We will start by examining one of the simplest structures of all, the Kelvin divider shown in Figure 1. An N-bit version of this DAC simply consists of 2N equal resistors in series. The output is taken from the appropriate tap by closing one of the 2N switches by decoding 1 of 2N switches from the N-bit data. Recent DACs using this architecture are referred to as "string DACs."
Figure 1. Simplest Voltage Output DAC: The Kelvin Divider ("String DAC").
This architecture is simple, has a voltage output (but a code-varying ZOUT), and is inherently monotonic (even if a resistor is zero, OUTPUTN cannot exceed OUTPUTN+1). It is linear if all the resistors are equal, but may be made deliberately nonlinear if a nonlinear DAC is required. Since only two switches operate during a transition, it is a low glitch architecture. Its major drawback is the large number of resistors required for high resolution, and as a result it is not commonly used—but, as we shall see later, it is used as a component in more complex DAC structures. There is an analogous current output DAC that consists, again, of 2N resistors (or current sources) but, in this case, they are all connected in parallel between the reference voltage input and the virtual ground output (see Figure 2).
Figure 2. The Simplest Current Output DAC.
In this DAC, once a resistor is switched into circuit by increasing digital code, any further increases do not switch it out again. The structure is thus inherently monotonic, regardless of inaccuracies in the resistors and, as in the previous case, may be made intentionally nonlinear where a specific nonlinearity is required. Again, as in the previous case, the architecture is rarely, if ever, used to fabricate a complete DAC because of the large numbers of resistors and switches required. However, it is often used as a component in a more complex DAC structure.
Unlike the Kelvin divider, this type of DAC does not have a unique name, although both types are referred to as fully decoded DACs or thermometer DACs or string DACs. Fully decoded DACs are often used as components of more complex DACs. The most common are "segmented DACs" where part of the output of a fully decoded DAC is further subdivided. The structure is used because the fully decoded DAC is inherently monotonic, so if the subdivision is also monotonic, the whole resulting DAC is also monotonic.
A voltage segmented DAC (see Figure 3) works by further subdividing the voltage across one resistor of a Kelvin divider. The subdivision may be done with a further Kelvin divider (in which case the whole structure is known as a "Kelvin-Varley divider") or with some other DAC structure.
Figure 3. Segmented Voltage DACs.
In all DACs, the output is the product of the reference voltage and the digital code, so in that sense, all DACs are multiplying DACs, but many DACs operate well only over a limited range of VREF. True MDACs, however, are designed to operate over a wide range of VREF. A strict definition of a multiplying DAC demands that its reference voltage range includes 0 V, and many, especially current mode ladder networks with CMOS switches, permit positive, negative, and ac VREF. DACs that do not work down to 0 V. VREF are still useful, however, and types where VREF can vary by 10:1 or so are often called MDACs, although a more accurate description might be "semimultiplying" DACs.
