Flavored DACs for every application: the digital image is identical to a real photograph. Well, try standing back a bit.(SIGNAL SOURCES)

Publication: EE-Evaluation Engineering

Publication Date: 01-NOV-05

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COPYRIGHT 2005 Nelson Publishing

Traditional analog man-machine interfaces disappeared years ago. First, knobs were replaced by push buttons. Then came the hand-held controller, now ubiquitous and driving everything from air conditioners and microwave ovens to music centers and blenders. More recently, controls that look like knobs have reappeared. In reality, they simply are finger grips that make it easier for human operators to turn digital shaft encoders.

For sure, there is an unstoppable digital conspiracy taking place. It can be argued that our basic physiology hasn't changed in response to the digital onslaught. Our senses still operate over the same ranges they have for millennia and respond best to analog inputs. But the complacency of this position only confirms the success of the ersatz-analog-interface marketing effort. We are living in a pixilated, digitized, interpolated, and decimated universe without realizing it.

Making possible new analog-like interfaces are modern conversion devices, both analog-to-digital converters (ADCs) and digital-to-analog converters (DACs). Precisely because humans haven't evolved to enjoy 8-b digitized sound, for example, digital audio required at least 16-b resolution to be successful. Low-cost, extremely precise, and relatively fast DACs only became practical with advanced semiconductor processes. A similar claim could be made for the high-resolution, high-speed converters used in modern communications systems.

Of course, DACs have been used for decades in test and measurement and control systems. Often, 8-b or 12-b resolution has been adequate, although in the last few years 16-b devices have replaced many 12-b solutions. Several types of architectures have been developed to meet different application requirements. In addition, a family of performance metrics has become standardized within the industry so that comparing devices is relatively straightforward.

[FIGURE 1 OMITTED]

DACs and Metrics (1)

String DAC

The easiest-to-understand architecture is based on a Kelvin divider. As shown in Figure 1, the output voltage is selected from the taps on a long string of equal-value resistors. Also called a string DAC, this structure has the advantage of guaranteed monotonicity. Even if one resistor were to have zero value, progressing from the bottom to the top of the divider string, the output voltage would never decrease. Monotonic means that an output level will never decrease for increasing input codes.

Ideally, all the resistors have exactly the same value, so one output step will be identical in size to any other. Although not perfect, actual string DACs obtain very good resistor matching, so specifying a low differential nonlinearity (DNL) value is easy. DNL describes the worst-case ratio of step sizes.

For example, if 128 10-[OMEGA] resistors made up a string DAC driven from a 12.8-V reference, each...

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