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DAC-Architecture-Slide8

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 segment DAC works by further sub-dividing the voltage across one resistor of a Kelvin divider. The sub-division may be done with a further Kelvin divider (in which case the whole structure is sometimes known as a "Kelvin-Varley divider“), or with some other DAC structure. In the past, the second DAC was isolated from the first by buffer amplifiers so that there was no loading on the first one. In the late 1990s, engineers at Analog Devices designed and patented the configuration shown in the diagram where the loading due to the second stage attenuated the voltage across the first stage resistor by exactly one LSB (least significant bit) so that one resistor fewer is needed in the second string and interstage buffer amplifiers are not necessary.

PTM Published on: 2009-08-10