Nyquist-Rate ADC and DAC:DAC Design Arts

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DAC Design Arts

There are many different circuit techniques used to implement DACs, but the popular ones widely used today are of the parallel type in which all bits change simultaneously upon the application of an input code word. Serial DACs, on the other hand, produce an analog output only after receiving all digital input data in a sequential form. When DACs are used as stand-alone devices, their output transient behaviors limited by glitch, slew rate, word clock jitter, settling, etc. are of paramount importance, but used as subblocks of ADCs, DACs need only to settle within a given time interval. An output S/H, usually called a deglitcher, is often used for better transient performance. Three of the most popular architectures of DACs are resistor string, ratioed current sources, and a capacitor array. The current-ratioed DAC finds most applications as a stand-alone DAC, while the resistor-string and capacitor-array DACs are mainly used as ADC subblocks.

For speeds over 100 MHz, most state-of-the-art DACs employ current sources switched directly to output resistors [19–23]. Furthermore, owing to the high bit counts (12 to 16 b), segmented architectures are employed, with the current sources broken into two or three segments. The CMOS design has the advantages of lower power, smaller area, and lower manufacturing costs. In all cases, it is of interest to note that the dynamic performance of the DACs degrades rapidly as input frequencies increase, and true dynamic performance is not attained except at low frequencies. Since a major application of wideband- width, high-resolution DACs is in communications, poor dynamic performance is undesirable, owing to the noise leakage from frequency multiplexed channels into other channels. The goal of better dynamic performance continues to be a target of ongoing research and development.

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