CMOS Analog Circuit Design : Digital-Analog and Analog-Digital Converters (#007)


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Digital-Analog Converters (DACs) and Analog-Digital Converters (ADCs) are excellent examples of analog IC design at the systems level. It is shown that the accuracy required to build the DAC and ADC depends on the bit. The accuracy required for the most significant bit (MSB) is always greater than the other bits in the DAC or ADC. The least significant bit (LSB) requires an accuracy of only ±50% if all other bits are idea. In the DAC, the primary active component is the op amp. The performance of the op amp must be such that the DAC is accurate to within 0.5LSB. DACs can be slow or fast in the time it takes to make the conversion from digital to analog. A DAC can be clocked or asynchronous (unclocked). In the ADC, the primary active component is the comparator. The performance of the comparator must be such that the accuracy of the ADC is within 0.5LSB. One important difference between the ADC and the DAC is that the unknown analog signal must be sampled and held in order to convert the analog signal to a digital word. This means that the highest bandwidth that can be applied to the ADC is ½ the sampling frequency. Like DACs, the ADCs can be slow or fast. A fast ADC is one that converts the analog to digital in one clock cycle. Oversampled DACs and ADCs work by keeping the bandwidth of the analog signal much less than the sampling frequency. Typically, the sampling frequency may be 32, 64, 128, etc. times the analog bandwidth. The oversampled converters work by noise shaping. They use a modulator that moves the quantization noise in the analog signal bandwidth to higher frequencies. In an ADC oversampled converter, the high frequency noise is filtered by digital circuits. The digital circuits work in such a manner as to output the digital equivalent of the analog input. This type of converter can have a high resolution although the bandwidth is small.

  1. Characterization of DACs and Current Scaling DACs
  2. Voltage, Charge Scaling and Serial DACs, Improved Resolution
  3. Characterization of ADCs and Sample and Hold Circuits
  4. Moderate Speed Nyquist ADCs
  5. High Speed Nyquist ADCs
  6. Oversampling ADCs – Part I
  7. Oversampling ADCs – Part II


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