Scrambling, or D 2 S) which overcomes this problem and produces excellent performance with respect to THD + N. The AD185x-series of audio DACs, however use a proprietary data scrambling technique ( called direct data In the past, multibit DACs have been difficult to design because of the accuracy requirement on the n-bit internal DAC (this DAC, although only n-bits, must have the linearity of the final number of bits, N). The concept is similar to that of the interpolating DAC previously discussed, with the addition of the digital sigma-delta modulator. It is possible to use more than one bit in the DAC, and this leads to the multibit architecture shown in Figure 4.12B. Because of the high oversampling frequency, the complexity of the LPF is much less than the case of traditional Nyquist operation. The output is filtered in an external analog LPF. It consists of an "interpolation filter" (a digital circuit which accepts data at a low rate, inserts zeros at a high rate, and then applies a digital filter algorithm and outputs data at a high rate), a Σ- ∆ modulator (which effectively acts as a low pass filter to the signal but as a high pass filter to the quantization noise, and converts the resulting data to a high speed bit stream), and a 1-bit DAC whose output switches between equal positive and negative reference voltages. The technique, known as sigma-delta ( Σ- ∆), is computation intensive, so has only recently become practical for the manufacture of high resolution DACs, but since it uses a 1-bit DAC, it is intrinsically linear and monotonic.Ī Σ- ∆ DAC, unlike the Σ- ∆ ADC, is mostly digital (see Figure 4.12A). Without oversampling, the image frequency occurs at 150MHz – 60MHz = 90MHz, and the filter transition band is 60MHz to 90MHz.Īnother way of obtaining high resolution is to use oversampling techniques and a 1-bit DAC. The transition band for the analog filter is therefore 60MHz to 240MHz.
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For an output frequency of 60MHz, and an update rate of 150MHz, and an oversampling ratio of 2, the image frequency occurs at 300MHz – 60MHz = 240MHz. The device is designed to handle 14-bit input word rates up to 150MSPS. The AD9772 is a 2-times oversampling interpolating 14-bit DAC, and a simplified block diagram is shown in Figure 4.11. This transition zone is a little greater than 2 octaves, implying that a 5- or 6-pole Butterworth filter is sufficient. The analog antialiasing filter transition zone is now 10 to 50MHz (the first image occurs at 2f c–f o=60–10=50MHz).
The response of the digital filter relative to the 2-times oversampling frequency is shown in Figure 4.10B. This is done by passing the 60MSPS data stream with the added zeros through a digital interpolation filter which computes the additional data points. The parallel data stream is now 60MSPS, but we must now determine the value of the zero-value data points. Filters become even more complex as the transition band becomes narrower.Īssume that we increase the DAC update rate to 60MSPS and insert a "zero" between each original data sample. Therefore, a minimum of 10 poles is required to provide the desired attenuation. A Butterworth filter design gives 6dB attenuation per octave for each pole. The filter must therefore go from a passband of 10MHz to 60dB stopband attenuation over the transition band lying between 10 and 20MHz (one octave). Assume that the image frequency must be attenuated by 60dB. The image frequency component at 30–10 = 20MHz must be attenuated by the analog antialiasing filter, and the transition band of the filter is 10 to 20MHz. Assume the DAC output frequency is 10MHz. Assume a traditional DAC is driven at an input word rate of 30MSPS (see Figure 4.10A). The same concept can be applied to a high speed DAC.
The sigma-delta 1-bit DAC architecture represents the ultimate extension of this concept and has become popular in modern CD players. The high oversampling rate moves the image frequencies higher, thereby allowing a less complex filter with a wider transition band. The 4x, 8x, or 16x data stream is passed through a digital interpolation filter which generates the extra data points. "Zeros" are inserted into the parallel data, thereby increasing the effective update rate to 4-times, 8-times, or 16-times the fundamental throughput rate. This concept is common in digital audio CD players, where the basic update rate of the data from the CD is about 44kSPS. In a DAC-based system (such as DDS), the concept of interpolation can be used in a similar manner.
In ADC-based systems, oversampling can ease the requirements on the antialiasing filter, and a sigma-delta ADC has this inherent advantage.
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