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Data Converters
Published in Wai-Kai Chen, Analog and VLSI Circuits, 2018
Ordinary DACs generate a discrete output level for every digital word applied to their input, and it is difficult to generate a large number of distinct output levels for long words. The oversampling interpolative DAC achieves fine resolution by covering the signal range with a few widely spaced levels and interpolating values between them. By rapidly oscillating between coarse output levels, the average output corresponding to the applied digital code can be generated with reduced noise in the signal band [8]. The general architecture of the interpolative oversampling DAC is shown in Figure 10.18. A digital filter interpolates sample values of the input signal in order to raise the word rate to a frequency well above the Nyquist rate. The core of the technique is a digital truncator to truncate the input words to shorter output words. These shorter words are then converted into analog form at the high sample rate so that the truncation noise in the signal band may be satisfactorily low. The sampling rate upconversion for this is usually done in stages using two upsampling digital filters. The first filter, usually a two to four times oversampling FIR, is to shape the signal band for sampling rate upconversion and to equalize the passband droop resulting from the second SINC filter for higher-rate oversampling.
Force-System Resultants and Equilibrium
Published in Richard C. Dorf, The Engineering Handbook, 2018
As in the example just discussed, there is usually a sample clock which generates a new output upon each rising clock edge. This is called synchronization and allows precise control of when the output is generated. This is necessary because there may be significant skew between the bits of the input word arriving at the input of the D/A converter. But even with synchronization, there is often enough skew within the circuitry of the D/A converter that the output may momentarily go through a variety of output states before the final state is reached. This produces a “glitch” in the analog output each time a new output is generated, as shown in graph (b) of Figure 121.5. These glitches can add significant noise and distortion to the output waveform. The sample-and-hold block of Figure 121.5 eliminates glitches from the D/A converter by sampling the analog output after each glitch has settled. This result is shown in graph (c). The final block, the inverse sinc/analog low-pass filter, provides frequency shaping and bandwidth limiting to smooth the analog output as evident in graph (d). The inverse sinc filter is necessary to compensate for the sinc envelope present in the output spectra of the D/A.
Oversampling Data Converters
Published in Bang-Sup Song, Micro CMOS Design, 2017
Figure 6.62 shows the general architecture of the interpolative oversampling DAC scheme. A digital filter interpolates the sampled input values in order to raise the sampling rate to a frequency well above the Nyquist rate. The core of the oversampling technique is the digital truncator to shorten the input words. These short words are then converted into analog values at high oversampling rates so that the truncation error in the signal band can be satisfactorily low. The sampling rate up-conversion for this is usually done in steps using two or three up-sampling digital filters. The first filter is typically a ×2 or ×4 oversampling finite impulse response (FIR) to shape the signal band for the sampling rate up-conversion, and to equalize the pass-band droop resulting from the second sinx/x (SINC) filter for higher-rate oversampling.
Medical image interpolation based on 3D Lanczos filtering
Published in Computer Methods in Biomechanics and Biomedical Engineering: Imaging & Visualization, 2020
Thiago Moraes, Paulo Amorim, Jorge Vicente Da Silva, Helio Pedrini
As can be observed from the overall results, the proposed 3D Lanczos resampling method presented satisfactory results for the majority of the medical images tested in this work. As rationale for its superiority, we highlight some advantages of the Lanczos filter (Turkowski 1990): (i) the sinc filter is a theoretically optimal reconstruction filter for band-limited signals; (ii) it allows a proper compromise in terms of aliasing, sharpness and ringing effects; (iii) it allows a choice between preservation of abrupt transition in the data and smoother interpolation.