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TDA1549T Datenblatt(PDF) 6 Page - NXP Semiconductors |
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TDA1549T Datenblatt(HTML) 6 Page - NXP Semiconductors |
6 / 20 page August 1994 6 Philips Semiconductors Objective specification Stereo 4fs data input up-sampling filter with bitstream continuous calibration DAC (BCC-DAC1) TDA1549T FUNCTIONAL DESCRIPTION General The TDA1549T CMOS digital-to-analog bitstream continuous calibration converter incorporates internal digital filtering which increases the oversampling rate of 4fs input data to 96fs, and removes the spectral data components around 4fs, 8fs, and 12fs. A 2nd order noise shaper operating at 96fs outputs a 5-bit data bitstream to the DACs. The filtering required for waveform smoothing and out-of-band noise reduction is achieved by simple 1st order analog post-filtering (see Fig.3). The combination of noise shaping and bitstream continuous calibration digital-to-analog conversion enables high performance and extremely low noise to be achieved. Input The circuit accepts four times oversampled data in 18-bit two's complement standard Japanese format with MSB first. Left and right data channel words are time multiplexed. The input format is illustrated in Fig.5. The bit clock (BCK) operates at 192fs, i.e. 48 times the word select (WS) frequency of 4fs. Oversampling filter The oversampling filter consists of: • A 7th order half-band low-pass FIR filter which increases the oversampling rate from 4 times to 8 times. This removes the spectral components around 4fs and 12fs (see Fig.3). • A linear interpolation section which increases the oversampling rate to 16 times. This removes the spectral components around 8fs. • A sample-and-hold section which provides another 6 times oversampling to 96 times. The zero-order hold characteristic of this sample-and-hold section plus the 1st order analog filtering removes the spectral components around 16fs. Passband ripple is within 0.1 dB. Stopband attenuation is >50 dB around multiples of the sampling frequency. Noise Shaper The 2nd-order digital noise shaper converts the 18-bit data at 96fs into a 5-bit bitstream, while shifting in-band quantization noise to frequencies well above the audio band. For low signal levels the noise shaper output is a 1-bit bitstream. This noise shaping technique used in combination with a special data code and bitstream DAC enables extremely high signal-to-noise ratios to be achieved. Data encoder The data encoder converts the 5-bit two's complement output data from the noise shaper to a 32-bit thermometer code. In traditional unidirectional current converters, half of the full-scale current flows to the output during small signal reproduction. The thermal noise and substrate crosstalk components present in this current severely restrict the dynamic range which can be attained. In this BCC-DAC1 true low-noise performance is achieved using a special data code and bidirectional current sources. The special data code guarantees that only small values of current flow to the output during small-signal passages while larger positive or negative signals are generated using the bidirectional current sources. For every change in the 18-bit input sample only one current source or current sink is switched on. This intrinsically monotonic thermometer code ensures the high differential linearity, zero crossover distortion and superior signal-to-noise ratio associated with bitstream conversion. |
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Ähnliche Beschreibung - TDA1549T |
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