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AD1877JR bảng dữ liệu(PDF) 6 Page - Analog Devices |
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6 / 18 page AD1877 REV. A –6– (Continued from Page 1 ) The flexible serial output port produces data in twos-comple- ment, MSB-first format. The input and output signals are TTL compatible. The port is configured by pin selections. Each 16-bit output word of a stereo pair can be formatted within a 32-bit field of a 64-bit frame as either right-justified, I 2S-compatible, Word Clock controlled or left-justified positions. Both 16-bit samples can also be packed into a 32-bit frame, in left-justified and I 2S-compatible positions. The AD1877 is fabricated on a single monolithic integrated circuit using a 0.8 µm CMOS double polysilicon, double metal process, and is offered in a plastic 28-lead SOIC package. Analog and digital supply connections are separated to isolate the analog cir- cuitry from the digital supply and reduce digital crosstalk. The AD1877 operates from a single 5 V power supply over the temperature range of 0 °C to 70°C, and typically consumes less than 260 mW of power. THEORY OF OPERATION Modulator Noise-Shaping The stereo, internally differential analog modulator of the AD1877 employs a proprietary feedforward and feedback archi- tecture that passes input signals in the audio band with a unity transfer function yet simultaneously shapes the quantization noise generated by the one-bit comparator out of the audio band. See Figure 1. Without the ∑∆ architecture, this quantiza- tion noise would be spread uniformly from dc to one-half the oversampling frequency, 64 × F S. DAC DAC SINGLE TO DIFFERENTIAL CONVERTER MODULATOR BITSTREAM OUTPUT VIN VIN VIN Figure 1. Modulator Noise-Shaper (One Channel) ∑∆ architectures “shape” the quantization noise-transfer function in a nonuniform manner. Through careful design, this transfer function can be specified to high-pass filter the quantization noise out of the audio band into higher frequency regions. The AD1877 also incorporates a feedback resonator from the fourth integrator’s output to the third integrator’s input. This resonator does not affect the signal transfer function but allows the flexible placement of a zero in the noise transfer function for more effec- tive noise shaping. Oversampling by 64 simplifies the implementation of a high per- formance audio analog-to-digital conversion system. Antialias requirements are minimal; a single pole of filtering will usually suffice to eliminate inputs near FS and its higher multiples. A fourth-order architecture was chosen both to strongly shape the noise out of the audio band and to help break up the idle tones produced in all ∑∆ architectures. These architectures have a tendency to generate periodic patterns with a constant dc input, a response that looks like a tone in the frequency domain. These idle tones have a direct frequency dependence on the input dc offset and indirect dependence on temperature and time as it affects dc offset. The AD1877 suppresses idle tones 20 dB or better below the integrated noise floor. The AD1877’s modulator was designed, simulated, and exhaus- tively tested to remain stable for any input within a wide tolerance of its rated input range. The AD1877 is designed to internally reset itself should it ever be overdriven, to prevent it from going instable. It will reset itself within 5 µs at a 48 kHz sampling frequency after being overdriven. Overdriving the inputs will produce a waveform “clipped” to plus or minus full scale. See TPCs 1 through 16 for illustrations of the AD1877’s typical analog performance as measured by an Audio Precision System One. Signal-to(distortion + noise) is shown under a range of conditions. Note that there is a small variance between the AD1877 analog performance specifications and some of the performance plots. This is because the Audio Precision System One measures THD and noise over a 20 Hz to 24 kHz band- width, while the analog performance is specified over a 20 Hz to 20 kHz bandwidth (i.e., the AD1877 performs slightly better than the plots indicate). The power supply rejection (TPC 5) graph illustrates the benefits of the AD1877’s internal differen- tial architecture. The excellent channel separation shown in TPC 6 is the result of careful chip design and layout. Digital Filter Characteristics The digital decimator accepts the modulator’s stereo bitstream and simultaneously performs two operations on it. First, the decimator low-pass filters the quantization noise that the modu- lator shaped to high frequencies and filters any other out-of audio-band input signals. Second, it reduces the data rate to an output word rate equal to FS. The high frequency bitstream is decimated to stereo 16-bit words at 48 kHz (or other desired FS). The out-of-band one-bit quantization noise and other high frequency components of the bitstream are attenuated by at least 90 dB. The AD1877 decimator implements a symmetric Finite Impulse Response (FIR) filter which possesses a linear phase response. This filter achieves a narrow transition band (0.1 × FS), high stopband attenuation (> 90 dB), and low passband ripple (< 0.006 dB). The narrow transition band allows the unattenu- ated digitization of 20 kHz input signals with FS as low as 44.1 kHz. The stopband attenuation is sufficient to eliminate modulator quantization noise from affecting the output. Low passband ripple prevents the digital filter from coloring the audio signal. See TPC 7 for the digital filter’s characteristics. The output from the decimator is available as a single serial output, multiplexed between left and right channels. Note that the digital filter itself is operating at 64 × FS. As a consequence, Nyquist images of the passband, transition band, and stopband will be repeated in the frequency spectrum at multiples of 64 × FS. Thus the digital filter will attenuate to greater than 90 dB across the frequency spectrum except for a window ±0.55 × F S wide centered at multiples of 64 × F S. Any input signals, clock noise, or digital noise in these frequency windows will not be attenuated to the full 90 dB. If the high frequency signals or noise appear within the passband images within these windows, they will not be attenuated at all, and therefore input antialias filtering should be applied. |
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