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AD1846 Fiches technique(PDF) 10 Page - Analog Devices |
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AD1846 Fiches technique(HTML) 10 Page - Analog Devices |
10 / 28 page AD1846 REV. A –10– Sixty-four steps of –1.5 dB attenuation are supported to –94.5 dB. The digital mix datapath can also be completely muted, preventing any mixing of the analog input with the digi- tal input. Note that the level of the mixed signal is also a func- tion of the input PGA settings, since they affect the ADCs’ output. The attenuated digital mix data is digitally summed with the DAC input data prior to the DACs’ datapath attenuators. The digital sum of digital mix data and DAC input data is clipped at plus or minus full scale and does not wrap around. Because both stereo signals are mixed before the output attenuators, mix data is attenuated a second time by the DACs’ datapath attenuators. In case the AD1846 is capturing data but ADC output data is not removed in time (“ADC overrun”), then the last sample captured before overrun will be used for the digital mix. In case the AD1846 is playing back data hut input digital DAC data fails to arrive in time (“DAC underrun”), then a midscale zero will be added to the digital mix data. Analog Outputs A stereo line level output is available at external pins. Each channel of this output can be independently muted. When muted, the outputs will settle to a dc value near VREF, the midscale reference voltage. Digital Data Types The AD1846 supports four data types: 16-bit twos-complement linear PCM, eight-bit unsigned linear PCM, companded µ-law, and 8-bit companded A-law, as specified by control register bits. Data in all four formats is always transferred MSB first. Stereo data is always transferred in the left-right order. All data formats that are less than 16 bits are properly aligned to insure the utili- zation of full system resolution. The 16-bit PCM data format is capable of representing 96 dB of dynamic range. Eight-bit PCM can represent 48 dB of dynamic range. Companded µ-law and A-law data formats use nonlinear coding with less precision for large amplitude signals. The loss of precision is compensated for by an increase in dynamic range to 64 dB and 72 dB, respectively. On input, 8-bit companded data is expanded to an internal lin- ear representation, according to whether µ-law or A-law was specified in the Codec’s internal registers. Note that when µ-law compressed data is expanded to a linear format, it requires 14 bits. A-law data expanded requires 13 bits. 3/2 2/1 15 0 15 0 MSB MSB 0 0 0 / 0 0 15 0 MSB DAC INPUT EXPANSION COMPRESSED INPUT DATA 3/2 2/1 LSB LSB 8 7 LSB Figure 2. A-Law or µ-Law Expansion When 8-bit companding is specified, the ADCs’ linear output is compressed to the format specified. LSB 3/2 2/1 15 0 15 0 MSB MSB 0 0 0 0 0 0 0 0 15 0 MSB ADC OUTPUT TRUNCATION COMPRESSION LSB 8 7 LSB Figure 3. A-Law or µ-Law Compression Note that all format conversions take place at input or output. Internally, the AD1846 always uses 16-bit linear PCM represen- tations to maintain maximum precision. Power Supplies and Voltage Reference The AD1846 operates from +5 V power supplies. Independent analog and digital supplies are recommended for optimal perfor- mance though excellent results can be obtained in single supply systems. A voltage reference is included on the Codec and its 2.25 V buffered output is available on an external pin (VREF). The reference output can be used for biasing op amps used in dc coupling. The internal reference must be externally bypassed to analog ground at the VREF_F pin. Clocks and Sample Rates The AD1846 operates from external crystals. Two crystal inputs are provided to generate a wide range of sample rates. The oscil- lators for these crystals are on the AD1846, as is a multiplexer for selecting between them. They can be overdriven with exter- nal clocks by the user, if so desired. The recommended crystal frequencies are 16.9344 MHz and 24.576 MHz. From them the following sample rates are divided down: 5.5125, 6.615, 8, 9.6, 11.025, 16, 18.9, 22.05, 27.42857, 32, 33.075, 37.8, 44.1, 48 kHz. OBSOLETE OBSOLETE OBSOLETE |
Numéro de pièce similaire - AD1846_15 |
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Description similaire - AD1846_15 |
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