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AD1848KST Datasheet(PDF) 10 Page - Analog Devices |
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AD1848KST Datasheet(HTML) 10 Page - Analog Devices |
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10 / 28 page  AD1848K REV. 0 –10– Digital Mixing Stereo digital output from the ADCs can be mixed digitally with the input to the DACs. Digital output from the ADCs going out of the data port is unaffected by the digital mix. Along the digi- tal mix datapath, the 16-bit linear output from the ADCs is attenuated by an amount specified with control bits. Both chan- nels of the monitor data are attenuated by the same amount. (Note that internally the AD1848K always works with 16-bit PCM linear data, digital mixing included; format conversions take place at the input and output.) 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 AD1848K 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 AD1848K is playing back data but 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 mid- scale reference voltage. Digital Data Types The AD1848K supports four global 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 trans- ferred MSB first. Eight-bit data is always left justified in 16-bit fields; said in other words, the MSBs of all data types are always aligned; in yet other words, full-scale representations in all four formats correspond to equivalent full-scale signals. The eight least significant bit positions of 8-bit data in 16-bit fields are ignored on input and zeroed on output. 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 AD1848K always uses 16-bit linear PCM repre- sentations to maintain maximum precision. Power Supplies and Voltage Reference The AD1848K operates from +5 V power supplies. Indepen- dent analog and digital supplies are recommended for optimal performance 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 AD1848K operates from external crystals. Two crystal in- puts are provided to generate a wide range of sample rates. The oscillators for these crystals are on the AD1848K, as is a multi- plexer for selecting between them. They can be overdriven with external 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. |
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