Datenblatt-Suchmaschine für elektronische Bauteile |
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AD1849K Datenblatt(PDF) 8 Page - Analog Devices |
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AD1849K Datenblatt(HTML) 8 Page - Analog Devices |
8 / 28 page AD1849K REV. A –8– FUNCTIONAL DESCRIPTION This section overviews the functionality of the AD1849K and is intended as a general introduction to the capabilities of the device. As much as possible, detailed reference information has been placed in “Control Registers” and other sections. The user is not expected to refer repeatedly to this section. Analog Inputs The AD1849K SoundPort Stereo Codec accepts stereo line-level and mic-level inputs. These analog stereo signals are multiplexed to the internal programmable gain amplifier (PGA) stage. The mic inputs can be amplified by 20 dB prior to the PGA to com- pensate for the voltage swing difference between line levels and typical condenser microphones. The mic inputs can bypass the 20 dB fixed gain block and go straight to the input multiplexer, which often results in an improved system signal-to-noise ratio. The PGA following the input multiplexer allows independent selectable gains for each channel from 0 to 22.5 dB in 1.5 dB steps. The Codec can operate either in a global stereo mode or in a global mono mode with left-channel inputs appearing at both channel outputs. Analog-to-Digital Datapath The AD1849K Σ∆ ADCs incorporate a proprietary fourth-order modulator. A single pole of passive filtering is all that is required for antialiasing the analog input because of the ADC’s high 64 times oversampling ratio. The ADCs include linear-phase digital decimation filters that low-pass filter the input to 0.45 × F S (“FS” is the word rate or “sampling frequency”). ADC input overrange conditions will cause a sticky bit to be set that can be read. Digital-to-Analog Datapath The Σ∆ DACs are preceded by a programmable attenuator and a low-pass digital interpolation filter. The attenuator allows independent control of each DAC channel from 0 dB to –94.5 dB in 1.5 dB steps plus full digital mute. The anti-imaging inter- polation filter oversamples by 64 and digitally filters the higher frequency images. The DACs’ Σ∆ noise shapers also oversample by 64 and convert the signal to a single-bit stream. The DAC outputs are then filtered in the analog domain by a combination of switched-capacitor and continuous-time filters. They remove the very high frequency components of the DAC bitstream output, including both images at the oversampling rate and shaped quantization noise. No external components are required. Phase linearity at the analog output is achieved by internally compensating for the group delay variation of the analog output filters. Attenuation settings are specified by control bits in the data stream. Changes in DAC output level take effect only on zero crossings of the digital signal, thereby eliminating “zipper” noise. Each channel has its own independent zero-crossing detector and attenuator change control circuitry. A timer guarantees that requested volume changes will occur even in the absence of an input signal that changes sign. The time-out period is 10.7 milliseconds at a 48 kHz sampling rate and 64 milliseconds at an 8 kHz sampling rate (Time-out [ms] ≈ 512/ Sampling Rate [kHz]). Monitor Mix A monitor mix is supported that digitally mixes a portion of the digitized analog input with the analog output (prior to digitiza- tion). The digital output from the ADCs going out of the serial data port is unaffected by the monitor mix. Along the monitor mix datapath, the 16-bit linear output from the ADCs is attenuated by an amount specified with control bits. Both channels of the monitor data are attenuated by the same amount. (Note that internally the AD1849K always works with 16-bit PCM linear data, digital mixing included; format conversions take place at the input and output.) Sixteen steps of –6 dB attenuation are supported to –94.5 dB. A “0” implies no attenuation, while a “14” implies 84 dB of attenuation. Specifying full scale “15” completely mutes the monitor datapath, preventing any mixing of the analog input with the digital input. Note that the level of the mixed output signal is also a function of the input PGA settings since they affect the ADCs’ output. The attenuated monitor data is digitally summed with the DAC input data prior to the DACs’ datapath attenuators. Because both stereo signals are mixed before the output attenuators, mix data is attenuated a second time by 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. Analog Outputs One stereo line-level output, one stereo headphone output, and one monaural (mono) speaker output are available at external pins. Each of these outputs can be independently muted. Muting either the line-level stereo output or the headphone stereo output mutes both left and right channels of that output. When muted, the outputs will settle to a dc value near CMOUT, the midscale reference voltage. The mono speaker output is differential. The chip can operate either in a global stereo mode or in a global mono mode with left channel inputs appearing at both outputs. Digital Data Types The AD1849K supports four global data types: 16-bit twos- complement linear PCM, 8-bit unsigned linear PCM, 8-bit 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. Sixteen-bit linear data output from the ADCs and input to the DACs is in twos-complement format. Eight-bit data is always left-justified in 16-bit fields; in other words, the MSBs of all data types are always aligned; in yet other words, full-scale representations in all three formats correspond to equivalent full-scale signals. The eight least-significant bit positions of 8-bit linear and companded 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. |
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