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AD1845 bảng dữ liệu(PDF) 10 Page - Analog Devices |
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AD1845 bảng dữ liệu(HTML) 10 Page - Analog Devices |
10 / 40 page AD1845 –10– REV. C Digital Mixing Stereo digital output from the ADCs can be digitally mixed 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 digital mix datapath, the 16-bit linear output from the ADCs is attenuated by an amount specified with control bits. Both channels of the digital mix datapath are attenuated by the same amount. (Note that internally the AD1845 always works with 16-bit PCM linear data, digital mixing included; format conver- sions 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. Note that the level of the mixed signal is also a function 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 AD1845 is capturing data, but ADC output data is not removed in time (“ADC overrun”), the last sample captured before overrun will be used for the digital mix. In case the AD1845 is playing back data, but input digital DAC data fails to arrive in time (“DAC underrun”), a midscale zero will be added to the digital mix data when the DACZ control bit is set to 0; otherwise, the DAC will output the previous valid sample in an underrun condition. Analog Outputs Stereo and mono line-level outputs are 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. The output is selectable for 2.0 V peak-to-peak or 2.8 V peak-to-peak. When selecting the LINE output as an input to the ADC, the ADC automatically com- pensates for the output level selection. Digital Data Types The AD1845 supports five global data types: 16-bit twos comple- ment linear PCM (little endian and big endian byte ordering), 8-bit unsigned linear PCM, companded µ-law, and 8-bit com- panded A-law, as specified by control register bits. Data in all formats is always transferred MSB first. All data formats that are less than 16 bits are MSB-aligned to ensure the use 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 dy- namic 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 dy- namic range to 64 dB and 72 dB, respectively. On input, 8-bit companded data is expanded to an internal linear 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. FUNCTIONAL DESCRIPTION This section overviews the functionality of the AD1845 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 AD1845 SoundPort Stereo Codec accepts stereo line-level and microphone-level inputs. The LINE, MIC, AUX1, and post-mixed DAC output are available to the ADC multiplexer. The DAC output can be mixed with LINE, MIC, AUX1, AUX2 and M_IN. Each channel of the MIC inputs can be amplified by +20 dB to compensate for the difference between line levels and typical condenser microphone levels. Analog Mixing The M_IN mono input signal, MIC, LINE, AUX1 and AUX2 analog stereo signals can be mixed in the analog domain with the DAC output. Each channel of each AUX, LINE and MIC analog input can be independently gained/attenuated from +12 dB to –34.5 dB in 1.5 dB steps or completely muted. M_IN can be attenuated from 0 dB to –45 dB in 3 dB steps or muted. The post-mixed DAC outputs are available on L_OUT and R_OUT and also to the ADC input multiplexer. Even if the AD1845 is not playing back data from its DACs, the analog mix function can still be active. Analog-to-Digital Datapath The PGA following the input multiplexer allows independent selectable gains for each channel from 0 dB 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. The AD1845 ∑∆ ADCs incorporate a fourth-order modulator. A single pole of passive filtering is all that is required for anti- aliasing the analog input because of the ADC’s high over sam- pling ratio. The ADCs include linear-phase digital decimation filters that low-pass filter the input to 0.4 × F S. (“FS” is the word rate or “sampling frequency.”) ADC input over range conditions are reported on status bits in the Test and Initializa- tion Register. Digital-to-Analog Datapath The ∑∆ DACs are preceded by a programmable attenuator and a low-pass digital interpolation filter. The anti-imaging interpo- lation filter over samples and digitally filters the higher fre- quency images. The attenuator allows independent control of each DAC channel from 0 dB to –94.5 dB in –1.5 dB steps plus full mute. The DACs’ ∑∆ noise shapers also over sample 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 bit stream output. No external components are required. Changes in DAC output attenuation take effect only on zero crossings, eliminating “zipper” noise on playback. Each chan- nel has its own independent zero-crossing detector and attenua- tor change control circuitry. A timer guarantees that requested volume changes will occur even in the absence of a zero cross- ing. The time-out period is 8 milliseconds at a 48 kHz sampling rate and 48 milliseconds at an 8 kHz sampling rate. (Timeout [ms] ≈ 384 ÷ F S [kHz].) |
Số phần tương tự - AD1845_15 |
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Mô tả tương tự - AD1845_15 |
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