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AD1846JP Scheda tecnica(PDF) 9 Page - Analog Devices |
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AD1846JP Scheda tecnica(HTML) 9 Page - Analog Devices |
9 / 28 page AD1846 REV. A –9– (Continued from page 1) 8 7 4 2 4 5 DIR G AEN SA19:2 SA1 SA0 IOWC IORC DATA7:0 ADDRESS DECODE DRQ<X> DRQ<Y> DAK<X> DAK<Y> IRQ<Z> A B ISA BUS 8 18 CS A1 A0 WR RD DATA7:0 DBDIR DBEN PDRQ CDRQ PDAK CDAK INT AD1846 Figure 1. Interface to ISA Bus The pair of 16-bit outputs from the ADCs is available over a byte-wide bidirectional interface that also supports 16-bit digital input to the DACs and control information. The AD1846 can accept and generate 16-bit twos-complement PCM linear digital data, 8-bit unsigned magnitude PCM linear data, and 8-bit µ-law or A-law companded digital data. The ∑∆ DACs are preceded by a digital interpolation filter. An attenuator provides independent user volume control over each DAC channel. Nyquist images and shaped quantization noise are removed from the DACs’ analog stereo output by on-chip switched-capacitor and continuous-time filters. Two stereo pairs of auxiliary line-level inputs can also be mixed in the analog do- main with the DAC output. AUDIO FUNCTIONAL DESCRIPTION This section overviews the functionality of the AD1846 and is intended as a general introduction to the capabilities of the de- vice. 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 AD1846 SoundPort Stereo Codec accepts stereo line-level and mic-level inputs. LINE, MIC, and AUX1 inputs and post- mixed DAC output analog stereo signals are multiplexed to the internal programmable gain amplifier (PGA) stage. 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 Mixing AUX1 and AUX2 analog stereo signals can be mixed in the ana- log domain with the DAC output. Each channel of each auxil- iary analog input can be independently gained/attenuated from +12 dB to –34.5 dB in –1.5 dB steps or completely muted. The post mixed DAC output is available on OUT externally and as an input to the ADCs. Even if the AD1846 is not playing back data from its DACs, the analog mix function can still be active. Analog-to-Digital Datapath The AD1846 ∑∆ 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 64 times oversampling ratio. The ADCs include linear phase digital deci- mation filters that low-pass filter the input to 0.4 FS. (“FS’’ is the word rate or “sampling frequency”). ADC input overrange conditions will cause bits to be set that can be read. Each channel of the mic inputs can be amplified digitally by +18 dB to compensate for the voltage swing differences between line levels and typical condenser microphone levels. This +18 dB digital gain is enabled with the same control bits (LMGE and RMGE) as the +20 dB analog gain in the AD1848. Digital-to-Analog Datapath The ∑∆ DACs contain a programmable attenuator and a low- pass digital interpolation filter. The anti-imaging interpolation filter oversamples by 64 and digitally filters the higher frequency 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 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. 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. Changes in DAC output attenuation take effect only on zero crossings, thereby eliminating “zipper” noise. Each channel has its own independent zero-crossing detector and attenuator change control circuitry. A timer guarantees that requested vol- ume changes will occur even in the absence of an input signal that changes sign. The time-out period is 8 milliseconds at a 48 kHz sampling rate and 48 milliseconds at an 8 kHz sampling rate. (Time out [ms] ≈ 384/F S [kHz].) 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 AD1846 always works with 16-bit PCM linear data, digital mixing included; format conversions take place at the input and output.) |
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