AD1848K Analog Devices, AD1848K Datasheet - Page 9

• Manufacturer Part Number: AD1848K
• Description: Parallel-Port 16-Bit SoundPort Stereo Codec
• Manufacturer: Analog Devices

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analog signals: line, microphone (“mic”), auxiliary (“aux”) #1,
and post-mixed DAC output. The microphone inputs can pass
through optional 20 dB gain blocks. A software-controlled pro-
grammable gain stage allows independent gain for each channel
going into the ADC. The ADCs’ output can be digitally mixed
with the DACs’ input.
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 AD1848K can
accept and generate 16-bit twos-complement PCM linear digital
data, 8-bit unsigned magnitude PCM linear data, and 8-bit
The
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 AD1848K 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 AD1848K 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 multi-
plexed to the internal programmable gain amplifier stage
(PGA). Each channel of the mic inputs can be amplified by
-law or A-law companded digital data.
AD1848K
DACs are preceded by a digital interpolation filter. An
DATA7:0
Figure 1. Interface to ISA Bus
DBDIR
CDRQ
DBEN
PDRQ
PDAK
CDAK
WR
INT
CS
RD
A1
A0
8
B
DIR
G
ADDRESS
DECODE
7
4
2
4
5
A
8
18
AEN
SA19:2
SA1
SA0
IOWC
IORC
DATA7:0
DRQ<X>
DRQ<Y>
DAK<X>
DAK<Y>
IRQ<Z>
ISA BUS
–9–
+20 dB prior to the PGA to compensate for the voltage swing
difference between line levels and typical condenser micro-
phones. Alternatively, the mic inputs can bypass the +20 dB
fixed gain block and go straight to the input multiplexer.
The PGA following the input multiplexer allows independent se-
lectable 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 AD1848K is not playing back data from its DACs,
the analog mix function can still be active.
Analog-to-Digital Datapath
The AD1848K
A single pole of passive filtering is all that is required for
anti-aliasing the analog input due to the ADC’s high 64 times
oversampling ratio. The ADCs include linear phase digital deci-
mation filters that low-pass filter the input to 0.45 F
the word rate or “sampling frequency.”) ADC input overrange
conditions will cause register bits to be set that can be read.
Digital-to-Analog Datapath
The
pass digital interpolation filter. The anti-imaging interpolation
filter nominally oversamples by 64 and digitally filters the higher
frequency images. The interpolation ratio is increased at low
sample rates to ensure that the shaped quantization noise is
inaudible. This feature of the AD1848K represents an improve-
ment over the earlier AD1848J. The attenuator allows indepen-
dent control of each DAC channel from 0 dB to –94.5 dB in
1.5 dB steps plus full mute. The DACs’
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 compen-
sating for the group delay variation of the analog output filters.
Changes in DAC output attenuation take effect only on zero
crossings of the digital signal, thereby eliminating “zipper”
noise. Each channel has its own independent zero-crossing de-
tector 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 8 milli-
seconds at a 48 kHz sampling rate and 48 milliseconds at an
8 kHz sampling rate. (Time out [ms] 384/F
DACs contain a programmable attenuator and a low-
ADCs incorporate a fourth order modulator.
noise shapers also
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AD1848K
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