AD1845 Analog Devices, AD1845 Datasheet - Page 10

• Manufacturer Part Number: AD1845
• Description: Parallel-port 16-Bit Soundport Stereo Codec
• Manufacturer: Analog Devices

AD1845
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 am-
plified 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
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
word rate or “sampling frequency.”) ADC input over range
conditions will cause bits to be set that can be read.
Digital-to-Analog Datapath
The
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’
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. (Time-out
[ms]
384
DACs are preceded by a programmable attenuator and
F
S
[kHz].)
ADCs incorporate a fourth-order modulator.
noise shapers also over sample and
S
. (“F
S
” is the
–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 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
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. 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”), then 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”), then 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 V
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 four global data types: 16-bit twos
complement linear PCM, 8-bit unsigned linear PCM, companded
bits. Data in all four formats is always transferred MSB first. All
data formats that are less than 16-bits are properly aligned to in-
sure 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
dynamic range. Companded -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 -law or A-law was
specified in the codec’s internal registers. Note that when -law
compressed data is expanded to a linear format, it requires 14
bits. A-law data expanded requires 13 bits.
-law, and 8-bit companded A-law, as specified by control register
REV. B
REF
, the