REV. 0
AD1955
–13–
Serial Data Format in External Digital Filter Mode
In the External Digital Filter Mode, the AD1955 will accept
up to 24-bit serial, twos complement, MSB-first data from an
external digital filter, an HDCD decoder, or a general-purpose
DSP. If the External Digital Filter Mode is selected by Control
Register 0, Bits 12 and 13, Pin 2 to Pin 5 are assigned as the word
clock input (EF_WCLK, Pin 2), bit clock input (EF_BCLK,
Pin 3), left channel data input (EF_LDATA, Pin 4), and right
channel data input (EF_RDATA, Pin 5), respectively, to accept
8fS (48 kHz), 4fS (96 kHz), or 2fS (196 kHz) oversampled data.
Left and right channel data should be valid on the rising edge
of EF_BCLK. The mode can be set to Left- or Right-Justified.
A burst mode BCLK can be used in Left-Justified Mode.
Serial Data Format in SACD Mode
In the SACD Mode, the AD1955 supports both normal mode
or phase modulation mode, which are selected by Control
Register 1, Bit 6. If normal mode is selected, DSD_SCLK,
DSD_LDATA, and DSD_RDATA are used to interface with
DSD decoder chip. In this mode, the DSD data is clocked in
the AD1955 using the rising edge of DSD_SCLK with a 64fS
rate, 2.8224 MHz. DSD_PHASE pin should be connected LOW.
If Phase Modulation Mode is selected, the DSD_PHASE pin is
also used to interface with the DSD decoder. In this mode, a
64fS DSD_PHASE signal is used as a reference signal to receive
the data from the decoder. The DSD data is clocked into the
AD1955 with a 128fS DSD_SCLK.
The AD1955 can operate as a master or slave device. In Master
Mode, the AD1955 will output DSD_SCLK and DSD_PHASE
(if in Phase Modulation Mode) to a DSD decoder and will
support Normal Mode and Phase Modulation Mode 0. In Slave
Mode, the AD1955 will accept DSD_SCLK and DSD_PHASE
(if in Phase Modulation Mode) from a DSD decoder and sup-
ports all of the normal and phase modulation modes.
When the SACD Port is not being used, the SACD pins (Pins
6, 7, 8, and 9) should be tied to a valid logic level. Please note
that there are weak pull-ups (0.6 mA typical) on DSD_SCLK
and DSD_PHASE.
Master Clock
The AD1955 must be set to the proper sample rate and master
clock rate using Control Registers 0 and 1. The allowable master
clock frequencies for each interpolation mode are shown below.
In the External Filter Mode, the AD1955 accepts master clock
frequencies depending on the input sample rate as shown below.
In the SACD Mode, the AD1955 accepts a 256fS, 512fS, or
768fS Master Clock, where fS is nominally 44.1 kHz. In Slave
Mode, by default, the rising edge of DSD_SCLK should coincide
with the rising edge of MCLK. Control Register 1, Bit 2 should
be set to 1 if the rising edge of DSD_SCLK coincides with the
falling edge of MCLK. In Master Mode this bit can be used to
select the MCLK edge used to generate the DSD clock outputs.
Zero Detection
When the AD1955 detects that the audio input data is continu-
ously zero during 1024 LRCLK periods in PCM Mode or 8192
LRCLK periods in 8fS External Digital Filter Mode, ZEROL
(Pin 21) or ZEROR (Pin 20) is set to active.
When the AD1955 is in SACD Mode, it will detect an SACD
mute pattern. If the input bit stream shows a mute pattern for
about 22 ms, the AD1955 will set ZEROL (Pin 21) or ZEROR
(Pin 20) to active. The outputs can be set to active high or low
using Control Register 1, Bit 8.
Reset/Power-Down
The AD1955 will be reset when the
PD/RST pin is set low. The
part may be powered down using Bit 15, Control Register 0.
Audio Outputs
Active I/V converters should be used, which will hold the DAC
outputs at a constant voltage level. Passive I/V conversion should
not be used, since the DAC performance will be seriously degraded.
For best THD + N performance over temperature, a reference
voltage of 2.80 V should be used with the I/V converters. For a
lower parts count, the voltage at FILTR can be used. In this
instance, THD + N performance at high temperature can be
improved by reducing IREF, with an attendant reduction in gain
(linear dependence) and DNR/SNR (square-root dependence).
The AD1955 audio outputs sink a current proportional to the
input signal, superimposed on a steady bias current. The cur-
rent-to-voltage (I/V) converters used need to be able to supply
this bias current, as well as the signal current, or a resistor or
current source can be used to a positive voltage to null this
current in order to center the range of the I/V converters.
If pull-up resistors are used to bring the output of the I/V convert-
ers to 0 V for maximum headroom and THD balance, as shown
in the applications circuits, the following equation can be used:
RV
V
I
V
R
PULLUP
SUPPLY
BIAS
I V
=
[]
+
[]
–
()
/
PCM Mode
Allowable Master Clock Frequencies (
fS)Nominal Input
Interpolation Mode
64
96
128
192
256
384
512
768
Sample Rate (kHz)
48 kHz (INT
8×) Mode
32, 44.1, 48
96 kHz (INT
4×) Mode
88.2, 96
192 kHz (INT
2×) Mode
176.4, 192
External Filter Mode
Allowable Master Clock Frequencies (
fS)Nominal Input Sample Rate
Input Sample Rate
64
96
128
192
256
384
512
768
(to External Filter) (kHz)
8
× fS
32, 44.1, 48
4
× fS
88.2, 96
2
× f
S
176.4, 192