AD1928
Rev. B | Page 15 of 32
POWER SUPPLY AND VOLTAGE REFERENCE
The AD1928 is designed for 3.3 V supplies. Separate power
supply pins are provided for the analog and digital sections.
These pins should be bypassed with 100 nF ceramic chip
capacitors, as close to the pins as possible, to minimize noise
pickup. A bulk aluminum electrolytic capacitor of at least 22 μF
should also be provided on the same PC board as the codec. For
critical applications, improved performance is obtained with
separate supplies for the analog and digital sections. If this is
not possible, it is recommended that the analog and digital
supplies be isolated by means of a ferrite bead in series with
each supply. It is important that the analog supply be as clean
as possible.
All digital inputs are compatible with TTL and CMOS levels.
All outputs are driven from the 3.3 V DVDD supply and are
compatible with TTL and 3.3 V CMOS levels.
The ADC and DAC internal voltage reference (V
REF
) is brought
out on FILTR and should be bypassed as close as possible to the
chip, with a parallel combination of 10 μF and 100 nF. Any
external current drawn should be limited to less than 50 μA.
The internal reference can be disabled in the PLL and Clock
Control 1 register, and FILTR can be driven from an external
source. This can be used to scale the DAC output to the clipping
level of a power amplifier based on its power supply voltage.
The ADC input gain varies by the inverse ratio. The total gain
from ADC input to DAC output remains constant.
The CM pin is the internal common-mode reference. It should
be bypassed as close as possible to the chip, with a parallel
combination of 47 μF and 100 nF. This voltage can be used to
bias external op amps to the common-mode voltage of the input
and output signal pins. The output current should be limited to
less than 0.5 mA source and 2 mA sink.
SERIAL DATA PORTS—DATA FORMAT
The eight DAC channels use a common serial bit clock (DBCLK)
and a common left-right framing clock (DLRCLK) in the serial
data port. The two ADC channels use a common serial bit clock
(ABCLK) and left-right framing clock (ALRCLK) in the serial
data port. The clock signals are all synchronous with the sample
rate. The normal stereo serial modes are shown in Figure 23.
The ADC and DAC serial data modes default to I
2
S. The ports
can also be programmed for left-justified, right-justified, and
TDM modes. The word width is 24 bits by default and can be
programmed for 16 or 20 bits. The DAC serial formats are
programmable according to DAC Control 0 register. The
polarity of the DBCLK and DLRCLK is programmable according
to the DAC Control 1 register. The ADC serial formats and
serial clock polarity are programmable according to ADC
Control 1 register. Both DAC and ADC serial ports are
programmable to become the bus masters according to the
DAC Control 1 register and the ADC Control 2 register. By
default, both ADC and DAC serial ports are in slave mode.
TIME-DIVISION MULTIPLEXED (TDM) MODES
The AD1928 serial ports also have several different TDM serial
data modes. The first and most commonly used configurations
are shown in Figure 12 and Figure 13. In Figure 12, the ADC
serial port outputs one data stream consisting of two on-chip
ADCs and unused slots. In Figure 13, the eight on-chip DAC
data slots are packed into one TDM stream. In this mode, both
DBCLK and ABCLK are 256 f
S
.
The input/output pins of the serial ports are defined according
to the serial mode selected. For a detailed description of the
function of each pin in TDM and auxilliary modes, see
Table 12.
The AD1928 allows systems with more than eight DAC channels
to be easily configured by the use of an auxiliary serial data port.
The DAC TDM-AUX mode is shown in Figure 14. In this mode,
the AUX channels are the last four slots of the TDM data stream.
These slots are extracted and output to the AUX serial port.
Note that due to the high DBCLK frequency, this mode is available
only in the 48 kHz/44.1 kHz/32 kHz sample rate.
The AD1928 also allows system configurations with more than
two ADC channels, as shown in Figure 15 and Figure 16, which
show configurations using 6 ADCs and 14 ADCs, respectively.
Again, due to the high ABCLK frequency, this mode is available
only in the 48 kHz/44.1 kHz/32 kHz sample rate.
Combining the AUX ADC and DAC modes results in a system
configuration of 6 ADCs and 12 DACs. The system, then, con-
sists of two external stereo ADCs, two external stereo DACs,
and one AD1928. This mode is shown in Figure 17 (combined
AUX DAC and ADC modes).
SLOT 1 SLOT 2
SLOT 3
LEFT 1
SLOT 4
RIGHT 1
MSB MSB – 1 MSB – 2
DATA
BCLK
LRCLK
SLOT 5 SLOT 6 SLOT 7 SLOT 8
LRCL
BCLK
DATA
256 BCLKs
32 BCLKs
06623-012
Figure 12. ADC TDM (6-Channel I
2
S Mode)
SLOT 1
LEFT 1
SLOT 2
RIGHT 1
SLOT 3
LEFT 2
SLOT 4
RIGHT 2
MSB MSB – 1 MSB – 2 DATA
BCLK
LRCLK
SLOT 5
LEFT 3
SLOT 6
RIGHT 3
SLOT 7
LEFT 4
SLOT 8
RIGHT 4
LRCL
BCLK
DATA
256 BCLKs
32 BCLKs
06623-013
Figure 13. DAC TDM (8-Channel I
2
S Mode)
