AD1857 Просмотр технического описания (PDF) - Analog Devices
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AD1857 Datasheet PDF : 16 Pages
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AD1857/AD1858
LRCLK
INPUT
BCLK
INPUT
SDATA
INPUT
LEFT CHANNEL
RIGHT CHANNEL
MSB MSB-1
LSB+2 LSB+1 LSB
MSB MSB-1
LSB+2 LSB+1 LSB
Figure 12. AD1858 Left-Justified DSP Serial Port Style
MSB MSB-1
LRCLK
INPUT
BCLK
INPUT
SDATA
INPUT
LEFT CHANNEL
LSB MSB MSB-1 MSB-2
RIGHT CHANNEL
LSB+2 LSB+1 LSB MSB MSB-1 MSB-2
LSB+2 LSB+1 LSB MSB MSB-1
Figure 13. AD1857/AD1858 32 ؋ FS Packed Mode
Figure 12 shows the AD1858 left-justified DSP serial port style
mode. LRCLK must pulse HI for at least one bit clock period
before the MSB of the left channel is valid, and LRCLK must
pulse HI again for at least one bit clock period before the MSB
of the right channel is valid. Data is valid on the falling edge of
BCLK. Note that in this mode, it is the responsibility of the DSP
to ensure that the left data is transmitted with the first LRCLK
pulse, the right data is transmitted with the second LRCLK pulse,
and synchronism is maintained from that point forward.
Note that in 16-bit input mode, the AD1857/AD1858 are
capable of a 32 × FS BCLK frequency “packed mode” where
the MSB is left-justified to an LRCLK transition, and the LSB
is right-justified to an LRCLK transition. LRCLK is HI for the
left channel, and LO for the right channel. Data is valid on the
rising edge of BCLK. Packed mode can be used when the
AD1857 is programmed in left-justified mode, or when the
AD1858 is programmed in right-justified mode. Packed mode
is shown in Figure 13.
Master Clock
The synchronous master clock of the AD1857/AD1858 is
supplied by an external clock source applied to MCLK. Figure
14 shows example connections. Do not change the state of the
384/256 pin while the AD1857/AD1858 is operational; this pin
should be hardwired LO or HI. Alternatively, its state may be
changed while the PD/RST pin is asserted LO.
MCLK FREQUENCY
12.288MHz
11.2896MHz
8.192MHz
18.432MHz
16.9344MHz
12.288MHz
256 MODE 384 MODE
384/256 = LO 384/256 = HI
SAMPLE RATE
48kHz
44.1kHz
32kHz
1
MCLK
6
384/256
Figure 14. AD1857/AD1858 Clock Connections
Digital Mute
The AD1857/AD1858 offer a control pin that mutes the analog
output. By asserting the MUTE (Pin 15) signal HI, both the
left channel and the right channel are muted. The AD1857/
AD1858 have been designed to minimize pops and clicks when
muting and unmuting the device. The AD1857/AD1858
include a zero crossing detector which attempts to implement
mute on waveform zero crossings only. If a zero crossing is not
found within 1024 input sample periods (approximately 23
milliseconds at 44.1 kHz), the output is muted regardless.
Output Drive, Buffering and Loading
The AD1857/AD1858 analog output stage is able to drive a 2 kΩ
load. If lower impedance loads must be driven, an external
buffer stage such as the Analog Devices SSM2142 should be
used. The analog output is generally ac coupled with a 10 µF
capacitor as shown in Figure 21. It is possible to dc couple the
AD1857/AD1858 output into an op amp stage using the
CMOUT signal as a bias point.
On-Chip Voltage Reference
The AD1857/AD1858 include an on-chip voltage reference that
establishes the output voltage range. The nominal value of this
reference is +2.25 V, which corresponds to a line output voltage
swing of 3 V p-p. The line output signal is centered around a
voltage established by the CMOUT (common-mode output)
(Pin 10). The reference must be bypassed both on the FILT
input (Pin 11) with 10 µF and 0.1 µF capacitors, and on the
CMOUT output (Pin 10) with 10 µF and 0.1 µF capacitors, as
shown in Figure 21. Both the FILT pin and the CMOUT pin
use the AGND ground. The on-chip voltage reference may be
overdriven with an external reference source by applying this
voltage to the FILT pin. CMOUT and FILT must still be
bypassed as shown in Figure 21. An external reference can be
useful to calibrate multiple AD1857/AD1858 DACs to the same
gain. Reference bypass capacitors larger than those suggested
can be used to improve the signal-to-noise performance of the
AD1857/AD1858.
Power-Down and Reset
The PD/RST input (Pin 2) is used to control the power consumed
by the AD1857/AD1858. When PD/RST is held LO, the
AD1857/AD1858 are placed in a low dissipation power-down
state. When PD/RST is brought HI, the AD1857/AD1858
become ready for normal operation. The master clock (MCLK,
Pin 1) must be running for a successful reset or power-down
operation to occur. The PD/RST signal must be LO for a
minimum of four master clock periods (326 ns with a 12.288 MHz
MCLK frequency).
When the PD/RST input (Pin 2) is brought HI, the AD1857/
AD1858 are reset. All registers in the AD1857/AD1858 digital
engine (serial data port, interpolation filter and modulator) are
zeroed, and the amplifiers in the analog section are shorted
during the reset operation. The AD1857/AD1858 have been
designed to minimize pops and clicks when entering and exiting
the power-down state.
REV. 0
–11–
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