EVAL-AD1939AZ Analog Devices Inc, EVAL-AD1939AZ Datasheet - Page 13

• Manufacturer Part Number: EVAL-AD1939AZ
• Description: Eval BD FOR MULTI CHANNEL 96KHz Codec
• Manufacturer: Analog Devices Inc

Specifications of EVAL-AD1939AZ

• Main Purpose: Audio, CODEC
• Utilized Ic / Part: AD1939
• Primary Attributes: 24-Bit, 192 kHz, 4 ADCs: 107dB Dynamic Range, 8 DACs: 112dB Dynamic Range
• Secondary Attributes: Time Division Multiplexed (TDM), SPI Interface
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Embedded: -
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

THEORY OF OPERATION
ANALOG-TO-DIGITAL CONVERTERS (ADCS)
There are four analog-to-digital converter (ADC) channels in
the AD1939 configured as two stereo pairs with differential
inputs. The ADCs can operate at a nominal sample rate of 48 kHz,
96 kHz, or 192 kHz. The ADCs include on-board digital anti-
aliasing filters with 79 dB stop-band attenuation and linear
phase response, operating at an oversampling ratio of 128
(48 kHz, 96 kHz, and 192 kHz modes). Digital outputs are
supplied through two serial data output pins (one for each
stereo pair) and a common frame clock (ALRCLK) and bit
clock (ABCLK). Alternatively, one of the TDM modes can be
used to access up to 16 channels on a single TDM data line.
The ADCs must be driven from a differential signal source for
best performance. The input pins of the ADCs connect to internal
switched capacitors. To isolate the external driving op amp from
the glitches caused by the internal switched capacitors, each in-
put pin should be isolated by using a series-connected external
100 Ω resistor together with a 1 nF capacitor connected from
each input to ground. This capacitor must be of high quality, for
example, ceramic NP0 or polypropylene film.
The differential inputs have a nominal common-mode voltage
of 1.5 V. The voltage at the common-mode reference pin (CM)
can be used to bias external op amps to buffer the input signals
(see the Power Supply and Voltage Reference section). The
inputs can also be ac-coupled and do not need an external dc
bias to CM.
A digital high-pass filter can be switched in line with the ADCs
under serial control to remove residual dc offsets. It has a 1.4 Hz,
6 dB per octave cutoff at a 48 kHz sample rate. The cutoff fre-
quency scales directly with sample frequency.
DIGITAL-TO-ANALOG CONVERTERS (DACS)
The AD1939 digital-to-analog converter (DAC) channels are
arranged as differential, four stereo pairs giving eight analog
outputs for improved noise and distortion performance. The
DACs include on-board digital reconstruction filters with 70 dB
stop-band attenuation and linear phase response, operating at an
oversampling ratio of 4 (48 kHz or 96 kHz modes) or 2 (192 kHz
mode). Each channel has its own independently programmable
attenuator, adjustable in 255 steps in increments of 0.375 dB.
Digital inputs are supplied through four serial data input pins
(one for each stereo pair) and a common frame clock (DLRCLK)
and bit clock (DBCLK). Alternatively, one of the TDM modes can
be used to access up to 16 channels on a single TDM data line.
Each output pin has a nominal common-mode dc level of 1.5 V
and swings ±1.27 V for a 0 dBFS digital input signal. A single op
amp, third-order, external, low-pass filter is recommended to
remove high frequency noise present on the output pins, as well
as to provide differential-to-single-ended conversion in the case
of the differential output. Note that the use of op amps with low
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slew rate or low bandwidth can cause high frequency noise and
tones to fold down into the audio band; exercise care in
selecting these components.
The voltage at CM, the common-mode reference pin, can be
used to bias the external op amps that buffer the output signals
(see the Power Supply and Voltage Reference section).
CLOCK SIGNALS
The on-chip phase-locked loop (PLL) can be selected to
reference the input sample rate from either of the LRCLK pins
or 256, 384, 512, or 768 times the sample rate, referenced to the
48 kHz mode from the MCLKI/XI pin. The default at power-up
is 256 × f
clock frequency stays at the same absolute frequency; therefore,
the actual multiplication rate is divided by 2. In 192 kHz mode,
the actual multiplication rate is divided by 4. For example, if a
device in the AD1939 family is programmed in 256 × f
frequency of the master clock input is 256 × 48 kHz = 12.288 MHz.
If the AD1939 is then switched to 96 kHz operation (by writing
to the SPI port), the frequency of the master clock should
remain at 12.288 MHz, which is 128 × f
192 kHz mode, this becomes 64 × f
The internal clock for the ADCs is 256 × f
The internal clock for the DACs varies by mode: 512 × f
mode), 256 × f
default, the on-board PLL generates this internal master clock
from an external clock. A direct 512 × f
mode) master clock can be used for either the ADCs or DACs if
selected in the PLL and Clock Control 1 register.
Note that it is not possible to use a direct clock for the ADCs set
to the 192 kHz mode. It is required that the on-chip PLL be
used in this mode.
The PLL can be powered down in the PLL and Clock Control 0
register. To ensure reliable locking when changing PLL modes,
or if the reference clock is unstable at power-on, power down
the PLL and then power it back up when the reference clock
stabilizes.
The internal master clock (MCLK) can be disabled in the PLL
and Clock Control 0 register to reduce power dissipation when
the AD1939 is idle. The clock should be stable before it is
enabled. Unless a standalone mode is selected (see the Serial
Control Port section), the clock is disabled by reset and must be
enabled by writing to the SPI port for normal operation.
To maintain the highest performance possible, limit the clock
jitter of the internal master clock signal to less than a 300 ps rms
time interval error (TIE). Even at these levels, extra noise or
tones can appear in the DAC outputs if the jitter spectrum
contains large spectral peaks. If the internal PLL is not used, it is
best to use an independent crystal oscillator to generate the
S
from the MCLKI/XI pin. In 96 kHz mode, the master
S
(96 kHz mode), or 128 × f
S
.
S
in this example. In
S
(referenced to 48 kHz
S
S
for all clock modes.
(192 kHz mode). By
S
AD1939
mode, the
S
(48 kHz