ADAV803
Rev. A | Page 22 of 60
PLL SECTION
The ADAV803 features a dual PLL configuration to generate
independent system clocks for asynchronous operation.
Figure 37 shows the block diagram of the PLL section. The PLL
generates the internal and system clocks from a 27 MHz clock.
This clock is generated either by a crystal connected between
XIN and XOUT, as shown in
Figure 35, or from an external
clock source connected directly to XIN. A 54 MHz clock can
also be used, if the internal clock divider is used.
CC
0
47
56-
035
XTAL
XI
N
XO
U
T
Figure 35. Crystal Connection
Both PLLs (PLL1 and PLL2) can be programmed independently
and can accommodate a range of sampling rates (32 kHz
/44.1 kHz/48 kHz) with selectable system clock oversampling
rates of 256 and 384. Higher oversampling rates can also be
selected by enabling the doubling of the sampling rate to give
512 or 768 × fS ratios. Note that this option also allows
oversampling ratios of 256 or 384 at double sample rates of
64 kHz /88.2 kHz/96 kHz.
The PLL outputs can be routed internally to act as clock sources
for the other component blocks such as the ADC and DAC. The
outputs of the PLLs are also available on the three SYSCLK pins.
Figure 38 shows how the PLLs can be configured to provide the
sampling clocks.
Table 7. PLL Frequency Selection Options
MCLK Selection
PLL
Sample Rate, fS
(kHz)
Normal fS
Double fS
1
32/44.1/48
256/384 × fS
512/768 × fS
64/88.2/96
256/384 × fS
2A
32/44.1/48
256/384 × fS
512/768 × fS
64/88.2/96
256/384 × fS
2B
Same as fS selected
for PLL 2A
256/512 × fS
The PLLs require some external components to operate
correctly. These components, shown in
Figure 36, form a loop
filter that integrates the current pulses from a charge pump and
produces a voltage that is used to tune the VCO. Good quality
capacitors, such as PPS film, are recommended.
Figure 37shows a block diagram of the PLL section, including the master
clock selection.
Figure 38 shows how the clock frequencies at
the clock output pins, SYSCLK1 to SYSCLK3, and the internal
PLL clock values, PLL1 and PLL2, are selected.
The clock nodes, PLL1 and PLL2, can be used as master clocks
for the other blocks in the ADAV803, such as the DAC or ADC.
The PLL has separate supply and ground pins, which should be
as clean as possible to prevent electrical noise from being
converted into clock jitter by coupling onto the loop filter pins.
047
56-
03
6
PLL BLOCK
PLL_LFx
732
1.2F
AVDD
10nF
Figure 36. PLL Loop Filter
04
75
6-
0
37
MCLKI
MCLKO
XOUT
XIN
REG 0x74
BIT 5
REG 0x74
BIT 4
REG 0x78
BIT 6
REG 0x78
BIT 7
PHASE
DETECTOR
AND LOOP
FILTER
PLL_LF1
PLL1
SYSCLK1
SYSCLK2
SYSCLK3
PLL_LF2
÷2
VCO
÷N
OUTPUT
SCALER N1
PHASE
DETECTOR
AND LOOP
FILTER
PLL2
VCO
÷N
OUTPUT
SCALER N2
OUTPUT
SCALER N3
Figure 37. PLL Section Block Diagram