AD6654/PCB Analog Devices Inc, AD6654/PCB Datasheet - Page 85

• Manufacturer Part Number: AD6654/PCB
• Description: BOARD EVALUATION FOR AD6654
• Manufacturer: Analog Devices Inc

Specifications of AD6654/PCB

• Module/board Type: Evaluation Board
• For Use With/related Products: AD6554
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

DDC DESIGN NOTES
The following guidelines describe circuit connections, layout
requirements, and programming procedures for the AD6654.
The designer should review these guidelines before starting the
system design and layout.
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The AD6654 requires the following power-up sequence.
The VDDCORE (1.8 V) is required to settle into nominal
voltage levels before the VDDIO attains the minimum.
The DDC input clock (CLK) and input (EXP) pins EXP[2:0]
are not 5 V tolerant. Care should be taken to drive these
pins within the limits of VDDIO (3.0 V to 3.6 V). This is
easily accomplished by using the ADC data ready (DR)
output to drive the DDC clock (CLK) input.
The number format used in this part is twos complement.
All input ports and output ports use twos complement data
format. The formats for individual internal registers are
given in the memory map description of these registers.
In both microport and serial port operation, the DTACK
(RDY, SDO) and IRP pins are open-drain outputs and
should be terminated externally to VDDIO through a 1 kΩ
pull-up resistor.
A simple RC circuit is used on the EXT_FILTER pin to
balance the internal RC circuit on this pin and maintain a
good PLL clock lock. The recommended circuit is given in
Figure 76. It is further recommended that this RC circuit be
placed as close as possible to the AD6654 part. This layout
consideration ensures that the PLL clock is clean and the
PLL lock is closely maintained.
Figure 75. DTACK , SDO, IRP Pull-Up Resistor Circuit
3.3V
VDD CORE
Figure 76. EXT_FILTER Circuit for PLL Clock
1kΩ
3.3V
0.01µF
250Ω
1kΩ
EXT_FILTER
DTACK (RDY, SDO)
IRP
AD6654
AD6654
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By default, the PLL CLK is disabled. It can be enabled by
programming the PLL multiplier and divider bits in the
ADC CLK control register. When the PLL CLK is enabled
by programming this register, it takes about 50 µs to 200 µs
to settle down. While the PLL loop settles down, the voltage
at the EXT_FILTER pin increases from 0 V to VDDCORE
(1.8 V) and settles there. Channel registers and output port
registers (Address 0x68 to Address 0xE7) should not be pro-
grammed before the PLL loop settles down.
To reset the AD6654, the user must provide a minimum
pulse of 30 ns to the RESET pin. The RESET pin should be
connected to GND (or pulled low) during power-up of the
part. The RESET pin can be pulled high after the power
supplies have settled to nominal values (1.8 V and
3.3 V). At this point, a pulse (pull low and high again)
should be provided to give a reset to the part.
The CPUCLK (SCLK) is the clock used for programming
via the microport (serial port). This clock needs to be
provided by the designer to the part (slave clock). The
designer should ensure that this clock’s frequency is less
than or equal to the frequency of the CLK signal. Addi-
tionally, the frequency of the CPUCLK (SCLK) should
always be less than 100 MHz.
The microport data bus is 16 bits wide. Both 8-bit and
16-bit modes are available using this part. If 8-bit mode is
used, the MSB of the data bus (D[15:8]) can be left floating
or connected to GND.
The output parallel port has a 1-clock cycle overhead. If two
channels (with the same data rates) are output on one
output port in 16-bit interleaved I/Q mode along with an
AGC word, this requires three clock cycles for one sample
from each channel (one clock each for I data, Q data, and
gain data). Therefore, the total number of clock cycles
required to output the data is 3 clocks/channel × 2 channels
+ 1 (overhead) = 7 clock cycles. The number of clock cycles
required for each channel can be 3 (interleaved I + Q + gain
word), or 2 (parallel I /Q + gain) or 2 (interleaved I + Q) or
1 (interleaved I/Q). Designers should make sure that suffi-
cient time is allowed to output these channels on one output
port. Also note that the I, Q, and gain for a particular
channel come out on a single output port and cannot be
divided among output ports.
When CRCF and DRCF filters are disabled, the coefficient
memory cannot be read back, because the clock to the
coefficient RAM is also cut off.
AD6654