Before starting design and layout of the AD9648 as a system,
it is recommended that the designer become familiar with these
guidelines, which discuss the special circuit connections and
layout requirements that are needed for certain pins.
Power and Ground Recommendations
When connecting power to the AD9648, it is recommended that
two separate 1.8 V supplies be used. Use one supply for analog
(AVDD); use a separate supply for the digital outputs (DRVDD).
For both AVDD and DRVDD several different decoupling capa-
citors should be used to cover both high and low frequencies.
Place these capacitors close to the point of entry at the PCB level
and close to the pins of the part, with minimal trace length.
A single PCB ground plane should be sufficient when using the
AD9648. With proper decoupling and smart partitioning of the
PCB analog, digital, and clock sections, optimum performance
is easily achieved.
The AD9648 defaults to CMOS output mode on power-up.
If LVDS operation is desired, this mode must be programmed,
using the SPI configuration registers after power-up. When the
AD9648 powers up in CMOS mode with LVDS termination
resistors (100 Ω) on the outputs, the DRVDD current can be
higher than the typical value until the part is placed in LVDS
mode. This additional DRVDD current does not cause damage
to the AD9648, but it should be taken into account when consid-
ering the maximum DRVDD current for the part.
To avoid this additional DRVDD current, the AD9648 outputs
can be disabled at power-up by taking the PDWN pin high.
After the part is placed into LVDS mode via the SPI port, the
PDWN pin can be taken low to enable the outputs.
Exposed Paddle Thermal Heat Slug Recommendations
It is mandatory that the exposed paddle on the underside of the
ADC be connected to analog ground (AGND) to achieve the
best electrical and thermal performance. A continuous, exposed
(no solder mask) copper plane on the PCB should mate to the
AD9648 exposed paddle, Pin 0.
The copper plane should have several vias to achieve the lowest
possible resistive thermal path for heat dissipation to flow through
the bottom of the PCB. These vias should be filled or plugged to
prevent solder wicking through the vias, which can compromise
To maximize the coverage and adhesion between the ADC and
the PCB, a silkscreen should be overlaid to partition the continuous
plane on the PCB into several uniform sections. This provides
several tie points between the ADC and the PCB during the reflow
process. Using one continuous plane with no partitions guarantees
only one tie point between the ADC and the PCB. For detailed
information about packaging and PCB layout of chip scale
packages, see the AN-772 Application Note, A Design and
Manufacturing Guide for the Lead Frame Chip Scale Package
(LFCSP), at www.analog.com.
The VCM pin should be decoupled to ground with a 0.1 μF
The VREF pin should be externally decoupled to ground with
a low ESR, 1.0 μF capacitor in parallel with a low ESR, 0.1 μF
The SPI port should not be active during periods when the full
dynamic performance of the converter is required. Because the
SCLK, CSB, and SDIO signals are typically asynchronous to the
ADC clock, noise from these signals can degrade converter
performance. If the on-board SPI bus is used for other devices,
it may be necessary to provide buffers between this bus and the
AD9648 to keep these signals from transitioning at the converter
inputs during critical sampling periods.
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