AD9644 Analog Devices, AD9644 Datasheet - Page 42

• Manufacturer Part Number: AD9644
• Description: 14-Bit, 80 MSPS/155 MSPS, 1.8V Dual, Serial Output A/D Converter
• Manufacturer: Analog Devices

Specifications of AD9644

• Resolution (bits): 14bit
• # Chan: 2
• Sample Rate: 155MSPS
• Analog Input Type: Diff-Bip
• Ain Range: 1.75 V p-p
• Adc Architecture: Pipelined

AD9644
APPLICATIONS INFORMATION
DESIGN GUIDELINES
Before starting design and layout of the AD9644 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 AD9644, 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
AD9644. With proper decoupling and smart partitioning of the
PCB analog, digital, and clock sections, optimum performance
is easily achieved.
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
AD9644 exposed paddle, Pin 0.
Rev. C | Page 42 of 44
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
the connection.
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
Manufacturing Guide for the Lead Frame Chip Scale Package
(LFCSP), at www.analog.com.
VCMA and VCMB
The VCMA and VCMB pins should be decoupled to ground
with a 0.1 μF capacitor, as shown in Figure 50.
SPI Port
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
AD9644 to keep these signals from transitioning at the converter
inputs during critical sampling periods.
AN-772
Application Note, A Design and
Data Sheet