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AD9284

Rev. 0 | Page 17 of 24

HARDWARE INTERFACE

The pins described in Table 9 constitute the physical interface

between the programming device of the user and the serial port

of the AD9284. The SCLK and CSB pins function as inputs

when using the SPI interface. The SDIO pin is bidirectional,

functioning as an input during write phases and as an output

during readback.

The SPI interface is flexible enough to be controlled by either

FPGAs or microcontrollers. One method for SPI configuration

is described in detail in the AN-812 Application Note, Micro-

controller-Based Serial Port Interface (SPI) Boot Circuit.

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

AD9284 to prevent these signals from transitioning at the converter

inputs during critical sampling periods.

SDIO/PWDN serves a dual function when the SPI interface is

not being used. When the pin is strapped to AVDD or ground

during device power-on, it is associated with a specific function.

The mode selection table (see Table 10) describes the strappable

functions that are supported on the AD9284.

Table 10. Mode Selection

External Voltage

Configuration

AVDD (default)

Chip in full power-down

SDIO/PWDN

AGND

Normal operation

AVDD

Outputs in high impedance

AGND (default)

Outputs enabled

CONFIGURATION WITHOUT THE SPI

In applications that do not interface to the SPI control registers,

the SDIO/PWDN pin serves as a standalone, CMOS-compatible

control pin. When the device is powered up, it is assumed that

the user intends to use the SDIO, SCLK, and CSB pins as static

control lines for the output enable and power-down feature control.

In this mode, connecting the CSB chip select to AVDD disables

the serial port interface.

SPI ACCESSIBLE FEATURES

Table 11 provides a brief description of the general features that

are accessible via the SPI. These features are described in detail

in the AN-877 Application Note, Interfacing to High Speed ADCs

via SPI. The AD9284 part-specific features are described in

detail in Table 12.

Table 11. Features Accessible Using the SPI

Feature

Description

Mode

Allows the user to set either power-down mode

or standby mode

Clock

Allows the user to access the DCS via the SPI

Offset

Allows the user to digitally adjust the converter

offset

Test I/O

Allows the user to set test modes to have known

data on output bits

Output Mode

Allows the user to set up outputs

Output Phase

Allows the user to set the output clock polarity

Output Delay

Allows the user to vary the DCO delay

Voltage

Reference

Allows the user to set the voltage reference

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