AD9626 datasheet(23/37 Pages) AD

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AD9626

Rev. 0 | Page 22 of 36

LAYOUT CONSIDERATIONS

POWER AND GROUND RECOMMENDATIONS

When connecting power to the AD9626, it is recommended

that two separate supplies be used: one for analog (AVDD, 1.8 V

nominal) and one for digital (DRVDD, 1.8 V nominal). If only a

single 1.8 V supply is available, it is routed to AVDD first, then

tapped off and isolated with a ferrite bead or filter choke with

decoupling capacitors proceeding connection to DRVDD. The

user can employ several different decoupling capacitors to cover

both high and low frequencies. These should be located close to

the point of entry at the PC board level and close to the parts

with minimal trace length.

A single PCB ground plane is sufficient when using the

AD9626. With proper decoupling and smart partitioning of

analog, digital, and clock sections of the PCB, optimum

performance is easily achieved.

Exposed Paddle Thermal Heat Slug Recommendations

It is required that the exposed paddle on the underside of the

ADC be connected to analog ground (AGND) to achieve the

best electrical and thermal performance of the AD9626. An

exposed, continuous copper plane on the PCB should mate to

the AD9626 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 solder-filled or plugged.

To maximize the coverage and adhesion between the ADC and

PCB, partition the continuous plane by overlaying a silkscreen

on the PCB into several uniform sections. This provides several

tie points between the two during the reflow process. Using one

continuous plane with no partitions guarantees only one tie

point between the ADC and PCB. See Figure 52 for a PCB layout

example. For detailed information on packaging and the PCB

layout of chip scale packages, see Application Note AN-772,

A Design and Manufacturing Guide for the Lead Frame Chip

Scale Package.

SILKSCREEN PARTITION

PIN 1 INDICATOR

Figure 52. Typical PCB Layout

CML

The CML pin should be decoupled to ground with a 0.1 μF

capacitor, as shown in Figure 54.

RBIAS

The AD9626 requires the user to place a 10 kΩ resistor between

the RBIAS pin and ground. This resistor sets the master current

reference of the ADC core and should have at least a 1% tolerance.

AD9626 CONFIGURATION USING THE SPI

The AD9626 SPI allows the user to configure the converter for

specific functions or operations through a structured register

space inside the ADC. This gives the user added flexibility to

customize device operation depending on the application.

Addresses are accessed (programmed or read back) serially in

one-byte words. Each byte can be further divided down into

fields, which are documented in the Memory Map section.

There are three pins that define the serial port interface or SPI

to this particular ADC. They are the SPI SCLK/DFS, SPI

SDIO/DCS, and CSB pins. The SCLK/DFS (serial clock) is used

to synchronize the read and write data presented the ADC. The

SDIO/DCS (serial data input/output) is a dual-purpose pin that

allows data to be sent and read from the internal ADC memory

map registers. The CSB is an active low control that enables or

disables the read and write cycles (see Table 8).

Table 8. Serial Port Pins

Mnemonic

Function

SCLK

SCLK (Serial Clock) is the serial shift clock in.

SCLK is used to synchronize serial interface

reads and writes.

SDIO

SDIO (Serial Data Input/Output) is a dual-purpose

pin. The typical role for this pin is an input and

output depending on the instruction being sent

and the relative position in the timing frame.

CSB

CSB (Chip Select Bar) is an active low control that

gates the read and write cycles.

RESET

Master Device Reset. When asserted, device

assumes default settings. Active low.

The falling edge of the CSB, in conjunction with the rising edge

of the SCLK, determines the start of the framing. An example of

the serial timing and its definitions can be found in Figure 53

and Table 10.

During an instruction phase, a 16-bit instruction is transmitted.

Data then follows the instruction phase and is determined by

the W0 and W1 bits, which is 1 or more bytes of data. All data is

composed of 8-bit words. The first bit of each individual byte of

serial data indicates whether this is a read or write command.

This allows the serial data input/output (SDIO) pin to change

direction from an input to an output.

Data can be sent in MSB or in LSB first mode. MSB first is

default on power-up and can be changed by changing the

configuration register. For more information about this feature

and others, see Interfacing to High Speed ADCs via SPI at

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