AD73311ARS datasheet(26/36 Pages) AD

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AD73311

–26–

REV. B

Cascade Operation

Where it is required to configure a cascade of up to eight de-

vices, it is necessary to ensure that the timing of the SE and

RESET signals is synchronized at each device in the cascade. A

simple D type flip flop is sufficient to sync each signal to the

master clock MCLK, as in Figure 30.

1/2

74HC74

CLK

DSP CONTROL

TO SE

MCLK

SE SIGNAL SYNCHRONIZED

TO MCLK

DSP CONTROL

RESET

MCLK

RESET SIGNAL SYNCHRONIZED

TO MCLK

1/2

74HC74

CLK

Figure 30. SE and

RESET Sync Circuit for Cascaded

Operation

Connection of a cascade of devices to a DSP, as shown in Fig-

ure 31, is no more complicated than connecting a single device.

Instead of connecting the SDO and SDOFS to the DSP’s Rx

port, these are now daisy-chained to the SDI and SDIFS of the

next device in the cascade. The SDO and SDOFS of the final

device in the cascade are connected to the DSP’s Rx port to

complete the cascade. SE and

RESETB on all devices are fed

from the signals that were synchronized with the MCLK using

the circuit as described above. The SCLK from only one device

need be connected to the DSP’s SCLK input(s) as all devices

will be running at the same SCLK frequency and phase.

SDIFS

SDI

SCLK

SDO

SDOFS

TFS

SCLK

RFS

MCLK

RESET

DEVICE 1

FL0

FL1

SDIFS

SDI

SCLK

SDO

SDOFS

MCLK

RESET

DEVICE 2

74HC74

CLK

AD73311

CODEC

AD73311

CODEC

ADSP-2181

DSP

Figure 31. Connection of Two AD73311s Cascaded to

ADSP-2181

Grounding and Layout

Since the analog inputs to the AD73311 are differential, most of

the voltages in the analog modulator are common-mode volt-

ages. The excellent common-mode rejection of the part will

remove common-mode noise on these inputs. The analog and

digital supplies of the AD73311 are independent and separately

pinned out to minimize coupling between analog and digital

sections of the device. The digital filters on the encoder section

will provide rejection of broadband noise on the power supplies,

except at integer multiples of the modulator sampling frequency.

The digital filters also remove noise from the analog inputs

provided the noise source does not saturate the analog modula-

tor. However, because the resolution of the AD73311’s ADC is

high, and the noise levels from the AD73311 are so low, care

must be taken with regard to grounding and layout.

The printed circuit board that houses the AD73311 should be

designed so the analog and digital sections are separated and

confined to certain sections of the board. The AD73311 pin

configuration offers a major advantage in that its analog and

digital interfaces are connected on opposite sides of the package.

This facilitates the use of ground planes that can be easily sepa-

rated, as shown in Figure 32. A minimum etch technique is

generally best for ground planes as it gives the best shielding.

Digital and analog ground planes should be joined in only one

place. If this connection is close to the device, it is recom-

mended to use a ferrite bead inductor as shown in Figure 32.

DIGITAL GROUND

ANALOG GROUND

Figure 32. Ground Plane Layout

Avoid running digital lines under the device for they will couple

noise onto the die. The analog ground plane should be allowed

to run under the AD73311 to avoid noise coupling. The power

supply lines to the AD73311 should use as large a trace as pos-

sible to provide low impedance paths and reduce the effects of

glitches on the power supply lines. Fast switching signals such as

clocks should be shielded with digital ground to avoid radiating

noise to other sections of the board, and clock signals should

never be run near the analog inputs. Traces on opposite sides of

the board should run at right angles to each other. This will

reduce the effects of feedthrough through the board. A microstrip

technique is by far the best but is not always possible with a

double-sided board. In this technique, the component side of

the board is dedicated to ground planes while signals are placed

on the other side.

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