REV. B

AD7663

–21–

Because the Serial Port within the ADSP-21065L will be seeing a

discontinuous clock, an initial word reading has to be done after

the ADSP-21065L has been reset to ensure that the Serial Port

is properly synchronized to this clock during each following data

read operation.

RFS

ADSP-21065L*

SHARC

CNVST

AD7663*

CS

SYNC

RD

DR

RCLK

FLAG OR TFS

SDOUT

SCLK

INVSYNC

INVSCLK

EXT/

INT

RDC/SDIN

SER/

PAR

DVDD

*ADDITIONAL PINS OMITTED FOR CLARITY

Figure 23. Interfacing to the ADSP-21065L Using the

Serial Master Mode

APPLICATION HINTS

Layout

The AD7663 has very good immunity to noise on the power

supplies as can be seen in Figure 9. However, care should still be

taken with regard to grounding layout.

The printed circuit board that houses the AD7663 should be

designed so the analog and digital sections are separated and con-

fined to certain areas of the board. This facilitates the use of

ground planes that can be easily separated. Digital and analog

ground planes should be joined in only one place, preferably

underneath the AD7663 or at least as close as possible to the

AD7663. If the AD7663 is in a system where multiple devices

require analog-to-digital ground connections, the connection

should still be made at one point only, a star ground point that

should be established as close as possible to the AD7663.

It is recommended to avoid running digital lines under the device

as these will couple noise onto the die. The analog ground plane

should be allowed to run under the AD7663 to avoid noise cou-

pling. Fast switching signals like

CNVST or clocks should be

shielded with digital ground to avoid radiating noise to other

sections of the board and should never run near analog signal paths.

Crossover of digital and analog signals should be avoided. Traces

on different but close layers of the board should run at right

angles to each other. This will reduce the effect of feedthrough

through the board.

The power supply lines to the AD7663 should use as large a trace

as possible to provide low impedance paths and reduce the effect of

glitches on the power supply lines. Good decoupling is also impor-

tant to lower the supplies’ impedance presented to the AD7663

and to reduce the magnitude of the supply spikes. Decoupling

ceramic capacitors, typically 100 nF, should be placed on all of

the power supply pins AVDD, DVDD, and OVDD close to, and

ideally right up against these pins and their corresponding ground

pins. Additionally, low ESR 10 µF capacitors should be located

in the vicinity of the ADC to further reduce low frequency ripple.

The DVDD supply of the AD7663 can be either a separate

supply or come from the analog supply, AVDD, or from the

digital interface supply, OVDD. When the system digital supply

is noisy or fast switching digital signals are present, it is recom-

mended, if no separate supply is available, to connect the DVDD

digital supply to the analog supply, AVDD, through an RC filter

as shown in Figure 5, and to connect the system supply to the

interface digital supply OVDD and the remaining digital circuitry.

When DVDD is powered from the system supply, it is useful to

insert a bead to further reduce high frequency spikes.

The AD7663 has five different ground pins: INGND, REFGND,

AGND, DGND, and OGND. INGND is used to sense the

analog input signal. REFGND senses the reference voltage and

should be a low impedance return to the reference because it

carries pulsed currents. AGND is the ground to which most internal

ADC analog signals are referenced. This ground must be con-

nected with the least resistance to the analog ground plane. DGND

must be tied to the analog or digital ground plane depending on

the configuration. OGND is connected to the digital system ground.

The layout of the decoupling of the reference voltage is important.

The decoupling capacitor should be close to the ADC and con-

nected with short and large traces to minimize parasitic inductances.

Evaluating the AD7663 Performance

A recommended layout for the AD7663 is outlined in the evalu-

ation board for the AD7663. The evaluation board package includes

a fully assembled and tested evaluation board, documentation,

and software for controlling the board from a PC via the Eval-

Control Board.