AD7665ASTRL datasheet(21/24 Pages) AD

21 / 24 page

REV. 0

AD7665

–21–

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

AD7665*

SYNC

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 AD7665 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 AD7665 should be

designed so the analog and digital sections are separated and

confined 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 AD7665, or, at least, as close as possible to the

AD7665. If the AD7665 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,

which should be established as close as possible to the AD7665.

It is recommended to avoid running digital lines under the de-

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

plane should be allowed to run under the AD7665 to avoid

noise coupling. 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 AD7665 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 important to lower the supplies impedance presented to the

AD7665 and reduce the magnitude of the supply spikes. Decoupling

ceramic capacitors, typically 100 nF, should be placed on each

power supplies 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 fre-

quency ripple.

The DVDD supply of the AD7665 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 available, to connect the DVDD

digital supply to the analog supply AVDD through an RC filter

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

interface digital supply OVDD and the remaining digital cir-

cuitry. When DVDD is powered from the system supply, it is

useful to insert a bead to further reduce high-frequency spikes.

The AD7665 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 connected 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 impor-

tant. The decoupling capacitor should be close to the ADC and

connected with short and large traces to minimize parasitic

inductances.

Evaluating the AD7665 Performance

A recommended layout for the AD7665 is outlined in the evalu-

ation board for the AD7665. The evaluation board package

includes a fully assembled and tested evaluation board, docu-

mentation, and software for controlling the board from a PC

via the Eval-Control Board.

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