AD7679 datasheet(21/24 Pages) AD

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AD7671

–21–

RFS

ADSP-21065L*

SHARC

CNVST

AD7671*

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 AD7671 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 AD7671 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 AD7671, or, at least, as close as possible to the AD7671. If

the AD7671 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 AD7671.

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 AD7671 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 AD7671 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 AD7671

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 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

capacitors should be located in the vicinity of the ADC to further

reduce low frequency ripple.

The DVDD supply of the AD7671 can be either a separate sup-

ply 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 recommended,

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 AD7671 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 important.

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

nected with short and large traces to minimize parasitic inductances.

Evaluating the AD7671 Performance

A recommended layout for the AD7671 is outlined in the evalua-

tion board for the AD7671. 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.

REV. C

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