AD7477AARM datasheet(21/24 Pages) AD

21 / 24 page

REV. C

AD7476A/AD7477A/AD7478A

–21–

To summarize,

MOD = 0

SYN = 1

WL2, WL1, and WL0 depend on the word length

FSL1 = 1 and FSL0 = 0

FSP = 1, Negative Frame Sync

SCD2 = 1

SCKD = 1

SHFD = 0

It should be noted that for signal processing applications, it is

imperative that the frame synchronization signal from the DSP563xx

will provide equidistant sampling.

AD7476A/

AD7477A

AD7478A*

SDATA

SCLK

DSP563xx*

SCK

SRD

SC2

*ADDITIONAL PINS OMITTED FOR CLARITY

Figure 19. Interfacing to the DSP563xx

APPLICATION HINTS

Grounding and Layout

The printed circuit board that houses the AD7476A/AD7477A/

AD7478A should be designed such that the analog and digi-

tal sections are separated and confined to certain areas of the

board. This facilitates the use of ground planes that can be

separated easily. A minimum etch technique is generally best

for ground planes because it gives the best shielding. Digital and

analog ground planes should be joined at only one place. If the

AD7476A/AD7477A/AD7478A is in a system where multiple

devices require an AGND to DGND connection, the connection

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

should be established as close as possible to the AD7476A/

AD7477A/AD7478A.

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 AD7476A/AD7477A/AD7478A to avoid noise

coupling. The power supply lines to the AD7476A/AD7477A/

AD7478A should use as large a trace as possible to provide low

impedance paths and reduce the effects of glitches on the power

supply line. Fast switching signals like clocks should be shielded

with digital grounds to avoid radiating noise to other sections of

the board, and clock signals should never be run near the analog

inputs. Avoid crossover of digital and analog signals. 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 solder side.

Good decoupling is also very important. The supply should be

decoupled with, for instance, a 680 nF 0805 to GND. When using

the SC70 package in applications where the size of the compo-

nents is of concern, a 220 nF 0603 capacitor, for example, could

be used instead. However, in that case, the decoupling may not be

as effective and may result in an approximate SINAD degradation

of 0.3 dB. To achieve the best performance from these decoupling

components, the user should endeavor to keep the distance

between the decoupling capacitor and the VDD and GND pins to a

minimum with short track lengths connecting the respective pins.

Figures 20 and 21 show the recommended positions of the decou-

pling capacitor for the MSOP and SC70 packages, respectively.

As can be seen in Figure 20, for the MSOP package, the decoup-

ling capacitor has been placed as close as possible to the IC with

short track lengths to VDD and GND pins. The decoupling capacitor

could also be placed on the underside of the PCB directly under-

neath the IC, between the VDD and GND pins attached by vias.

This method is not recommended on PCBs above a standard

1.6 mm thickness. The best performance will be seen with the

decoupling capacitor on the top of the PCB next to the IC.

Figure 20. Recommended Supply Decoupling Scheme

for the AD7476A/AD7477A/AD7478A MSOP Package

Similarly, for the SC70 package, the decoupling capacitor

should be located as close as possible to the VDD and the GND

pins. Because of its pinout, i.e., VDD being next to GND, the

decoupling capacitor can be placed extremely close to the IC.

The decoupling capacitor could be placed on the underside of

the PCB directly under the VDD and GND pins, but as before,

the best performance will be achieved with the decoupling

capacitor on the same side as the IC.

Figure 21. Recommended Supply Decoupling Scheme

for the AD7476A/AD7477A/AD7478A SC70 Package

Evaluating the AD7476A/AD7477A Performance

The evaluation board package includes a fully assembled and tested

evaluation board, documentation, and software for controlling

the board from the PC via the EVAL-BOARD CONTROLLER.

The EVAL-BOARD CONTROLLER can be used in conjunction

with the AD7476ACB/AD7477ACB evaluation board, as well

as many other Analog Devices evaluation boards ending in the

CB designator, to demonstrate/evaluate the ac and dc performance

of the AD7476A/AD7477A.

The software allows the user to perform ac (fast Fourier transform)

and dc (histogram of codes) tests on the AD7476A/AD7477A. See

the evaluation board application note for more information.

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