AD7490BCP datasheet(20/24 Pages) AD

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AD7490

REV. A

APPLICATION HINTS

Grounding and Layout

The AD7490 has very good immunity to noise on the power

supplies as can be seen by the PSRR vs. Supply Ripple Frequency

plot, TPC 3. However, care should still be taken with regard to

grounding and layout.

The printed circuit board that houses the AD7490 should be

designed such that 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 separated easily. A minimum

etch technique is generally best for ground planes as it gives the

best shielding. All three AGND pins of the AD7490 should be

sunk in the AGND plane. Digital and analog ground planes

should be joined at only one place. If the AD7490 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 which should be established as close as possible to

the AD7490.

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 AD7490 to avoid noise coupling. The power supply

lines to the AD7490 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 ground 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 important. All analog supplies should be

decoupled with 10

µF tantalum in parallel with 0.1 µF capacitors

to AGND. To achieve the best from these decoupling components,

they must be placed as close as possible to the device, ideally right

up against the device. The 0.1

µF capacitors should have low

Effective Series Resistance (ESR) and Effective Series Induc-

tance (ESI), such as the common ceramic types or surface mount

types, which provide a low impedance path to ground at high

frequencies to handle transient currents due to internal logic

switching.

PCB Design Guidelines For Chip Scale Package

The lands on the chip scale package (CP-32), are rectangular. The

printed circuit board pad for these should be 0.1 mm longer than

the package land length and 0.05 mm wider than the package

land width. The land should be centered on the pad. This will

ensure that the solder joint size is maximized. The bottom of the

chip scale package has a central thermal pad. The thermal pad

on the printed circuit board should be at least as large as this

exposed pad. On the printed circuit board, there should be a

clearance of at least 0.25 mm between the thermal pad and the

inner edges of the pad pattern. This will ensure that shorting is

avoided. Thermal vias may be used on the printed circuit board

thermal pad to improve thermal performance of the package. If vias

are used, they should be incorporated in the thermal pad at

1.2 mm pitch grid. The via diameter should be between 0.3 mm

and 0.33 mm and the via barrel should be plated with 1 oz. copper

to plug the via. The user should connect the printed circuit

board thermal pad to AGND.

Evaluating the AD7490 Performance

The recommended layout for the AD7490 is outlined in the

evaluation board for the AD7490. The evaluation board package

includes a fully assembled and tested evaluation board, docu-

mentation, 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 AD7490

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 AD7490.

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

and dc (histogram of codes) tests on the AD7490. The software and

documentation are on a CD shipped with the evaluation board.

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