AD7854LARS datasheet(26/28 Pages) AD

26 / 28 page

AD7854/AD7854L

–26–

REV. B

APPLICATION HINTS

Grounding and Layout

The analog and digital supplies of the AD7854/AD7854L are

independent and separately pinned out to minimize coupling

between the analog and digital sections of the device. The part

has very good immunity to noise on the power supplies as can

be seen by the PSRR versus frequency graph. However, care

should still be taken with regard to grounding and layout.

The printed circuit board on which the AD7854/AD7854L is

mounted 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

easily separated. A minimum etch technique is generally best

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

analog ground planes should only be joined in one place. If

the AD7854/AD7854L is the only device requiring an AGND

to DGND connection, then the ground planes should be

connected at the AGND and DGND pins of the AD7854/

AD7854L. If the AD7854/AD7854L is in a system where

multiple devices require AGND to DGND 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

AD7854/AD7854L.

Avoid running digital lines under the device as these couple

noise onto the die. The analog ground plane should be allowed

to run under the AD7854/AD7854L to avoid noise coupling.

The power supply lines to the AD7854/AD7854L 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 and the data inputs 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 reduces 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 a 10

µF tantalum capacitor in parallel with

0.1

µF disc ceramic capacitor to AGND. All digital supplies

should have a 0.1

µF disc ceramic capacitor to DGND. To

achieve the best performance from these decoupling compo-

nents, they must be placed as close as possible to the device,

ideally right up against the device. In systems where a common

supply voltage is used to drive both the AVDD and DVDD of the

AD7854/AD7854L, it is recommended that the system’s AVDD

supply is used. In this case an optional 10

Ω resistor between

the AVDD pin and DVDD pin can help to filter noise from digital

circuitry. This supply should have the recommended analog

supply decoupling capacitors between the AVDD pin of the

AD7854/AD7854L and AGND and the recommended digital

supply decoupling capacitor between the DVDD pin of the

AD7854/AD7854L and DGND.

Evaluating the AD7854/AD7854L Performance

The recommended layout for the AD7854/AD7854L is outlined

in the evaluation board for the AD7854/AD7854L. The evalua-

tion board package includes a fully assembled and tested

evaluation board, documentation, and software for controlling

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

The EVAL-CONTROL BOARD can be used in conjunction

with the AD7854/AD7854L Evaluation board, as well as many

other Analog Devices evaluation boards ending in the CB desig-

nator, to demonstrate/evaluate the ac and dc performance of the

AD7854/AD7854L.

The software allows the user to perform ac (fast Fourier trans-

form) and dc (histogram of codes) tests on the AD7854/

AD7854L. It also gives full access to all the AD7854/AD7854L

on-chip registers allowing for various calibration and power-

down options to be programmed.

AD785x Family

All parts are 12 bits, 200 kSPS, 3.0 V to 5.5 V.

AD7853 – Single Channel Serial

AD7854 – Single Channel Parallel

AD7858 – Eight Channel Serial

AD7859 – Eight Channel Parallel

AD7854LARS Datasheet(PDF) 26 Page - Analog Devices

AD7854LARS Datasheet(HTML) 26 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

Datasheet Download

Link URL