AD7853_15 datasheet(32/34 Pages) AD

32 / 34 page

REV. B

–32–

AD7853/AD7853L

APPLICATION HINTS

Grounding and Layout

The analog and digital supplies to the AD7853/AD7853L 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 vs. Frequency graph. However, care

should still be taken with regard to grounding and layout.

The printed circuit board that houses the AD7853/AD7853L

should be designed such that the analog and digital sections are

separated and confined to certain areas of the board. This facili-

tates 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. Digital and analog ground planes

should only be joined in one place. If the AD7853/AD7853L 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 AD7853/AD7853L. If the AD7853/AD7853L

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 AD7853/AD7853L.

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 AD7853/AD7853L to avoid noise coupling.

The power supply lines to the AD7853/AD7853L 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 ca-

pacitors to AGND. All digital supplies should have a 0.1

µF disc

ceramic capacitor to AGND. To achieve the best from these

decoupling components, they must be placed as close as pos-

sible 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 AD7853/AD7853L, it is recommended that

the system’s AVDD supply is used. In this case there should be a

Ω resistor between the AV

DD pin and DVDD pin. This supply

should have the recommended analog supply decoupling capaci-

tors between the AVDD pin of the AD7853/AD7853L and

AGND and the recommended digital supply decoupling ca-

pacitor between the DVDD pin of the AD7853/AD7853L and

DGND.

Evaluating the AD7853/AD7853L Performance

The recommended layout for the AD7853/AD7853L is outlined

in the evaluation board for the AD7853/AD7853L. The evalua-

tion board package includes a fully assembled and tested evalua-

tion 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 AD7853/AD7853L 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

AD7853/AD7853L.

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

form) and dc (histogram of codes) tests on the AD7853/AD7853L.

It also gives full access to all the AD7853/AD7853L on-chip

registers allowing for various calibration and power-down op-

tions 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

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