AD7859LAS datasheet(27/28 Pages) AD

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AD7859/AD7859L

REV. A

–27–

DGND connection, then the ground planes should be con-

nected at the AGND and DGND pins of the AD7859/

AD7859L. If the AD7859/AD7859L is in a system where mul-

tiple devices require AGND to DGND connections, the con-

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

point which should be established as close as possible to the

AD7859/AD7859L.

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 AD7859/AD7859L to avoid noise coupling.

The power supply lines to the AD7859/AD7859L 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 al-

ways 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

AD7859/AD7859L, 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

AD7859/AD7859L and AGND and the recommended digital

supply decoupling capacitor between the DVDD pin of the

AD7859/AD7859L and DGND.

Evaluating the AD7859/AD7859L Performance

The recommended layout for the AD7859/AD7859L is outlined

in the evaluation board for the AD7859/AD7859L. 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 AD7859/AD7859L 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

AD7859/AD7859L.

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

form) and dc (histogram of codes) tests on the AD7859/

AD7859L. It also gives full access to all the AD7859/AD7859L

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

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