AD7892

REV. C

–13–

Grounding and Layout

plies both the analog and digital circuitry on the part. For opti-

mum performance from the part, it is recommended that this

+5 V is taken from the +5 V analog supply in the system. The

analog and digital grounds to the AD7892 are independent and

separately pinned out to minimize coupling between the analog

and digital sections of the device. The part exhibits good immu-

nity to noise on the supplies but care must still be taken with

regard to grounding and layout especially when using switching

mode supplies.

The printed circuit board which houses the AD7892 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 which 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 AD7892 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

AD7892. If the AD7892 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 AD7892.

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 AD7892 to avoid noise coupling. The power

supply lines to the AD7892 should use as large a trace as pos-

sible 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 important when using high resolution ADCs.

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 have to be placed as

close as possible to the device, ideally right up against the device.

All logic chips should be decoupled with 0.1

µF disc ceramic

capacitors to DGND. It is recommended that the system’s

supply should have the recommended analog supply decoupling

the recommended digital supply decoupling capacitor between

Evaluating the AD7892 Performance

The recommended layout for the AD7892 is outlined in the

evaluation board for the AD7892. The evaluation board package

includes a fully assembled and tested evaluation board, docu-

mentation and software for controlling the board from a PC

using the EVAL-CONTROL BOARD. The EVAL-CONTROL

BOARD can be used in conjunction with the AD7892 evalua-

tion board, as well as many other Analog Devices evaluation

boards ending in the CB designator. Using the EVAL-CONTROL

BOARD with the AD7892 evaluation board allows the user to

evaluate the ac and dc performance of the AD7892 on a PC.

The software provided with the evaluation board allows the user

to perform ac (Fast Fourier Transform) and dc (histogram of

codes) tests on the AD7892. The evaluation board can also

be used in a stand-alone fashion without the EVAL-CONTROL

BOARD but in this case, the user has to write their own software

to evaluate the part. There are two versions of the evaluation

board available, one for the AD7892-2 and one for the AD7892-3.

To order the AD7892-2 evaluation board, the order number is

EVAL-AD7892-2CB and to order the AD7892-3 evaluation

board, the order number is EVAL-AD7892-3CB.