AD7910ARMZ2 datasheet(22/24 Pages) AD

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AD7910/AD7920

Rev. C | Page 22 of 24

APPLICATION HINTS

GROUNDING AND LAYOUT

The printed circuit board that houses the AD7910/AD7920

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 be joined at only one place. If the AD7910/AD7920 is in

a system where multiple devices require an AGND to DGND

connection then the connection should still be made at one

point only, a star ground point that should be established as

close to the AD7910/AD7920 as possible.

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 AD7910/AD7920 to avoid noise coupling. The

power supply lines to the AD7910/AD7920 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

reduces the effects of feedthrough through the board. A micro-

strip 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 very important. The supply should be

decoupled with, for example, a 680 nF 0805 to GND. When

using the SC70 package in applications where the size of the

components is of concern, a 220 nF 0603 capacitor, for example,

could be used instead. However, in that case, the decoupling can

not be as effective and can result in an approximate SINAD

degradation of 0.3 dB. To achieve the best performance from

these decoupling components, the user should endeavor to keep

the distance between the decoupling capacitor and the VDD and

GND pins to a minimum with short track lengths connecting

the respective pins. Figure 28 and Figure 29 show the

recommended positions of the decoupling capacitor for the

MSOP and SC70 packages, respectively.

As can be seen in Figure 28, for the MSOP package, the

decoupling capacitor is placed as close as possible to the IC,

with short track lengths to VDD and GND pins. The decoupling

capacitor could also be placed on the underside of the PCB

directly underneath the IC, between the VDD and GND pins

attached by vias. This method would not be recommended on

PCBs above a standard 1.6 mm thickness. The best performance

is seen with the decoupling capacitor on the top of the PCB next

to the IC.

Figure 28. Recommended Supply Decoupling Scheme for the

AD7910/AD7920 MSOP Package

Similarly, for the SC70 package, the decoupling capacitor should

be located as close as possible to the VDD and GND pins.

Because of its pinout, that is, VDD being next to GND, the

decoupling capacitor can be placed extremely close to the IC.

The decoupling capacitor could be placed on the underside of

the PCB directly under the VDD and GND pins, but, as before,

the best performance is seen with the decoupling capacitor on

the same side as the IC.

Figure 29. Recommended Supply Decoupling Scheme for the

AD7910/AD7920 SC70 Package

EVALUATING PERFORMANCE

The evaluation board package includes a fully assembled and

tested evaluation board, documentation, and software for

controlling the board from the PC via the Eval-Board

Controller. To demonstrate/evaluate the ac and dc performance

of the AD7910/AD7920, the evaluation board controller can be

used in conjunction with the AD7910/AD7920CB evaluation

boards as well as many other Analog Devices’ evaluation boards

ending in the CB designator.

The software allows the user to perform ac (fast Fourier

transform) and dc (histogram of codes) tests on the

AD7910/AD7920. See the evaluation board technical note for

more information.

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