Data Sheet AD7798/AD7799
Rev. B | Page 25 of 28
GROUNDING AND LAYOUT
Because the analog inputs and reference inputs of the ADC are
differential, most of the voltages in the analog modulator are
common-mode voltages. The excellent common-mode reject-
ion of the parts removes common-mode noise on these inputs.
The digital filter provides rejection of broadband noise on the
power supply, except at integer multiples of the modulator
sampling frequency. The digital filter also removes noise from
the analog and reference inputs, provided that these noise
sources do not saturate the analog modulator. As a result, the
AD7798/AD7799 are more immune to noise interference than
conventional high resolution converters. However, because the
resolution of the AD7798/AD7799 is so high and the noise
levels from the AD7798/AD7799 are so low, care must be taken
with regard to grounding and layout.
The printed circuit board that houses the AD7798/AD7799
should be designed such that the analog and digital sections are
separated and confined to certain areas of the board. A mini-
mum etch technique is generally best for ground planes because
it provides the best shielding.
It is recommended that the GND pin be tied to the AGND plane
of the system. In any layout, it is important that the user keep in
mind the flow of currents in the system, ensuring that the return
paths for all currents are as close as possible to the paths the
currents took to reach their destinations. Avoid forcing digital
currents to flow through the AGND sections of the layout.
The ground planes should be allowed to run under the AD7798/
AD7799 to prevent noise coupling. The power supply lines to
the AD7798/AD7799 should use as wide a trace as possible to
provide low impedance paths and reduce the effects of glitches
on the power supply line. Fast switching signals, such as clock
signals, 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 works best, but it is not always
possible to use this method with a double-sided board. In this
technique, the component side of the board is dedicated to
ground planes, and signals are placed on the solder side.
Good decoupling is important when using high resolution
ADCs. AV
DD
should be decoupled with 10 µF tantalum in
parallel with 0.1 µF capacitors to GND. DV
DD
should be
decoupled with 10 µF tantalum in parallel with 0.1 µF
capacitors to the system’s DGND plane, with the system’s
AGND-to-DGND connection being close to the AD7798/
AD7799. To achieve the best from these decoupling components,
they should 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 ceramic capacitors to DGND.
