AD7170 Data Sheet
Rev. B | Page 12 of 14
GROUNDING AND LAYOUT
Because the analog input and reference input 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 part 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 AD7170
is more immune to noise interference than conventional high
resolution converters. However, because the noise levels from
the AD7170 are so low, care must be taken with regard to
grounding and layout.
The printed circuit board that houses the AD7170 should be
designed such that the analog and digital sections are separated
and confined to certain areas of the board. A minimum etch
technique is generally best for ground planes because it gives
the best shielding.
It is recommended that the GND pin of the AD7170 be tied to
the analog ground (AGND) plane of the system. In any layout,
it is important that the user pay attention to the flow of currents
in the system and ensure 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 plane of the AD7170 should be allowed to run
under the AD7170 to prevent noise coupling. The power supply
lines to the AD7170 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 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 microstrip technique is by far
the best, but it 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.
V
DD
should be decoupled with 10 µF tantalum capacitors in
parallel with 0.1 µF capacitors to GND, with the system’s analog
ground to digital ground (DGND) connection being close to
the AD7170. To achieve the best results 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.
