AD7781
Rev. 0 | Page 15 of 16
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-
tion 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 AD7781
is more immune to noise interference than conventional high
resolution converters. However, because the resolution of the
AD7781 is so high and the noise levels from the AD7781 are so
low, care must be taken with regard to grounding and layout.
The printed circuit board (PCB) that houses the AD7781 should
be designed so 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 AD7781 be tied
to the AGND plane of the system. In any layout, 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 that 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 AD7781 should be allowed to run under
the AD7781 to prevent noise coupling. The power supply lines
to the AD7781 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,
and the 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 capacitors in
parallel with 0.1 µF capacitors to GND. DV
DD
should be decoupled
with 10 µF tantalum capacitors in parallel with 0.1 µF capac-
itors to GND, with the system’s AGND to DGND connection
being close to the AD7781. To achieve the best results from
these decoupling components, place them 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.
