AD7484 Analog Devices, AD7484 Datasheet - Page 17

• Manufacturer Part Number: AD7484
• Description: 14-Bit, 3 MSPS SAR ADC
• Manufacturer: Analog Devices

Specifications of AD7484

• Resolution (bits): 14bit
• # Chan: 1
• Sample Rate: 3MSPS
• Interface: Par
• Analog Input Type: SE-Uni
• Ain Range: Uni 2.5V
• Adc Architecture: SAR
• Pkg Type: QFP

BOARD LAYOUT AND GROUNDING
For optimum performance from the AD7484, it is recommended
that a PCB with a minimum of three layers be used. One of
these layers, preferably the middle layer, should be as complete a
ground plane as possible to give the best shielding. The board
should be designed in such a way that the analog and digital
circuitry is separated and confined to certain areas of the board.
This practice, along with not running digital and analog lines close
together, helps to avoid coupling digital noise onto analog lines.
The power supply lines to the AD7484 are to be approximately
3 mm wide to provide low impedance paths and reduce the effects
of glitches on the power supply lines. It is vital that good decoupling
also be present. A combination of ferrites and decoupling capa-
citors should be used, as shown in Figure 23.The decoupling
capacitors are to be as close to the supply pins as possible. This
is made easier by the use of multilayer boards. The signal traces
from the AD7484 pins can be run on the top layer, while the
Figure 26. Top and Bottom Routing Layers
Figure 25. Bottom Layer Silkscreen
Figure 24. Top Layer Routing
Rev. C | Page 17 of 20
decoupling capacitors and ferrites can be mounted on the bottom
layer where the power traces exist. The ground plane between
the top and bottom planes provides excellent shielding.
Figure 24 to Figure 28 show a sample layout of the board area
immediately surrounding the AD7484. Pin 1 is the bottom left
corner of the device. The black area in each figure indicates the
ground plane present on the middle layer. Figure 24 shows the
top layer where the AD7484 is mounted with vias to the bottom
routing layer highlighted. Figure 25 shows the bottom layer
silkscreen where the decoupling components are soldered
directly beneath the device. Figure 26 shows the top and bottom
routing layers overlaid. Figure 27 shows the bottom layer where
the power routing is with the same vias highlighted. Figure 28
shows the silkscreen overlaid on the solder pads for the
decoupling components, which are C1 to C6: 100 nF, C7 to C8:
470 nF, C9: 1 nF, and L1 to L4: Meggit-Sigma Chip Ferrite
Beads (BMB2A0600RS2).
Figure 28. Silkscreen and Bottom Layer Routing
Figure 27. Bottom Layer Routing
AD7484