ad73422 Analog Devices, Inc., ad73422 Datasheet - Page 36

• Manufacturer Part Number: ad73422
• Description: Dual Low Power Cmos Analog Front End With Dsp Microcomputer
• Manufacturer: Analog Devices, Inc.

AD73422
Grounding and Layout
As the analog inputs to the AD73422’s AFE section are differ-
ential, most of the voltages in the analog modulator are common-
mode voltages. The excellent common-mode rejection of the
part will remove common-mode noise on these inputs. The
analog and digital supplies of the AD73422 are independent and
separately pinned out to minimize coupling between analog and
digital sections of the device. The digital filters on the encoder
section will provide rejection of broadband noise on the power
supplies, except at integer multiples of the modulator sampling
frequency. The digital filters also remove noise from the analog
inputs provided the noise source does not saturate the analog
modulator. However, because the resolution of the AD73422’s
ADC is high, and the noise levels from the AD73422 are so low,
care must be taken with regard to grounding and layout.
The printed circuit board that houses the AD73422 should be
designed so the analog and digital sections are separated and
confined to certain sections of the board. The AD73422 ball-
out configuration offers a major advantage in that its analog
interfaces are confined to the last three rows of the package.
This facilitates the use of ground planes that can be easily sepa-
rated, as shown in Figure 35. A minimum etch technique is
generally best for ground planes as it gives the best shielding.
Digital and analog ground planes should be joined in only one
place. If this connection is close to the device, it is recommended
to use a ferrite bead inductor.
Avoid running digital lines under the AFE section of the device
for they will couple noise onto the die. The analog ground plane
should be allowed to run under the AD73422’s AFE section to
avoid noise coupling (see Figure 35). The power supply lines to
the AD73422 should use as large a trace as possible to provide
low impedance paths and reduce the effects of glitches on the
power supply lines. 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
0.089 (2.27)
0.073 (1.85)
0.559 (14.20)
0.543 (13.80)
119-Ball Plastic Ball Grid Array (PBGA)
TOP VIEW
A1
Dimensions shown in inches and (mm).
OUTLINE DIMENSIONS
DETAIL A
0.874 (22.20)
0.858 (21.80)
B-119
–36–
0.028 (0.70)
0.020 (0.50)
SEATING
(0.84)
PLANE
0.033
the analog inputs. Traces on opposite sides of the board should
run at right angles to each other. This will reduce the effects of
feedthrough through the board. A microstrip 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 other side.
Good decoupling is important when using high speed devices.
On the AD73422 both the reference (REFCAP) and supplies
need to be decoupled. It is recommended that the decoupling
capacitors used on both REFCAP and the supplies, be placed as
close as possible to their respective ball connections to ensure
high performance from the device. All analog and digital sup-
plies should be decoupled to AGND and DGND respectively,
with 0.1 µF ceramic capacitors in parallel with 10 µF tantalum
capacitors. The AFE’s digital section supply (DVDD) should be
connected to the digital supply that feeds the DSP’s VDD(Ext)
connections while the AFE’s digital ground DGND should be
returned to the digital ground plane.
REF
(1.27)
0.050
BOTTOM
BSC
VIEW
0.126 (3.19)
BALL DIAMETER
DSP
REF
0.035 (0.90)
0.024 (0.60)
0.300 (7.62) BSC
7 6 5 4 3 2 1
DETAIL A
A B C D E F G H J K L M N P R T U
0.050 (1.27)
Figure 35. Ground Plane Layout
GROUND PLANE
BSC
DIGITAL
0.022 (0.56)
A
B
C
D
E
F
G
H
J
K
L
M
N
P
R
T
U
0.037 (0.95)
0.033 (0.85)
REF
(20.32)
0.800
BSC
AFE DIGITAL
GROUND PLANE
AFE ANALOG
ANALOG
7
6
5
4
3
2
1
REV. 0