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FN6640.1
September 11, 2008
degraded for traces greater than one inch, unless
controlled impedance (50
Ω or 75Ω) strip lines or
microstrips are used.
Match channel-to-channel analog I/O trace lengths and
layout symmetry. This will minimize propagation delay
mismatches.
Maximize use of AC de-coupled PCB layers. All signal I/O
lines should be routed over continuous ground planes (i.e. no
split planes or PCB gaps under these lines). Avoid vias in the
signal I/O lines.
Use proper value and location of termination resistors.
Termination resistors should be as close to the device as
possible.
When testing, use good quality connectors and cables,
matching cable types and keeping cable lengths to a
minimum.
A minimum of 2 power supply decoupling capacitors are
recommended (1000pF, 0.01F) as close to the devices as
possible. Avoid vias between the capacitor and the device
because vias add unwanted inductance. Larger capacitors
can be farther away. When vias are required in a layout, they
should be routed as far away from the device as possible.
The NIC pins are placed on both sides of the input pins.
These pins are not internally connected to the die. It is
recommended these pins be tied to ground to minimize
crosstalk.
The QFN Package Requires Additional PCB Layout
Rules for the Thermal Pad
The thermal pad is electrically connected to power supply
ground through the high resistance IC substrate. Its primary
function is to provide heat sinking for the IC. However,
because of the connection to the power ground pins through
the substrate, the thermal pad must be tied to the power
supply ground to prevent unwanted current flow through the
thermal pad. Maximum AC performance is achieved if the
thermal pad has good contact to the IC ground pins. Heat
sinking requirements can be satisfied using thermal vias
directly beneath the thermal pad to a heat dissipating layer
of a square at least 1” on a side.
FIGURE 26. BASIC APPLICATION CIRCUIT
V+(1,2,3)
EN(1,2,3)
V+_OUT(1,2,3)
OUT_2
IN+_5
IN+_2
IN+_3
GND_IN(1,2,3)
EN(4,5,6)
V+ (4,5,6)
IN+_4
OUT_3
GND_OUT(4,5,6)
GND_PWR(1,2,3)
OUT_6
+
-
+
-
+
-
DIE 1
+
-
+
-
+
-
DIE 2
IN+_1
IN+_6
OUT_1
GND_OUT(4,5,6)
V+_OUT(4,5,6)
OUT_5
OUT_4
GND_PWR(4,5,6)
GND_IN(4,5,6)
ROUT 1
ROUT 2
ROUT 3
ROUT 4
ROUT 5
ROUT 6
RIN 4
RIN 6
RIN 5
RIN 4
RIN 6
RIN 5
DECOUPLING
CAPACITORS
DECOUPLING
CAPACITORS
ISL55036