Atmel AVR10004: RCB256RFR2 – Hardware User Manual [APPLICATION NOTE]
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4.7.3
PCB detail 2 – balun ground connection
The filter-balun requires a solid ground connection (refer to
. Because the filter-balun has to drive a single-ended line
towards the SMA connector, each current injected into this line creates a counterpart current into the ground plane. A
parasitic inductance to ground is, therefore, directly inserted into the signal path and increases the insertion loss.
Further, the integrated harmonic low-pass filter has to perform well at much higher frequencies to reduce harmonics.
Any parasitic inductance causes limited harmonic filter performance.
Lowest inductance is achieved with large copper areas on the top and bottom planes. Both planes are to be sewn
together with sufficient through holes, especially in close proximity of GND pins of critical RF components. Resulting
through-hole inductances are to be considered as parallel connected, resulting in the lowest possible overall
inductance.
4.7.4
PCB detail 3 – bias DC block, AC ground
The RCB uses an integrated filter-balun (refer to
). The component provides pin 2 as a bias port towards the
differential pins. To avoid a DC connection of the radio transceiver circuitry (refer to
), filter-balun pin 2 requires a DC
blocking capacitor, C1, to create an AC GND connection only.
This capacitor is to be placed as close as possible to the filter-balun to ensure a low-impedance AC connection. For the
RCB256RFR2, the minimum distance was limited by the minimum width of a solder mask separator in between the
balun and the capacitor pad. The actual value is related to the PCB manufacturer capabilities.
The grounded pad of the capacitor is surrounded by three through holes in close proximity to ensure the lowest possible
impedance.
The capacitor itself should be of size 0402, or smaller.
4.7.5
PCB detail 4 – analog GND routing
It is recommended to design the ground trace as wide as possible to avoid parasitic inductances.
Atmel Atmega256RFR2 analog ground pins are to be routed to the paddle underneath the IC. The GND trace width is
recommended to be similar to the pad width.
Each GND pin should be connected to the bottom plane with at least one through hole in direct proximity to the IC.
Soldering technology allows placing small through holes (0.15mm drill) within the ground paddle underneath the
Atmega256RFR2. Through holes are filled with solder during reflow soldering; solder paste loss is low, however, due to
the small drill size.
For pin 55 an exception should be considered since there are special conditions in its adjacent area:
•
In this area a solid GND plane does not exist on top since there are many parts located there (XTAL, load and
blocking capacitors). The GND in most cases is limited to one via and maybe some traces. This is not really
low inductance, especially on a 1.5mm double layer substrate
•
The crystal is right next to it. An increased noise level at this pin may crosstalk into the crystal
Therefore, the rule for pin 55 is that it should be handled like the digital ground pins when a solid grounding is possible
next to the package. The term ‘solid grounding’ shall be understood as a close connection to a ground plane on top and
more than one via.
If only one or no via is possible the pin shall also be connected to the paddle.
4.7.6
PCB detail 5 – digital GND routing
The digital GND pins are not directly connected to the paddle. This is to avoid dispersion of digital noise from I/O pad
cells or other digital processing units.
