LISA-U1 series - System Integration Manual
3G.G2-HW-10002-3
Preliminary
Design-In
Page 97 of 125
Avoid coupling of any noisy signals to microphone input lines
It is strongly recommended to route MIC signals away from battery and RF antenna lines. Try to skip fast
switching digital lines as well
Keep ground separation from other noisy signals. Use an intermediate GND layer or vias wall for coplanar
signals
MIC_P
and
MIC_N
are sensed differentially within the module. Therefore they should be routed as a
differential pair up to the audio signal source
Cross other signals lines on adjacent layers with 90° crossing
Place bypass capacitor for RF very close to active microphone. The preferred microphone should be designed
for GSM applications which typically have internal built-in bypass capacitor for RF very close to active device.
If the integrated FET detects the RF burst, the resulting DC level will be in the pass-band of the audio
circuitry and cannot be filtered by any other device
The bias for an external electret active microphone is not provided by the module. Verify that microphone is
properly biased from an external low noise supply and verify that the supply noise is properly filtered
Output audio lines have two separated configurations.
SPK_P
/
SPK_N
are high level balanced output. They are DC coupled and must be used with a speaker
connected in bridge configuration
Route
SPK_P
/
SPK_N
as differential pair, to reduce differential noise pick-up. The balanced configuration
will help reject the common mode noise
Consider enlarging PCB lines, to reduce series resistive losses, when the audio output is directly connected to
low impedance speaker transducer
Use twisted pair cables for balanced audio usage
If DC decoupling is required, a large capacitor needs to be used, typically in the microFarad range,
depending on the load impedance, in order to not increase the lower cut-off frequency of its High-Pass RC
filter response
2.2.1.5
Module grounding
Good connection of the module with application board solid ground layer is required for correct RF
performance. It significantly reduces EMC issues and provides a thermal heat sink for the module.
Connect each
GND
pin with application board solid GND layer. It is strongly recommended that each
GND
pad surrounding
VCC
pins have one or more dedicated via down to the application board solid ground layer
The shielding metal tabs are connected to GND, and are a fundamental part of electrical grounding and
thermal heat-sink. Connect them to board solid ground layer, by soldering them on the baseboard using
PCB plated through holes connected to
GND
net
If the application board is a multilayer PCB, then it is required to connect together each GND area with
complete via stack down to main board ground layer
It is recommended to implement one layer of the application board as ground plane
Good grounding of
GND
pads will also ensure thermal heat sink. This is critical during call connection, when
the real network commands the module to transmit at maximum power: proper grounding helps prevent
module overheating
2.2.1.6
Other sensitive pins
A few other pins on the LISA-U1 series modules requires careful layout.
RTC supply (V_BCKP):
avoid injecting noise on this voltage domain as it may affect the stability of sleep
oscillator
