Sierra Wireless AIRPRIME WISMO218 - Manual (Page 53)

WA_DEV_W218_PTS_002 Rev 006 April 29, 2010 53
Product Technical Specification &
Customer Design Guidelines Interfaces
3.10.4.2.2. SPK Single-Ended Connection
Typical implementation:
SPKP
C1
+
SPKN
Z hp Speaker
C3 33 pF
to
100 pF
X
Figure 20. Example of Speaker Single-Ended Connection
4.7µF < C1 < 47 µF (Depending on speaker characteristics and output power.)
Using a single-ended connection includes losing output power (-6dB) as compared to a differential
connection.
The connection between the AirPrime WISMO218 pins and the speaker must be designed to keep the
serial impedance lower than 1.5
 in a single-ended connection.
SPKN can be left open in a single-ended connection.
3.10.5. Design Recommendation
3.10.5.1. General
When both speaker and microphone are exposed to the external environment, it is recommended to
add ESD protection as close as possible to the speaker or microphone, connected between the audio
lines and a good ground.
When using the single-ended connection of MICP, ensure to have a good ground plane, good filtering
as well as shielding, in order to avoid any disturbance on the audio path.
It is important to select an appropriate microphone, speaker and filtering components to avoid TDMA
noise.
3.10.5.2. Recommended Microphone Characteristics
The impedance of the microphone has to be around 2K .
Sensitivity is from -40dB to –50 dB.
SNR > 50 dB.
Frequency response is compatible with the GSM specifications.
To suppress TDMA noise, it is highly recommended to use microphones with two internal decoupling
capacitors:
 CM1=56pF (0402 package) for the TDMA noise coming from the demodulation of the
GSM900 frequency signal
 CM2=15pF (0402 package) for the TDMA noise coming from the demodulation of the DCS
frequency signal