Sierra Wireless AIRPRIME WISMO218 - Manual (Page 47)

WA_DEV_W218_PTS_002 Rev 006 April 29, 2010 47
Product Technical Specification &
Customer Design Guidelines Interfaces
3.10. Analog Audio Interface
The AirPrime WISMO218 supports one microphone input and one speaker output. It also includes an
echo cancellation feature which allows hands free function.
In some cases, ESD protection must be added on the audio interface lines.
3.10.1. Microphone Features
The microphone, MIC, can either have a single-ended or a differential connection. However, it is
strongly recommended to use a differential connection in order to reject common mode noise and
TDMA noise.
When using a single-ended connection, be sure to have a very good ground plane, very good filtering
as well as shielding in order to avoid any disturbance on the audio path.
The gain of MIC inputs is internally adjusted and can be tuned using AT commands.
The MIC already includes suitable biasing for an electret microphone. The electret microphone can
then be connected directly on the inputs for easy connection.
AC coupling is also already embedded in the AirPrime WISMO218.
DC Equivalent Circuit AC Equivalent Circuit
Figure 13. DC and AC Equivalent Circuits of MIC
3.10.1.1. Electrical Characteristics
Table 16: Electrical Characteristics of MIC
Parameters Min Typ Max Unit
Internal biasing
DC Characteristics
MICP - 2.4 - V
MICN without 2.2K 
to GND
- 2.4 - V
MICN with 2.2K  to
GND
- 1.2 - V
Output current mA
R2 - 2.2 - K 
AC Characteristics
200 Hz
Z2 MICP
(MICN=Open)
2.2 K 
Z2 MICN
(MICP=Open)
MICP
MICN
Do
cu
me
nt
Titl
e
Do
cu
me
nt
Titl
e
Do
cu
Z2
Z2
GND
MIC2+
MICP
MICN
R2
R2
GND