Sierra Wireless AIRPRIME WISMO218 - Manual (Page 51)

WA_DEV_W218_PTS_002 Rev 006 April 29, 2010 51
Product Technical Specification &
Customer Design Guidelines Interfaces
3.10.4.1.1.1. Recommended Components
Component Description/Details Notes
C1 12pF to 33pF needs to be tuned depending on the design
C2, C3, C4 47pF needs to be tuned depending on the design
L1, L2 100nH needs to be tuned depending on the design
3.10.4.1.2. Microphone Single-Ended Connection Example
When single-ended connection is used for MIC, MICN is just left open.
Figure 17. Example of MIC Input Single-Ended connection with LC Filter
*Z2 is from 200Hz to 4kHz. For more characteristics refer to section 3.2.2 Electrical Characteristics.
Note that:
 Internal input impedance value becomes 1100, due to the connection of the other end to
ground.
 The single ended design is very sensitive to TDMA noise.
 It is recommended to add L1 and C2 footprint as an LC filter to try to eliminate TDMA noise.
 A very good grounding on the MIC is a must in order to ensure good audio performance
against TDMA. Also, special care on the PCB layout must be taken.
 When not used, the filter can be removed by replacing L1 with a 0 resistor and by
disconnecting C2, as shown in the following schematic.