Sierra Wireless WISMO218 - Manual (Page 54)

WA_DEV_W218_PTS_002 Rev 005 Page 54 of 109
Product Technical Specification &
Customer Design Guidelines
Figure 17. Example of MIC Input Single-Ended connection with LC Filter
*Z2 is from 200Hz to 4kHz. For more characteristics refer to the Electrical Characteristics section.
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.