User's Manual

SB555 Hardware Integration Guide
62 Proprietary and Confidential 2130075
Microphone input (headset)
The microphone input is a capacitively
connected differential input, with input
impedance greater than 4 k
Ω. Microphone
signals should be -44 dBV (18 mV
p-p
) nominal.
Note: Single-ended
drive will reduce input
impedance by 50% to
2.1 kΩ typical.
If a single-ended drive is desired, the MIC- input
must be connected to the Audio Common
ground (pin 16) as close to the microphone, or its
connector, as possible. Do not use a general
system ground, but rather the Audio
Common (
AGND) provided by the module.
The modem provides a microphone DC bias of
just under 1 mA of current for a standard micro-
phone.
Speaker output (headset)
The speaker output is a single-ended signal used
to interface to a headset. The output signal is
AC-coupled -21 dBV (250 mV
p-p
) nominal into a
32
Ω load.
If additional amplification is needed, it is the
responsibility of the integrator.
Sample headset integration
Note: The Audio
Common Ground is
independent of the
system ground used for
other operations.
The simplest integration of the modem’s voice
service uses the conventional analog micro-
phone and speaker pins (13–16). These can be
connected directly to a standard 3-wire cellular
headset.