Silicon Laboratories SI2493/57/34/15/04 - Manual (Page 13)

Si2493/57/34/15/04
Global ISOmodem-EVB
Rev. 0.6 13
2.2.1. Audio Output
Audio output is provided from the Si2457/34/15 on the
AOUT pin. This signal allows the user to monitor call
progress signals, such as dial tone, DTMF dialing, ring,
busy signals, and modem negotiation. Control of this
signal is provided by AT commands and register
settings described in the introduction. The AOUT signal
can be connected to an amplifier, such as the LM386
(the default stuffing option on the Si2457/34/15URT-
EVB), for high-quality output. AOUT can also be
connected to a summing amplifier or multiplexer in an
embedded application as part of an integrated audio
system.
2.2.2. Amplifier (LM386)
The audio amplifier circuit consists of U10 (LM386),
C20, R3, R4, C21, C22, C23, R5, C24, and an optional
loudspeaker, LS1. The LM386 has an internally-set
voltage gain of 20. R3 and R4 provide a voltage divider
to reduce the AOUT signal to prevent overdriving the
LM386. C20 provides dc blocking for the input signal
and forms a high-pass filter with R3+R4 while R4 and
C21 form a low-pass filter. These four components limit
the bandwidth of the AOUT signal. C22 provides high-
frequency power supply bypassing for the LM386 and
should be connected to a hard ground and located very
close to the amplifier’s power supply and ground pins.
C23 and R5 form a compensation circuit to prevent
oscillation of the high current PNP transistor in the
LM386 output stage on negative signal peaks. These
oscillations can occur between 2–5 MHz and can pose
a radiation compliance problem if C23 and R5 are
omitted. C24 provides dc blocking for the output of the
LM386, which is biased at approximately 2.5 V (V
CC
/2),
and forms a high-pass filter with the impedance of the
loudspeaker (LS1). The output from the LM386 amplifier
circuit is available on the RCA jack, J2 (not installed).
Install jumper JP13 to enable the on-board speaker,
LS1.
2.3. Modem Module Operation
The Si2457/34/15URT-EVB daughter card is a complete
modem solution perfectly suited for use in an embedded
system.
The daughter card requires a 3.3 V supply capable of
providing at least 35 mA and communicates with the
system via LVCMOS/TTL-compatible digital signals on
JP1. The RJ-11 jack (TIP and RING) is connected via
JP2. Be sure to provide the proper power-on reset pulse
to the daughter card if it is used in the stand-alone
mode.
2.3.1. Reset Requirements
The Si2457/34/15 ISOmodem
®
daughter card must be
properly reset at powerup. The reset pin (pin 8) of the
Si2457/34/15 (JP1, pin 13) must be held low for at least
5.0 ms after power is applied and stabilized to ensure
the device is properly reset.
2.3.2. Crystal Requirements
Clock accuracy and stability are important in modem
applications. To ensure reliable communication between
modems, the clock must remain within ±100 ppm of the
design value over the life of the modem. The crystal
selected for use in a modem application must have a
frequency tolerance of less than ±100 ppm for the
combination of initial frequency tolerance, drift over the
normal operating temperature range, and five year
aging. Other considerations, such as production
variations in PC board capacitance and the tolerance of
loading capacitors, must also be taken into account.
2.3.3. Protection
The Si2493/57/34/15/04-EVB meets or exceeds all FCC
and international PTT requirements and
recommendations for high-voltage surge and isolation
testing without any modification. The protection/isolation
circuitry includes C1, C2, C8, C9, FB1, FB2, and RV1.
The PCB layout is also a key “component” in the
protection circuitry. The Si2493/57/34/15/04-EVB
provides isolation to 3 kV. Contact Silicon Laboratories
for information about designing to higher levels of
isolation.
3. Design
The following sections contain the schematics, bill of
materials, and layout for the Si2493/57/34/15/04
including the daughter card and motherboard.