msm7715 Oki Semiconductor, msm7715 Datasheet - Page 19

• Manufacturer Part Number: msm7715
• Description: Multi-function Telecommunication Lsi
• Manufacturer: Oki Semiconductor

¡ Semiconductor
Analog Interface
When R1 = R2 in the modem mode, the AOUT output level is at its maximum value of about –9
dBm. When R1 = R3 in DTMF mode, the AOUT output level is at its maximum value of –11 dBm
(sum of the low group and high group). To increase the transmit output level in the line to a high
level, use an external amplifier (AMP1).
The receive signal levels can be adjusted by the values of R13 and R14.
Clock noise of about –70 dBm will be generated from AOUT as out-of-band noise. (Clock noise
has a frequency of 27.965 kHz and its odd harmonics.) The clock noise level is nearly always
constant even if the output level of AOUT is lowered by varying the values of resistors R1 to R3
that are connected to GAT1, GAT2, DTO, and MOD. Therefore, to suppress this noise output to
the line, build a LPF at the AMP1. Note, however, that setting the cutoff frequency of the LPF
too low affects the output signal level.
Note that too large a time constant determined by the values of C9 and R15 allows the longer time
required for reaching a stable DC level, which may result in the violation of specification for the
DTMF signal receive output delay time.
Example: R15=100 kW, C9=0.022 mF.
Larger circuit-return levels of the transmit signal to the AIN pin can cause receive data errors.
If the line impedance is equal to the R10 impedance, R8=R9 can not induce the circuit-return
levels of the transmit signal to the AIN pin.
The peripheral circuits should be designed so that the circuit-return level, including variations
of the line impedance, of the transmit signal to the AIN pin is –9 dBm or less.
Line
600 W : 600 W
V
V
T
R
AMP2
R11
R9
Figure 12 Analog Interface Circuit Example
+
–
R12
R8
600 W
R10
R13
R7
AMP1
C7
+
–
–
+
R6
R14
R15≥60 kW
C8
C9
R15
AOUT
AIN
SGO
r
4
+
+
–
–
SG
4 dB
r
3
MSM7715
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