MSM7715 データシートの表示（PDF） - Oki Electric Industry

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MSM7715

Multi-Function Telecommunication LSI

Oki Electric Industry 

¡ Semiconductor

MSM7715

Analog Interface

VT

VR

Line

R9

–

+

AMP2

600 W : 600 W

R8

R10

600 W

C7

R7

–

+

AMP1

R13

R11 R12

C8

R6

R14

–

C9

+

R15

R15≥60 kW

r4 4 dB

r3

–

+

AOUT

AIN

SGO SG

Figure 12 Analog Interface Circuit Example

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.

19/21

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