AN1500A South African Micro-Electronic Systems, AN1500A Datasheet - Page 12

Manufacturer Part Number

AN1500A

Description

Single Chip Telephone Demo Board

Manufacturer

South African Micro-Electronic Systems

Datasheet

1.AN1500A.pdf (27 pages)

13.1 Sidetone cancellation

Perfect sidetone cancellation is achieved, when the sidetone balance network is matched to the line impedance by

a factor of 10, which is equal to the matching of the resistors Z2 and Z1.

With ideal matching, no differential potential of the transmitted signal occurs at nodes RI and STB and the output of

the RX amplifier is 0.

13.2 AC impedance

An internal amplifier controls the impedance of Zsyn by matching LI to an internal reference. Part of this internal

reference is accessible by pin CI. When no external component is connected at that pin, the AC impedance of the

circuit is synthesized to 600 .

When a capacitor is connected at CI, the AC impedance of the chip becomes complex. For correct adaption, the

capacitance of CCI should be 1/10 of the line’s complex part.

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AN1500A

sames

Fig. 5: Double Wheatstone bridge principle