AN1500A Datasheet, PDF (12/27 Pages) Sames – SINGLE CHIP TELEPHONE DEMO BOARD

AN1500A
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 600Ω.
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.
Fig. 5: Double Wheatstone bridge principle
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