Thin Film Silicon Monolithic Integrated Circuit
The MC33121 is designed to provide the interface between the 4–wire side of
a central office, or PBX, and the 2–wire subscriber line. Interface functions
include battery feed, proper loop termination AC impedance, hookswitch
detection, adjustable transmit, receive, and transhybrid gains, and single/
double fault indication. Additionally, the MC33121 provides a minimum of 58 dB
of longitudinal balance (4–wire and 2–wire).
The transmit and receive signals are referenced to analog ground, while
digital signals are referenced to digital ground, easing the interface to codecs,
filters, etc. The 2 status outputs (hookswitch and faults) and the Power Down
Input are TTL/CMOS compatible. The Power Down Input permits local
shutdown of the circuit.
Internal drivers allow the external loop current pass transistors to be standard
bipolar transistors (non–Darlington).
The MC33121 is available in a 20 pin DIP and a 28 pin PLCC surface mount
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