LXT381 — Octal E1 Line Interface

14

Datasheet

When TCLK is held high for at least 12

µs, TPOS and TNEG become RZ formatted inputs. In this

mode, TPOS and TNEG control the pulse width and polarity on TTIP and TRING.

If TCLK is held Low, the output drivers enter a low power, high impedance mode. The OE pin can also

be used to set all the output drivers to an high impedance mode. This feature is useful for

redundancy/protection applications.

Each output driver is supplied by a separate power supply (TVCC and TGND).

2.2.1

Transmit Pulse Shaping

In NRZ mode, the transmitted pulse shape is internally generated using a high speed D/A

converter. Shaped pulses are further applied to the line driver for transmission onto the line at TTIP

and TRING. The line driver provides a constant low output impedance regardless of whether it is

driving marks, spaces or if it is in transition. This well controlled dynamic impedance provides

excellent return loss when used with external precision resistors (± 1% accuracy) in series with the

transformer. See Figure 5.

The LXT381 produces 2.048 MHz pulses for both 75

Ω coaxial (2.37 V) or 120 Ω shielded

twisted-pair (3.0 V) lines through an output transformer with a 1:2 step up pulse transformer and 11

Ω series resistors. No transmit component changes are required in 75 or 120 Ω operation as the

output driver dynamically adjusts its output pulse amplitude.

2.3

Interfacing with 5V logic

The LXT381 can interface with 5V logic. In this case, the VCCIO pins should be connected to a

5V power supply. The VCCIO pins feed the digital I/O pads making the input/output voltage levels

consistent with 5V logic. See Table 10. The internal logic will still operate from the 3.3V supply

(VCC0 and VCC1) to minimize the power consumption.

2.4

Line Protection

In the receive side, the 1 k

Ω series resistors protect the receiver against current surges coupled into

the device. Due to the high receiver impedance (70 k

Ω typ.) the resistors do not affect the receiver

sensitivity. In the transmit side, the Schottky diodes D1-D4 protect the output driver.While not

mandatory for normal operation, these protection elements are strongly recommended to improve

the design robustness.

2.5

Loopbacks

The LXT381 offers two loopback modes for diagnostic purposes. The loopback mode is selected

with the LOOPn pins.