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April 2004

AN585

APPLICATION NOTE

ISDN INTERFACE PROTECTION

REV. D2A - 3597

INTRODUCTION

The choice of a suitable protection device for an ISDN line interface requires consideration of a parameter

which is not critical in analogue line applications: the parasitic capacitance that the device introduces. Be-

cause of the high data rates used, parasitic capacitances must be minimized in order to ensure correct

signal transmission. In particular, attention must be paid to the capacitance imbalance in the line which

can cause considerable signal degradation. Such imbalance most frequently results from the presence of

common-mode protection, in which the capacitance introduced between each line and earth is frequently

unequal.

STMicroelectronics has developed a complete range of specific protection devices for ISDN applications:

the "TRIBALANCED PROTECTION" TPIxx series.

These devices introduce only a minimum of capacitance imbalance (30pF), which does not affect the

transmission performance of the line.

TRIBALANCED PROTECTION = TPIXX SERIES

The use of TRIBALANCED protection is mandatory under the following conditions:

– The bias voltage on line A and line B is different (line A = GND, Line B = –Vbat)

– The protection is realized in common mode, as illustrated in Figure 1 (b).

In this case, two conditions must be satisfied:

Figure 1. ISDN interface protection

U INTERFACE

OR

S INTERFACE

U INTERFACE

OR

S INTERFACE

(a)

FLOATING GROUND

U INTERFACE

OR

S INTERFACE

U INTERFACE

OR

S INTERFACE

(b)

ABSOLUTE GROUND REFERENCE

TPIxx