AN585 Datasheet, PDF (1/5 Pages) STMicroelectronics – ISDN INTERFACE PROTECTION

AN585
APPLICATION NOTE
ISDN INTERFACE PROTECTION
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:
1. Low capacitance from line to ground → No signal attenuation.
2. Good capacitance balance between line a and line b → Good longitudinal balance on the line.
Figure 1. ISDN interface protection
U INTERFACE
OR
S INTERFACE
(a) FLOATING GROUND
TPIxx
U INTERFACE
OR
S INTERFACE
(b) ABSOLUTE GROUND REFERENCE
April 2004
REV. D2A - 3597
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