DS88C120 Datasheet, PDF (6/10 Pages) National Semiconductor (TI) – Dual CMOS Compatible Differential Line Receiver

Application Hints (Continued)
LINE DRIVERS
Line drivers which will interface with the DS78C120
DS88C120 are listed below
Balanced Drivers
DS26LS31
DS7830 DS8830
DS7831 DS8831
DS7832 DS8832
DS1691A DS3691
DS1692 DS3692
DS3587 DS3487
Quad RS-422 Line Driver
Dual TTL
Dual TRI-STATE TTL
Dual TRI-STATE TTL
Quad RS-423 Dual RS-422 TTL
Quad RS-423 Dual TRI-STATE
RS-422 TTL
Quad TRI-STATE RS-422
Unbalanced Drivers
DS1488
DS14C88
DS75150
Quad RS-232
Quad RS-232
Dual RS-232
RESPONSE CONTROL AND HYSTERESIS
In unbalanced (RS-232 RS-423) applications it is recom-
mended that the rise time and fall time of the line driver be
controlled to reduce cross-talk Elimination of switching
noise is accomplished in the DS78C120 DS88C120 by the
50 mV of hysteresis incorporated in the output gate This
eliminates the oscillations which may appear in a line receiv-
er due to the input signal slowly varying about the threshold
level for extended periods of time
High frequency noise which is superimposed on the input
signal which may exceed 50 mV can be reduced in ampli-
tude by filtering the device input On the DS78C120
DS88C120 a high impedance response control pin in the
input amplifier is available to filter the input signal without
affecting the termination impedance of the transmission
line Noise pulse width rejection vs the value of the re-
sponse control capacitor is shown in Figures 1 and 2 This
combination of filters followed by hysteresis will optimize
performance in a worse case noise environment
TRANSMISSION LINE TERMINATION
On a transmission line which is electrically long it is advisa-
ble to terminate the line in its characteristic impedance to
prevent signal reflection and its associated noise cross-
talk A 180X termination resistor is provided in the
DS78C120 DS88C120 line receiver To use the termination
resistor connect pins 2 and 3 together and pins 13 and 14
together The 180X resistor provides a good compromise
between line reflections power dissipation in the driver and
IR drop in the transmission line If power dissipation and IR
drop are still a concern a capacitor may be connected in
series with the resistor to minimize power loss
The value of the capacitor is recommended to be the line
length (time) divided by 3 times the resistor value Example
if the transmission line is 1 000 feet long (approximately
1000 ns) the capacitor value should be 1852 pF For addi-
tional application details refer to application notes AN-22
and AN-108
TL F 5801 – 9
FIGURE 1 Noise Pulse Width vs
Response Control Capacitor
TL F 5801 – 10
FIGURE 2
TL F 5801 – 11
http www national com
6