DS78LS120QML Datasheet, PDF (5/12 Pages) Texas Instruments – Dual Differential Line Receiver (Noise Filtering and Fail-Safe)

DS78LS120 is OBSOLETE
DS78LS120QML, DS78LS120
www.ti.com
SNLS370C – SEPTEMBER 1999 – REVISED APRIL 2013
LINE DRIVERS
Line drivers which will interface with the DS78LS120QML are listed below.
Balanced Drivers
DS26LS31: Quad RS-422 Line Driver, Dual CMOS
DS7830, DS8830: Dual TTL
DS7831, DS8831: Dual TRI-STATE TTL
DS7832, DS8832: Dual TRI-STATE TTL
DS1691A, DS3691: Quad RS-423/Dual RS-422 TTL
DS1692, DS3692: Quad RS-423/Dual TRI-STATE RS-422 TTL
DS3487: Quad TRI-STATE RS-422
Unbalanced Drivers
DS1488: Quad RS-232
DS75150: Dual RS-232
RESPONSE CONTROL AND HYSTERESIS
In unbalanced (RS-232/RS-423) applications it is recommended 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 DS78LS120QML by the
50 mV of hysteresis incorporated in the output gate. This eliminates the oscillations which may appear in a line
receiver 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
amplitude by filtering the device input. On the DS78LS120QML, 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 response control capacitor is shown in Figure 6 and Figure 7.
This combination of filters followed by hysteresis will optimize performance in a worse case noise environment.
Figure 6. Noise Pulse Width vs
Response Control Capacitor
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