Datasheet

LTC2854/LTC2855
11
285455fb
Switchable Termination
Proper cable termination is very important for good
signal fi delity. If the cable is not terminated with its char-
acteristic impedance, refl ections will result in distorted
waveforms.
The LTC2854/LTC2855 are the fi rst 3.3V RS485/RS422
transceivers to offer integrated switchable termination
resistors on the receiver input pins. This provides the
advantage of being able to easily change, through logic
control, the line termination for optimal performance when
confi guring transceiver networks.
When the TE pin is high, the termination resistor is enabled
and the differential resistance from A to B is 120Ω. Figure
10 shows the I/V characteristics between pins A and B
with the termination resistor enabled and disabled. The
resistance is maintained over the entire RS485 common
mode range of –7V to +12V as shown in Figure 11.
The integrated termination resistor has a high frequency
response which does not limit performance at the maximum
specifi ed data rate. Figure 12 shows the magnitude and
phase of the termination impedance vs frequency.
Figure 10. Curve Trace Between A and B
with Termination Enabled and Disabled
Figure 11. Typical Resistance of the Enabled
Terminator vs Voltage on B Pin
Figure 12. Termination Magnitude
and Phase vs Frequency
10
–1
10
0
FREQUENCY (MHz)
MAGNITUDE (Ω)
PHASE (°)
10
1
80
95
110
125
140
155
170
185
–75
–60
–45
–30
–15
0
15
30
285455 F12
MAGNITUDE
PHASE
COMMON MODE VOLTAGE (V)
–10
RESISTANCE (Ω)
130
140
150
10
285455 F11
120
110
100
–5
0
5
15
V
AB
= 2V
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