LTC2855 Linear Technology, LTC2855 Datasheet - Page 11

• Manufacturer Part Number: LTC2855
• Description: 3.3V 20Mbps RS485/RS422 Transceivers
• Manufacturer: Linear Technology

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APPLICATIONS INFORMATION
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.
Figure 10. Curve Trace Between A and B
with Termination Enabled and Disabled
185
170
155
140
125
110
95
80
10
Figure 12. Termination Magnitude
and Phase vs Frequency
–1
FREQUENCY (MHz)
MAGNITUDE
10
0
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.
When the TE pin is high, the termination resistor is enabled
PHASE
150
140
120
110
100
130
10
Figure 11. Typical Resistance of the
Enabled Terminator vs Voltage on B Pin
285455 F12
1
–10
30
15
0
–15
–30
– 45
– 60
– 75
V
AB
= 2V
–5
COMMON MODE VOLTAGE (V)
LTC2854/LTC2855
0
5
10
285455 F11
15
11
285455fa