LTC2854/LTC2855

11

285455fc

For more information www.linear.com/LTC2854

Switchable Termination

Proper cable termination is very important for good signal

fidelity. If the cable is not terminated with its characteristic

impedance, reflections will result in distorted waveforms.

The LTC2854/LTC2855 are the first 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,thelineterminationforoptimalperformancewhen

configuring transceiver networks.

When the TE pin is high, the termination resistor is en-

abled 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 com-

mon 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 maxi-

mum specified 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

100

FREQUENCY (MHz)

101

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

130

140

150

10

285455 F11

120

110

100

–5

0

5

15

VAB = 2V

APPLICATIONS INFORMATION