LTC2859/LTC2861

12

285961fc

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. The

termination resistor cannot be enabled by TE if the device

is unpowered or in thermal shutdown mode.

Supply Current

The unloaded static supply currents in the LTC2859/

LTC2861areverylow—typicallyunder700µAforallmodes

of operation without the internal terminator enabled. In

applications with resistively terminated cables, the supply

currentisdominatedbythedriverload.Forexample,when

usingtwo120Ωterminatorswithadifferentialdriveroutput

voltage of 2V, the DC current is 33mA, which is sourced

by the positive voltage supply. This is true whether the

terminatorsareexternalorinternalsuchasintheLTC2859/

LTC2861. Power supply current increases with toggling

data due to capacitive loading and this term can increase

significantly at high data rates. Figure 13 shows supply

current vs data rate for two different capacitive loads (for

the circuit configuration of Figure 3).

High Speed Considerations

A ground plane layout is recommended for the LTC2859/

LTC2861. A 0.1µF bypass capacitor less than one quarter

board traces connected to signals A/B and Z/Y (LTC2861)

shouldbesymmetricalandasshortaspossibletomaintain

good differential signal integrity. To minimize capacitive

effects, the differential signals should be separated by

more than the width of a trace and should not be routed

on top of each other if they are on different signal planes.

Care should be taken to route outputs away from any sen-

sitive inputs to reduce feedback effects that might cause

noise, jitter, or even oscillations. For example, in the full

duplex LTC2861, DI and A/B should not be routed near

the driver or receiver outputs.

The logic inputs of the LTC2859/LTC2861 have 50mV of

hysteresis to provide noise immunity. Fast edges on the

outputscancauseglitchesinthegroundandpowersupplies

whichareexacerbatedbycapacitiveloading.Ifalogicinput

is held near its threshold (typically 1.5V), a noise glitch

Figure 11. Termination Resistance

vs Common Mode Voltage

Figure 12. Termination Magnitude

and Phase vs Frequency

Figure 13. Supply Current vs Data Rate

applicaTions inFormaTion

COMMON MODE VOLTAGE (V)

–10

130

140

10

285961 F11

120

110

–5

0

5

15

150

FREQUENCY (MHz)

80

95

110

125

140

155

170

185

–75

–60

–45

–30

–15

0

15

30

285455 F12

MAGNITUDE

PHASE

DATA RATE (kbps)

45

50

55

60

65

70

75

285961 F13