Datasheet
LTC2854/LTC2855
12
285455fc
For more information www.linear.com/LTC2854
285455 F14
DATA RATE (bps)
CABLE LENGTH (FT)
10k 1M 10M100k 100M
100
1k
10
10k
RS485/RS422
MAX DATA RATE
LTC2854/LTC2855
MAX DATA RATE
Figure 14. Cable Length vs Data Rate (RS485/
RS422 Standards Shown in Vertical Solid Line)
Figure 13. Supply Current vs Data Rate
DATA RATE (Mbps)
0.1
20
SUPPLY CURRENT (mA)
60
70
80
1 10 100
285455 F13
50
40
30
R
DIFF
= 54Ω
C
L
= 1000pF
C
L
= 100pF
APPLICATIONS INFORMATION
Supply Current
The unloaded static supply currents in the LTC2854/
LTC2855 are very low —typically under 500µA for all modes
of operation. In applications with resistively terminated
cables, the supply current is dominated by the driver load.
For example, when using two 120Ω terminators with a
differential driver output voltage of 2V, the DC current is
33mA, which is sourced by the positive voltage supply.
This
is true whether the terminators are external or internal
such as in the LTC2854/LTC2855. Power supply current
increases with toggling rate due to capacitive loading and
this term can increase significantly at high data rates. Fig-
ure 13 shows supply current vs data rate for two different
capacitive loads for the circuit configuration of Figure 4.
High Speed Considerations
A ground plane layout is recommended for the LTC
2854/
LTC2855. A 0.1µF bypass capacitor less than one-quarter
inch away from the V
CC
pin is also recommended. The PC
board traces connected to signals A/B and Z/Y (LTC2855)
should be symmetrical and as short as possible to maintain
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 LTC2855, DI and A/B should not be routed near
the driver or receiver outputs.
The logic inputs of the
LTC2854/LTC2855 have 150mV of
hysteresis to provide noise immunity. Fast edges on the
outputs can cause glitches in the ground and power supplies
which are exacerbated by capacitive loading. If a logic input
is held near its threshold (typically 1.5V), a noise glitch
from a driver transition may exceed the hysteresis levels
on the logic and data input pins causing an unintended
state change. This
can be avoided by maintaining normal
logic levels on the pins and by slewing inputs through
their thresholds by faster than 1V/µs when transitioning.
Good supply decoupling and proper line termination also
reduces glitches caused by driver transitions.
Cable Length vs Data Rate
For a given data rate, the maximum transmission distance
is bounded by the cable properties. A typical curve of
cable length vs data rate compliant with the RS
485/RS422
standards is shown in Figure 14. Three regions of this
curve reflect different performance limiting factors in data
transmission. In the flat region of the curve, maximum
distance is determined by resistive losses in the cable. The
downward sloping region represents limits in distance and
data rate due to AC losses in the cable. The solid vertical
line represents the specified maximum data rate in the
RS
485/RS422 standards. The dashed lines at 20Mbps
show the maximum data rates of the LTC2854/LTC2855.
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