LTC2856-1 Datasheet, PDF (13/20 Pages) Linear Technology – 20Mbps and Slew Rate Limited 15kV RS485/RS422 Transceivers

LTC2856-1/LTC2856-2
LTC2857-1/LTC2857-2
LTC2858-1/LTC2858-2
APPLICATIONS INFORMATION
75
R_DIFF = 54Ω
70
65
C_L = 1000pF
60
55
C_L = 100pF
50
4510₂
10³
10⁴
DATA RATE (kbps)
10⁵
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Figure 8. Supply Current vs Data Rate
High Speed Considerations
A ground plane layout is recommended. 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 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
sensitive inputs to reduce feedback effects that might
cause noise, jitter or even oscillations. For example, in the
full-duplex LTC2857-1, DI and A/B should not be routed
near the driver or receiver outputs.
The logic inputs have 100mV 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 reduce 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 RS485 standard is
shown in Figure 9. Three regions of this curve reflect differ-
ent 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 RS485 standard. The
dashed line at 250kbps shows the maximum data rate of
the low-EMI LTC2856-2, LTC2857-2, and LTC2858-2. The
dashed line at 20Mbps shows the maximum data rates of
the LTC2856-1, LTC2857-1 and LTC2858-1.
10k
LOW-EMI MODE
1k
MAX DATA RATE
NORMAL
MODE MAX
100
DATA RATE
10
10k
RS485 MAX
DATA RATE
100k
1M
10M
100M
DATA RATE (bps)
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Figure 9. Cable Length vs Data Rate
(RS485 Standard Shown in Solid Vertical Lines)
Cable Termination
Proper cable termination is very important for good
signal fidelity. If the cable is not terminated with its char-
acteristic impedance, reflections will result in distorted
waveforms.
RS485 transceivers typically communicate over twisted-
pair cables with characteristic impedance ranging from
100Ω to 120Ω. Each end of the network should be termi-
nated with a discrete resistor matching the characteristic
impedance or with an LTC2859/LTC2861 transceiver with
integrated termination capability.
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