Datasheet

LTC2862/LTC2863/
LTC2864/LTC2865
16
2862345f
curve of cable length versus maximum data rate is shown
in Figure 9. Various regions of this curve refl ect different
performance limiting factors in data transmission.
At frequencies below 100kbps, the maximum cable length
is determined by DC resistance in the cable. In this ex-
ample, a cable longer than 4000ft will attenuate the signal
at the far end to less than what can be reliably detected
by the receiver.
APPLICATIONS INFORMATION
It should be emphasized that the plot in Figure 9 shows
a typical relation between maximum data rate and cable
length. Results with the LTC2862 series will vary, de-
pending on cable properties such as conductor gauge,
characteristic impedance, insulation material, and solid
versus stranded conductors.
Low EMI 250kbps Data Rate
The LTC2862-2, LTC2863-2, and the LTC2864-2 feature
slew rate limited transmitters for low electromagnetic
interference (EMI) in sensitive applications. In addition,
the LTC2865 has a logic-selectable 250kbps transmit rate.
The slew rate limit circuit maintains consistent control of
transmitter slew rates across voltage and temperature to
ensure low EMI under all operating conditions. Figure 10
demonstrates the reduction in high frequency content
achieved by the 250kbps mode compared to the 20Mbps
mode.
Figure 9. Cable Length vs Data Rate (RS485/RS422 Standard
Shown in Vertical Solid Line)
Figure 10. High Frequency EMI Reduction of Slew Limited
250kbps Mode Compared to Non Slew Limited 20Mbps Mode
For data rates above 100kbps the capacitive and inductive
properties of the cable begin to dominate this relation-
ship. The attenuation of the cable is frequency and length
dependent, resulting in increased rise and fall times at
the far end of the cable. At high data rates or long cable
lengths, these transition times become a signifi cant part
of the signal bit time. Jitter and intersymbol interference
aggravate this so that the time window for capturing valid
data at the receiver becomes impossibly small.
The boundary at 20Mbps in Figure 9 represents the guar-
anteed maximum operating rate of the LTC2862 series. The
dashed vertical line at 10Mbps represents the specifi ed
maximum data rate in the RS485 standard. This boundary
is not a limit, but refl ects the maximum data rate that the
specifi cation was written for.
DATA RATE (bps)
10k
10
CABLE LENGTH (FT)
100
1k
10k
100k 1M 10M
2862345 F09
100M
LOW EMI
MODE
SLO = GND
RS485
STANDARD
SPEC
The 250kbps mode has the added advantage of reducing
signal reflections in an unterminated network, and there-
by increasing the length of a network that can be used
without termination. Using the rule of thumb that the rise
time of the transmitter should be greater than four times
the one-way delay of the signal, networks of up to 140
feet can be driven without termination.
FREQUENCY (MHz)
0
–120
Y–Z (NON SLEW LIMITED) (dB)
–40
–60
–80
–100
–20
0
20
–60
Y–Z (SLEW LIMITED) (dB)
20
0
–20
–40
40
60
80
2
46810
2862345 F10
12
NON SLEW LIMITED
SLEW LIMITED