10
LTC1690
Cables and Data Rate
The transmission line of choice for RS485 applications is
a twisted pair. There are coaxial cables (twinaxial) made
for this purpose that contain straight pairs, but these are
less flexible, more bulky and more costly than twisted
pairs. Many cable manufacturers offer a broad range of
120Ω cables designed for RS485 applications.
Losses in a transmission line are a complex combination
of DC conductor loss, AC losses (skin effect), leakage and
AC losses in the dielectric. In good polyethylene cables
such as Belden 9841, the conductor losses and dielectric
losses are of the same order of magnitude, leading to
relatively low overall loss (Figure 8).
When using low loss cable, Figure 9 can be used as a
guideline for choosing the maximum length for a given
data rate. With lower quality PVC cables, the dielectric loss
factor can be 1000 times worse. PVC twisted pairs have
terrible losses at high data rates (>100kbits/s), reducing
the maximum cable length. At low data rates, they are
acceptable and are more economical. The LTC1690 is
tested and guaranteed to drive CAT 5 cable and termina-
tions as well as common low cost residential telephone
wire.
APPLICATIONS INFORMATION
WUU
U
FREQUENCY (MHz)
0.1
0.1
LOSS PER 100 FT (dB)
1.0
10
1.0 10 100
1690 F08
Figure 8. Attenuation vs Frequency for Belden 9841
DATA RATE (bps)
10k
10
CABLE LENGTH (FT)
100
1k
10k
100k 1M 10M
1690 F09
2.5M
Figure 9. RS485 Cable Length Recommended. Applies
for 24 Gauge, Polyethylene Dielectric Twisted Pair
ESD PROTECTION
The ESD performance of the LTC1690 driver outputs (Z, Y)
and the receiver inputs (A, B) is as follows:
a) Meets ±15kV Human Body Model (100pF, 1.5kΩ).
b) Meets IEC1000-4-2 Level 4 (±8kV) contact mode speci-
fications.
c) Meets IEC1000-4-2 Level 3 (±8kV) air discharge speci-
fications.
This level of ESD performance means that external voltage
suppressors are not required in many applications, when
compared with parts that are only protected to ±2kV. The
LTC1690 driver input (D) and receiver output are pro-
tected to ±2kV per the Human Body Model.
When powered up, the LTC1690 does not latch up or
sustain damage when the Z, Y, A or B pins are subjected
to any of the conditions listed above. The data during the
ESD event may be corrupted, but after the event the
LTC1690 continues to operate normally.
The additional ESD protection at the LTC1690 Z, Y, A and
B pins is important in applications where these pins are
exposed to the external world via socket connections.
