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

120

Ω 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.

FREQUENCY (MHz)

0.1

0.1

1.0

10

1.0

10

100

1690 F08

Figure 8. Attenuation vs Frequency for Belden 9841

DATA RATE (bps)

10k

10

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)contactmodespeci-

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.