Datasheet

9
LTC485
sn 485LTC485ffs
APPLICATIO S I FOR ATIO
UU W U
LTC485 Line Length vs Data Rate
The maximum line length allowable for the RS422/RS485
standard is 4000 feet.
Figures 17 and 18 show that the LTC485 is able to
comfortably drive 4000 feet of wire at 110kHz.
Figure 14. Line Length Test Circuit
TTL
OUT
LTC485LTC485
NOISE
GENERATOR
100Ω
C
D
4000 FT 26AWG
TWISTED PAIR
A
B
TTL
IN
Using the test circuit in Figure 14, Figures 15 and 16 show
that with ~20V
P-P
common mode noise injected on the
line, The LTC485 is able to reconstruct the data stream at
the end of 4000 feet of twisted pair wire.
RO
DIFFERENTIAL
VOLTAGE A – B
DI
LTC485 • F16
Figure 16. System Differential Voltage at 19.2kHz
Figure 15. System Common Mode Voltage at 19.2kHz
COMMON MODE
VOLTAGE (A + B)/2
RO
DI
LTC485 • F15
Figure 17. System Common Mode Voltage at 110kHz
RO
COMMON MODE
VOLTAGE (A + B)/2
DI
LTC485 • F17
RO
COMMON MODE
VOLTAGE (A – B)
DI
LTC485 • F18
Figure 18. System Differential Voltage at 110kHz
When specifying line length vs maximum data rate the
curve in Figure 19 should be used:
Figure 19. Cable Length vs Maximum Data Rate
MAXIMUM DATA RATE
10k
10
CABLE LENGTH (FT)
100
1k
10k
100k 1M 10M
LTC485 • F19
2.5M