13
LTC1343
APPLICATIONS INFORMATION
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Figure 19. V.28 Receiver Configuration
Figure 21. V.35 Receiver Configuration
The generator differential impedance must be 50Ω to
150Ω and the impedance between shorted terminals (A
and B) and ground C must be 150Ω ±15Ω. For the
generator termination, switches S1 and S2 are both on and
the top side of the center resistor is brought out to a pin so
it can be bypassed with an external capacitor to reduce
common mode noise as shown in Figure 22.
Any mismatch in the driver rise and fall times or skew in
the driver propagation delays will force current through
the center termination resistor to ground, causing a high
frequency common mode spike on the A and B terminals.
The common mode spike can cause EMI problems that are
reduced by capacitor C1 which shunts much of the com-
mon mode energy to ground rather than down the cable.
V.35 Interface
A typical V.35 balanced interface is shown in Figure 20. A
V.35 differential generator with outputs A and B with
ground C is connected to a differential receiver with
ground C', inputs A' connected to A, B' connected to B. The
V.35 interface requires a T or delta network termination at
the receiver end and the generator end. The receiver
differential impedance measured at the connector must be
100Ω␣ ±10Ω, and the impedance between shorted termi-
nals (A' and B) and ground C' must be 150Ω ±15Ω.
In V.35 mode, both switches S1 and S2 inside the LTC1344
are on, connecting the T network impedance as shown in
Figure 21. Both switches in the LTC1343 are off. The 30k
input impedance of the receiver is placed in parallel with
the T network termination, but does not affect the overall
input impedance significantly.
Figure 20. Typical V.35 Interface
Figure 22. V.35 Driver Using the LTC1344
R3
124Ω
R5
20k
LTC1344
LTC1343
RECEIVER
1343 F19
A
B
A
'
B
'
C
'
R1
51.5Ω
R8
6k
S2
S3
S4
R2
51.5Ω
R6
10k
R7
10k
GND
R4
20k
S1
R3
124Ω
R5
20k
LTC1344
LTC1343
RECEIVER
1343 F21
A
B
A
'
B
'
C
'
R1
51.5Ω
R8
6k
S2
S3
S4
R2
51.5Ω
R6
10k
R7
10k
GND
R4
20k
S1
A
A
'
B
C
B
'
C
'
GENERATOR
BALANCED
INTERCONNECTING
CABLE
LOAD
CABLE
TERMINATION
RECEIVER
1343 F20
50Ω
125Ω
50Ω
50Ω
125Ω
50Ω
V.35 DRIVER
A
B
C
51.5Ω
S2
ON
S1
ON
1343 F22
51.5Ω
LTC1344
124Ω
C1
100pF
