Datasheet

MAX3172/MAX3174
+3.3V Multiprotocol Software-Selectable
Cable Terminators and Transceivers
______________________________________________________________________________________ 13
In Figure 9, the MAX3172/MAX3174 are used to imple-
ment the resistive T-network that is needed to properly
terminate the V.35 driver and receiver. Internal to the
MAX3172/MAX3174, S1 and S2 are closed to connect
the T-network resistors to the circuit. The V.28 termina-
tion resistor, internal to the MAX3170, is disabled by
opening S3 to avoid interference with the T-network
impedance.
The V.35 specification allows for ±4V of ground differ-
ence between the V.35 generator and V.35 load. The
V.35 data rates may be as high as 10Mbps. The
MAX3172/MAX3174 maintain correct terminal imped-
ances over these conditions.
V.35 EMI Reduction
For applications where EMI reduction is especially
important, the MAX3172/MAX3174 termination net-
works provide a pin for shunting common-mode driver
currents to GND (Figure 10). Mismatches between A
and B driver output propagation delays create a com-
mon-mode disturbance on the cable. This common-
mode energy can be shunted to GND by placing a
100pF capacitor (C1 to GND) from the center point of
the T-network termination (R1C, R2C, and R3C).
V.28 Termination
Most industry-standard V.28 receivers (including the
MAX3170) do not require external termination because
the receiver includes an internal 5kΩ termination resis-
tor. When the MAX3172/MAX3174 are placed in V.28
mode, all five of the termination networks are placed in
a high-Z mode. In high-Z mode, the MAX3172/
MAX3174 termination networks will not interfere with the
MAX3170's internal 5kΩ termination.
Figure 9. V.35 Termination and Internal Resistance Networks
R6
10kΩ
R8
5kΩ
R3
127Ω
R2
52Ω
R1
52Ω
A′
B′
C′
A
B
GND
R5
30kΩ
R7
10kΩ
R4
30kΩ
MAX3172
MAX3174
MAX3170
S3
S2
S1
RECEIVER
Figure 10. V.35 Driver
R3
127Ω
R2
52Ω
R1
52Ω
A
B
C(GND)
MAX3172
MAX3174
S1
S2
C1
100pF
DRIVER
R_C