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LT1713IMS8 Datasheet PDF : 16 Pages
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LT1713/LT1714
TYPICAL APPLICATIO S
Simultaneous Full Duplex 75Mbaud Interface
with Only Two Wires
The circuit of Figure 3 shows a simple, fully bidirectional,
differential 2-wire interface that gives good results to
75Mbaud, using the LT1714. Eye diagrams under condi-
tions of unidirectional and bidirectional communication
are shown in Figures 4 and 5. Although not as pristine as
the unidirectional performance of Figure⣠4, the perfor-
mance under simultaneous bidirectional operation is still
excellent. Because the LT1714 input voltage range ex-
tends 100mV beyond both supply rails, the circuit works
with a full ±3V (one whole VS up or down) of ground
potential difference.
The circuit works well with the resistor values shown, but
other sets of values can be used. The starting point is the
characteristic impedance, ZO, of the twisted-pair cable.
The input impedance of the resistive network should
match the characteristic impedance and is given by:
RIN
=
2
•
RO
•
RO
R1||(R2 + R3)
+ 2 • [R1||(R2 +
R3)]
This comes out to 120Ω for the values shown. The
Thevenin equivalent source voltage is given by:
VTH
=
VS
•
(R2
(R2
+
+
R3
R3
–
+
R1)
R1)
•
RO
+
2
•
RO
[R1||
(R2
+
R3)]
750k
3V
4
2
14
RxD
1/2
LE LT1714 1
13 16
3
15
750k
100k
3V
49.9Ω 7 +
5
11
TxD
49.9Ω
1/2
8
LT1714
– LE
10
12
6
9
100k
R2A
2.55k
R3A
124Ω
R3B
124Ω
R2B
2.55k
3V
R1A
499Ω
ROA
140Ω
R1B
499Ω
6-FEET
TWISTED PAIR
ZO ≈ 120Ω
DIODES: BAV99
×4
3V
R1C
499Ω
ROB
140Ω
R1D
499Ω
R2C
2.55k
R3C
124Ω
R3D
124Ω
R2D
2.55k
Figure 3. 75Mbaud Full Duplex Interface on Two Wires
750k
3V
2+
4
14
1/2
1
LT1714 LE
–
16
13
RxD
15
3
750k
100k
3V
5
7
11
1/2
LT1714 8
12 6 LE
10
9
49.9Ω
TxD
49.9Ω
100k
171314 F03
11
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