Datasheet

11
LT1713/LT1714
TYPICAL APPLICATIO S
U
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 V
S
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, Z
O
, of the twisted-pair cable.
The input impedance of the resistive network should
match the characteristic impedance and is given by:
RR
RRR
RRRR
IN O
O
=
+
++
[]
2
123
2123
••
||( )
•||( )
This comes out to 120Ω for the values shown. The
Thevenin equivalent source voltage is given by:
VV
RRR
RRR
R
RRRR
TH S
O
O
=
+
++
++
[]
•
(–)
()
•
•||( )
231
231
2123
–
+
1/2
LT1714
TxD
RxD
7
8
9
LE
6
5
49.9Ω
750k
750k
100k
100k
49.9Ω
2
1
15
3
16
13
14
3V
3V
4
11
12
R2A
2.55k
R3A
124Ω
R
OA
140Ω
R
OB
140Ω
R1B
499Ω
6-FEET
TWISTED PAIR
Z
O
≈ 120Ω
R1D
499Ω
R1C
499Ω
R3B
124Ω
R2C
2.55k
3V
100k
5
11
2
1
12
6
10
9
8
7
171314 F03
14
3
15
16
13
4
TxD
RxD
3V
R3C
124Ω
R3D
124Ω
R2D
2.55k
R2B
2.55k
R1A
499Ω
10
–
+
1/2
LT1714
LE
–
+
750k
750k
49.9Ω
49.9Ω
100k
–
+
LE
1/2
LT1714
LE
1/2
LT1714
3V 3V
DIODES: BAV99
×4
Figure 3. 75Mbaud Full Duplex Interface on Two Wires