Datasheet

t + L
T
C
T
Ǹ
(2)
R
LOAD
R
OUT
Z
O
V
IN
V
OUT
BUF602
SELF-BIASED, LOW-IMPEDANCE
V
T
+ V
OUT
) V
REFL
(3)
V
REFL
+ V
OUT
G
(4)
Z Z-
1 O
Z Z
1 O
+
G =
(5)
50k
Ω
50k
Ω
V
S
V
S
/2
BUF602
200
Ω
0.1
µ
F
20
Ω
x1
x1
SELF-REFERENCED, AC-COUPLED
BUF602
SBOS339B – OCTOBER 2005 – REVISED MAY 2008 ...................................................................................................................................................... www.ti.com
In the same manner, line inductance and capacitance The figure shown in Figure 33 makes use of the
determine the delay time of a transmission line as BUF602 as a line driver. The BUF602 exhibits high
shown in Equation 2 : input impedance and low output impedance, making it
ideal whenever a buffer is required.
Typical values for Z
O
are 240 Ω for symmetrical traces
and 75 Ω or 50 Ω for coaxial cables. Z
O
sometimes
decreases to 30 Ω to 40 Ω in high data rate bus
systems for bus lines on printed circuit boards
(PCBs). In general, the more complex a bus system
is, the lower Z
O
will be. Because it increases the
capacitance of the transmission medium, a complex
system lowers the typical line impedance, resulting in
higher drive requirements for the line drivers used
Figure 33. Typical Line Driver Circuit
here.
Transmission lines are almost always terminated on
the transmitter line and always terminated on the
MID-SUPPLY VOLTAGE REFERENCE
receiver side. Unterminated lines generate signal
reflections that degrade the pulse fidelity. The driver
Using the midpoint reference in conjunction with the
circuit transmits the output voltage (V
OUT
) over the
BUF602 allows the creation of a low-impedance
line. The signal appears at the end of the line and will
reference from DC to 250MHz.
be reflected when not properly terminated. The
The 0.1 µ F external capacitor is used in Figure 34 to
reflected portion of V
OUT
, called V
REFL
, returns to the
filter the noise.
driver. The transmitted signal is the sum of the
original signal V
OUT
and the reflected V
REFL
.
The magnitude of the reflected signal depends upon
the typical line impedance (Z
O
) and the value of the
termination resistor Z
1
.
Γ denotes the reflection factor and is described by
Equation 5 .
Γ can vary from – 1 to +1.
The conditions at the corner points of Equation 5 are
Figure 34. Self-Biased, Low Impedance
as follows:
Mid-Supply Voltage Reference
Z
O
= Z
1
→ Γ = 0 V
REFL
= 0
Z
O
= ∞ → Γ = – 1 V
REFL
= – V
OUT
Z
O
= 0 → Γ = +1 V
REFL
= +V
OUT
WIDEBAND BUFFER
An unterminated driver circuit complicates the
Whenever a high-speed AC-coupled buffer is
situation even more. V
REFL
is reflected a second time
required, you should consider the BUF602. One
on the driver side and wanders like a ping-pong ball
feature of the BUF602 is the mid-supply reference
back and forth over the line. When this happens, it is
voltage, saving external components and power
usually impossible to recover the output signal V
OUT
dissipation. A capacitor on the output of the
on the receiver side.
mid-supply reference is recommended to bandlimit
the noise contribution of the mid-supply reference
voltage generated by the two 50k Ω internal resistors.
This circuit is shown on the front page of the
datasheet.
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