BUF602 Datasheet, PDF (15/22 Pages) Texas Instruments – High-Speed, Closed-Loop Buffer

BUF602
www.ti.com
In the same manner, line inductance and capacitance
determine the delay time of a transmission line as
shown in Equation 2:
t + ^ǸL_T C_T
(2)
Typical values for Z_O are 240Ω for symmetrical traces
and 75Ω or 50Ω for coax cables. Z_O sometimes
decreases to 30Ω to 40Ω 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
here.
Transmission lines are almost always terminated on
the transmitter line and always terminated on the
receiver side. Unterminated lines generate signal
reflections that degrade the pulse fidelity. The driver
circuit transmits the output voltage (V_OUT) over the
line. The signal appears at the end of the line and will
be reflected when not properly terminated. The
reflected portion of V_OUT, called V_REFL, returns to the
driver. The transmitted signal is the sum of the
original signal V_OUT and the reflected V_REFL.
V_T + V_OUT ) V_REFL
(3)
The magnitude of the reflected signal depends upon
the typical line impedance (Z₀) and the value of the
termination resistor Z₁.
V_REFL + V_OUT G
(4)
Γ denotes the reflection factor and is described by
Equation 5.
ǒ Ǔ G +
Z₁ * Z₀
Z₁ ) Z₀
(5)
Γ can vary from –1 to +1.
The conditions at the corner points of Equation 5 are
as follows:
Z₀ = Z₁
Z₀ = ∞
Z₀ = 0
→ Γ=0
V_REFL = 0
→
Γ = –1 V_REFL = –V_OUT
→
Γ = +1 V_REFL = +V_OUT
An unterminated driver circuit complicates the
situation even more. V_REFL is reflected a second time
on the driver side and wanders like a ping-pong ball
back and forth over the line. When this happens, it is
usually impossible to recover the output signal V_OUT
on the receiver side.
The figure shown in Figure 31 makes use of the
BUF602 as a line driver. The BUF602 exhibits high
input impedance and low output impedance, making it
ideal whenever a buffer is required.
SBOS339 – OCTOBER 2005
R_OUT
Z_O
V_IN
BUF602
V_OUT
R_LOAD
Figure 31. Typical Line Driver Circuit
SELF-BIASED, LOW-IMPEDANCE MID
SUPPLY VOLTAGE REFERENCE
Using the mid-point reference in conjunction with the
BUF602 allows the creation of a low-impedance
reference from DC to 250MHz.
The 0.1µF external capacitor is used in Figure 32 to
filter the noise.
V_S
50kΩ
x1
50kΩ
200Ω
BUF602
x1
V_S/2
20Ω
0.1µF
Figure 32. Self-Biased, Low Impedance Mid
Supply Voltage Reference
SELF-REFERENCED, AC-COUPLED
WIDEBAND BUFFER
Whenever a high-speed AC-coupled buffer is
required, you should consider the BUF602. One
feature of the BUF602 is the mid-supply reference
voltage, saving external components and power
dissipation. A capacitor on the output of the
mid-supply reference is recommended to bandlimit
the noise contribution of the mid-supply reference
voltage generated by the two 50kΩ internal resistors.
This circuit is shown on the front page of the
datasheet.
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