THS6204 Datasheet, PDF (32/43 Pages) Texas Instruments – Dual-Port, Differential VDSL2 Line Driver Amplifiers

THS6204
SBOS416C – OCTOBER 2007 – REVISED APRIL 2009 ................................................................................................................................................... www.ti.com
c) Careful selection and placement of external
components preserve the high-frequency
performance of the THS6204. Resistors should be a
very low reactance type. Surface-mount resistors
work best and allow a tighter overall layout. Metal film
and carbon composition axially leaded resistors can
also provide good high-frequency performance.
Again, keep leads and PCB trace length as short as
possible. Never use wire-wound type resistors in a
high-frequency application. Although the output pin
and inverting input pin are the most sensitive to
parasitic capacitance, always position the feedback
and series output resistor, if any, as close as possible
to the output pin. Other network components, such as
noninverting input termination resistors, should also
be placed close to the package. Where double-side
component mounting is allowed, place the feedback
resistor directly under the package on the other side
of the board between the output and inverting input
pins. The frequency response is primarily determined
by the feedback resistor value as described
previously. Increasing the value reduces the
bandwidth, whereas decreasing it gives a more
peaked frequency response. The 1.24kΩ feedback
resistor used in the Typical Characteristics at a gain
of +10V/V on ±12V supplies is a good starting point
for design. Note that a 1.5kΩ feedback resistor,
rather than a direct short, is recommended for the
unity-gain follower application. A current-feedback op
amp requires a feedback resistor even in the
unity-gain follower configuration to control stability.
d) Connections to other wideband devices on the
board may be made with short direct traces or
through onboard transmission lines. For short
connections, consider the trace and the input to the
next device as a lumped capacitive load. Relatively
wide traces (50mils to 100mils) should be used,
preferably with ground and power planes opened up
around them. Estimate the total capacitive load and
set R_S from the plot of Recommended R_S vs
Capacitive Load (Figure 6, Figure 24, and Figure 36).
Low parasitic capacitive loads (< 5pF) may not need
an R_S because the THS6204 is nominally
compensated to operate with a 2pF parasitic load. If a
long trace is required, and the 6dB signal loss
intrinsic to a doubly-terminated transmission line is
acceptable, implement a matched impedance
transmission line using microstrip or stripline
techniques (consult an ECL design handbook for
microstrip and stripline layout techniques). A 50Ω
environment is normally not necessary on board; in
fact, a higher impedance environment improves
distortion (see the distortion versus load plots). With a
characteristic board trace impedance defined based
on board material and trace dimensions, a matching
series resistor into the trace from the output of the
THS6204 is used, as well as a terminating shunt
resistor at the input of the destination device.
Remember also that the terminating impedance is the
parallel combination of the shunt resistor and the
input impedance of the destination device.
This total effective impedance should be set to match
the trace impedance. The high output voltage and
current capability of the THS6204 allows multiple
destination devices to be handled as separate
transmission lines, each with their own series and
shunt terminations. If the 6dB attenuation of a
doubly-terminated transmission line is unacceptable,
a long trace can be series-terminated at the source
end only.
Treat the trace as a capacitive load in this case and
set the series resistor value as shown in the plot of
R_S vs Capacitive Load (Figure 6, Figure 24, and
Figure 36). However, this does not preserve signal
integrity as well as a doubly-terminated line. If the
input impedance of the destination device is low,
there is some signal attenuation due to the voltage
divider formed by the series output into the
terminating impedance.
e) Socketing a high-speed part like the THS6204
is not recommended. The additional lead length and
pin-to-pin capacitance introduced by the socket can
create an extremely troublesome parasitic network,
which can make it almost impossible to achieve a
smooth, stable frequency response. Best results are
obtained by soldering the THS6204 directly onto the
board.
f) Use the –V_S plane to conduct heat out of the
QFN-24 and TSSOP-24 PowerPAD packages. These
packages attach the die directly to an exposed
thermal pad on the bottom, which should be soldered
to the board. This pad must be connected electrically
to the same voltage plane as the most negative
supply applied to the THS6204 (in Figure 82, this
would be –12V).
32
Submit Documentation Feedback
Product Folder Link(s): THS6204
Copyright © 2007–2009, Texas Instruments Incorporated