Datasheet

BOARD LAYOUT GUIDELINES
THS6214
SBOS431 – MAY 2009 ....................................................................................................................................................................................................... www.ti.com
peaked frequency response. The 1.24k Ω feedback
resistor used in the Typical Characteristics at a gain
Achieving optimum performance with a
of +10V/V on ± 12V supplies is a good starting point
high-frequency amplifier such as the THS6214
for design. Note that a 1.5k Ω feedback resistor,
requires careful attention to board layout parasitic and
rather than a direct short, is recommended for a
external component types. Recommendations that
unity-gain follower application. A current-feedback op
optimize performance include:
amp requires a feedback resistor to control stability
even in the unity-gain follower configuration.
a) Minimize parasitic capacitance to any ac ground
for all of the signal I/O pins. Parasitic capacitance on
d) Connections to other wideband devices on the
the output and inverting input pins can cause
board may be made with short direct traces or
instability; on the noninverting input, it can react with
through onboard transmission lines. For short
the source impedance to cause unintentional band
connections, consider the trace and the input to the
limiting. To reduce unwanted capacitance, a window
next device as a lumped capacitive load. Relatively
around the signal I/O pins should be opened in all of
wide traces (50mils to 100mils [.050in to .100in, or
the ground and power planes around those pins.
1,27mm to 2,54mm]) should be used, preferably with
Otherwise, ground and power planes should be
ground and power planes opened up around them.
unbroken elsewhere on the board.
Estimate the total capacitive load and set R
S
from the
plot of Recommended R
S
vs Capacitive Load (see
b) Minimize the distance (less than 0.25in, or
Figure 6 , Figure 24 , Figure 36 , Figure 48 , Figure 61 ,
6,35mm) from the power-supply pins to
and Figure 73 ). Low parasitic capacitive loads (less
high-frequency 0.1 µ F decoupling capacitors. At the
than 5pF) may not need an isolation resistor because
device pins, the ground and power plane layout
the THS6214 is nominally compensated to operate
should not be in close proximity to the signal I/O pins.
with a 2pF parasitic load. If a long trace is required,
Avoid narrow power and ground traces to minimize
and the 6dB signal loss intrinsic to a
inductance between the pins and the decoupling
doubly-terminated transmission line is acceptable,
capacitors. The power-supply connections should
implement a matched-impedance transmission line
always be decoupled with these capacitors. An
using microstrip or stripline techniques (consult an
optional supply decoupling capacitor across the two
ECL design handbook for microstrip and stripline
power supplies (for bipolar operation) improves
layout techniques). A 50 Ω environment is not
second-harmonic distortion performance. Larger
necessary on board; in fact, a higher impedance
(2.2 µ F to 6.8 µ F) decoupling capacitors, effective at
environment improves distortion (see the distortion
lower frequencies, should also be used on the main
versus load plots). With a characteristic board trace
supply pins. These capacitors can be placed
impedance defined based on board material and
somewhat farther from the device and may be shared
trace dimensions, a matching series resistor into the
among several devices in the same area of the PCB.
trace from the output of the THS6214 is used, as well
c) Careful selection and placement of external
as a terminating shunt resistor at the input of the
components preserve the high-frequency
destination device. Remember also that the
performance of the THS6214. Resistors should be a
terminating impedance is the parallel combination of
very low reactance type. Surface-mount resistors
the shunt resistor and the input impedance of the
work best and allow a tighter overall layout. Metal film
destination device.
and carbon composition, axially-leaded resistors can
This total effective impedance should be set to match
also provide good high-frequency performance.
the trace impedance. The high output voltage and
Again, keep leads and PCB trace length as short as
current capability of the THS6214 allows multiple
possible. Never use wire-wound type resistors in a
destination devices to be handled as separate
high-frequency application. Although the output pin
transmission lines, each with their own series and
and inverting input pin are the most sensitive to
shunt terminations. If the 6dB attenuation of a
parasitic capacitance, always position the feedback
doubly-terminated transmission line is unacceptable,
and series output resistor, if any, as close as possible
a long trace can be series-terminated at the source
to the output pin. Other network components, such as
end only.
noninverting input termination resistors, should also
Treat the trace as a capacitive load in this case and
be placed close to the package. Where double-side
set the series resistor value as shown in the plot of
component mounting is allowed, place the feedback
R
S
vs Capacitive Load. However, this configuration
resistor directly under the package on the other side
does not preserve signal integrity as well as a
of the board between the output and inverting input
doubly-terminated line. If the input impedance of the
pins. The frequency response is primarily determined
destination device is low, there is some signal
by the feedback resistor value as described
attenuation as a result of the voltage divider formed
previously. Increasing the value reduces the
by the series output into the terminating impedance.
bandwidth, whereas decreasing it leads to a more
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