THS4500IDR Datasheet, PDF (31/48 Pages) Texas Instruments – WIDEBAND, LOW-DISTORTION, FULLY DIFFERENTIAL AMPLIFIERS

THS4500
THS4501
www.ti.com
SLOS350F – APRIL 2002 – REVISED OCTOBER 2011
PRINTED CIRCUIT BOARD LAYOUT
TECHNIQUES FOR OPTIMAL
PERFORMANCE
Achieving optimum performance with high frequency
amplifier-like devices in the THS4500 family requires
careful attention to PCB layout parasitic and external
component types.
Recommendations that optimize performance include:
• Minimize parasitic capacitance to any ac ground
for all of the signal I/O pins. Parasitic capacitance
on the output and input pins can cause instability.
To reduce unwanted capacitance, a window
around the signal I/O pins should be opened in all
of the ground and power planes around those
pins. Otherwise, ground and power planes should
be unbroken elsewhere on the board.
• Minimize the distance (< 0.25”, 6.35 mm) from the
power-supply pins to high frequency 0.1-μF
decoupling capacitors. At the device pins, the
ground and power-plane layout should not be in
close proximity to the signal I/O pins. Avoid
narrow power and ground traces to minimize
inductance between the pins and the decoupling
capacitors. The power supply connections should
always be decoupled with these capacitors.
Larger (6.8 μF or more) tantalum decoupling
capacitors, effective at lower frequency, should
also be used on the main supply pins. These may
be placed somewhat farther from the device and
may be shared among several devices in the
same area of the PCB. The primary goal is to
minimize the impedance seen in the
differential-current return paths.
• Careful selection and placement of external
components preserve the high-frequency
performance of the THS4500 family. 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 the leads and PCB trace length as short as
possible. Never use wirewound type resistors in a
high-frequency application. Since the output pin
and inverting input pins are the most sensitive to
parasitic capacitance, always position the
feedback and series output resistors, if any, as
close as possible to the inverting input pins and
output pins. Other network components, such as
input termination resistors, should be placed close
to the gain-setting resistors. Even with a low
parasitic capacitance shunting the external
resistors, excessively high resistor values can
create significant time constants that can degrade
performance. Good axial metal-film or
surface-mount resistors have approximately
0.2 pF in shunt with the resistor. For resistor
values greater than 2.0 kΩ, this parasitic
capacitance can add a pole and/or a zero below
400 MHz that can affect circuit operation. Keep
resistor values as low as possible, consistent with
load driving considerations.
• 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 (50 mils to 100 mils, or 1.27
mm to 2.54 mm) should be used, preferably with
ground and power planes opened up around
them. Estimate the total capacitive load and
determine if isolation resistors on the outputs are
necessary. Low parasitic capacitive loads (less
than 4 pF) may not need an R_S since the
THS4500 family is nominally compensated to
operate with a 2-pF parasitic load. Higher parasitic
capacitive loads without an R_S are allowed as the
signal gain increases (increasing the unloaded
phase margin). If a long trace is required, and the
6-dB 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 50-Ω environment is normally not necessary
onboard, and in fact, a higher impedance
environment improves distortion as shown in the
distortion versus load plots. With a characteristic
board trace impedance defined based onboard
material and trace dimensions, a matching series
resistor into the trace from the output of the
THS4500 family 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. If the 6-dB 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. This
configuration 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.
• Socketing a high-speed part such as the THS4500
family is not recommended. The additional lead
length and pin-to-pin capacitance introduced by
the socket can create an extremely troublesome
parasitic network that can make it almost
impossible to achieve a smooth, stable frequency
response. Best results are obtained by soldering
Copyright © 2002–2011, Texas Instruments Incorporated
Product Folder Link(s): THS4500 THS4501
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