Datasheet

THS3091
THS3095
SLOS423G – SEPTEMBER 2003 – REVISED OCTOBER 2008 ........................................................................................................................................ www.ti.com
farther from the device and may be shared among signal loss intrinsic to a doubly terminated
several devices in the same area of the PC board. transmission line is acceptable, implement a
matched impedance transmission line using
• Careful selection and placement of external
microstrip or stripline techniques (consult an ECL
components preserve the high-frequency
design handbook for microstrip and stripline layout
performance of the THS3091/5. Resistors should
techniques). A 50- Ω environment is not necessary
be a low reactance type. Surface-mount resistors
onboard, and in fact, a higher impedance
work best and allow a tighter overall layout. Again,
environment improves distortion as shown in the
keep their leads and PC board trace length as
distortion versus load plots. With a characteristic
short as possible. Never use wirebound type
board trace impedance based on board material
resistors in a high-frequency application. Because
and trace dimensions, a matching series resistor
the output pin and inverting input pins are the
into the trace from the output of the THS3091/5 is
most sensitive to parasitic capacitance, always
used as well as a terminating shunt resistor at the
position the feedback and series output resistors,
input of the destination device. Remember also
if any, as close as possible to the inverting input
that the terminating impedance is the parallel
pins and output pins. Other network components,
combination of the shunt resistor and the input
such as input termination resistors, should be
impedance of the destination device; this total
placed close to the gain-setting resistors. Even
effective impedance should be set to match the
with a low parasitic capacitance shunting the
trace impedance. If the 6-dB attenuation of a
external resistors, excessively high resistor values
doubly terminated transmission line is
can create significant time constants that can
unacceptable, a long trace can be series-
degrade performance. Good axial metal-film or
terminated at the source end only. Treat the trace
surface-mount resistors have approximately 0.2
as a capacitive load in this case. This does not
pF in shunt with the resistor. For resistor values >
preserve signal integrity as well as a doubly
2 k Ω , this parasitic capacitance can add a pole
terminated line. If the input impedance of the
and/or a zero that can effect circuit operation.
destination device is low, there is some signal
Keep resistor values as low as possible,
attenuation due to the voltage divider formed by
consistent with load-driving considerations.
the series output into the terminating impedance.
• Connections to other wideband devices on the
• Socketing a high-speed part like the THS3091/5 is
board may be made with short direct traces or
not recommended. The additional lead length and
through onboard transmission lines. For short
pin-to-pin capacitance introduced by the socket
connections, consider the trace and the input to
can create an extremely troublesome parasitic
the next device as a lumped capacitive load.
network which can make it almost impossible to
Relatively wide traces [0.05 inch (1,3 mm) to 0.1
achieve a smooth, stable frequency response.
inch (2,54 mm)] should be used, preferably with
Best results are obtained by soldering the
ground and power planes opened up around
THS3091/5 parts directly onto the board.
them. Estimate the total capacitive load and
determine if isolation resistors on the outputs are
necessary. Low parasitic capacitive loads ( < 4 pF)
may not need an R
S
because the THS3091/5 are
nominally compensated to operate with a 2-pF
parasitic load. Higher parasitic capacitive loads
without an RS are allowed as the signal gain
increases (increasing the unloaded phase
margin). If a long trace is required, and the 6-dB
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Product Folder Link(s): THS3091 THS3095