THS3095_14 Datasheet, PDF (22/37 Pages) Texas Instruments – HIGH-VOLTAGE, LOW-DISTORTION, CURRENT-FEEDBACK OPERATIONAL AMPLIFIERS

THS3091
THS3095
SLOS423G – SEPTEMBER 2003 – REVISED OCTOBER 2008........................................................................................................................................ www.ti.com
farther from the device and may be shared among
several devices in the same area of the PC board.
• Careful selection and placement of external
components preserve the high-frequency
performance of the THS3091/5. Resistors should
be a low reactance type. Surface-mount resistors
work best and allow a tighter overall layout. Again,
keep their leads and PC board trace length as
short as possible. Never use wirebound type
resistors in a high-frequency application. Because
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 >
2 kΩ, this parasitic capacitance can add a pole
and/or a zero that can effect 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 [0.05 inch (1,3 mm) to 0.1
inch (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 (< 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
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 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 based on board material
and trace dimensions, a matching series resistor
into the trace from the output of the THS3091/5 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 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 like the THS3091/5 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
THS3091/5 parts directly onto the board.
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