Datasheet

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PowerPAD™ DESIGN CONSIDERATIONS
THS4302
SLOS403H – OCTOBER 2002 – REVISED AUGUST 2006
possible is recommended. set the series resistor value as shown in the plot
of R
ISO
vs Capacitive Load. This does not
3. Careful selection and placement of external
preserve signal integrity as well as a doubly
components preserves the high frequency
terminated line. If the input impedance of the
performance of the THS4302. Resistors should
destination device is low, there is some signal
be a low reactance type. Surface-mount resistors
attenuation due to the voltage divider formed by
work best and allow a tighter overall layout.
the series output into the terminating impedance.
Axially-leaded parts do not provide good high
A 50- Ω environment is normally not necessary on
frequency performance, because they have ~0.8
board as long as the lead lengths are short, and
nH of inductance for every mm of current path
in fact, a higher impedance environment
length. Again, keep PC board trace length as
improves distortion as shown in the distortion
short as possible. Never use wirewound type
versus load plots. Uncontrolled impedance traces
resistors in a high frequency application. Because
without double termination results in reflections at
the output pin and inverting input pin are the most
each end, and hence, produces PCB resonances.
sensitive to parasitic capacitance, always position
It is recommended that if this approach is used,
the terminating resistors, if any, as close as
the trace length be kept short enough to avoid
possible to the noninverting and output pins.
resonances in the band of interest. For guidance
Even with a low parasitic capacitance shunting
on useful lengths, use equation (1) given in the
the external resistors, excessively high resistor
Power Supply Decoupling Techniques section for
values can create significant time constants that
approximate resonance frequencies vs trace
can degrade performance. Good axial metal-film
length. This relation provides an upper bound on
or surface-mount resistors have approximately
the resonant frequency, because additional
0.2 pF in shunt with the resistor.
capacitive coupling to the trace from other leads
4. Connections to other wideband devices on
or the ground plane causes extra distributed
the board may be made with short direct
loading and slows the signal propagation along
traces or through onboard transmission lines.
the trace.
For short connections, consider the trace and the
5. Socketing a high-speed part like the THS4302
input to the next device as a lumped capacitive
is not recommended. The additional lead length
load. Relatively wide traces (50 mils to 100 mils)
inductance and pin-to-pin capacitance introduced
should be used, preferably with ground and
by the socket creates an extremely troublesome
power planes opened up around them. Estimate
parasitic network, which can make it almost
the total capacitive load and set R
ISO
from the
impossible to achieve a smooth, stable frequency
plot of recommended R
ISO
vs Capacitive Load.
response. Best results are obtained by soldering
Low parasitic capacitive loads (<4 pF) may not
the THS4302 onto the board.
need an R
ISO
because THS4302 amplifiers are
nominally compensated to operate with a 2-pF
parasitic load. Higher parasitic capacitive loads
without an R
ISO
are allowed as the signal gain
The THS4302 is available in a thermally enhanced
increases (increasing the unloaded phase
PowerPAD family of packages. These packages are
margin). If a long trace is required, and the 6-dB
constructed using a downset leadframe on which the
signal loss intrinsic to a doubly-terminated
die is mounted [see Figure 49 (a) and Figure 49 (b)].
transmission line is acceptable, implement a
This arrangement results in the lead frame being
matched impedance transmission line using
exposed as a thermal pad on the underside of the
microstrip or stripline techniques (consult an ECL
package [see Figure 49 (c)]. Because this thermal pad
design handbook for microstrip and stripline
has direct thermal contact with the die, excellent
layout techniques). With a characteristic board
thermal performance can be achieved by providing a
trace impedance defined based on board material
good thermal path away from the thermal pad.
and trace dimensions, a matching series resistor
The PowerPAD package allows both assembly and
into the trace from the output of the THS4302 is
thermal management in one manufacturing operation.
used as well as a terminating shunt resistor at the
input of the destination device. Remember also
During the surface-mount solder operation (when the
that the terminating impedance is the parallel
leads are being soldered), the thermal pad can also
combination of the shunt resistor and the input
be soldered to a copper area underneath the
impedance of the destination device: this total
package. Through the use of thermal paths within this
effective impedance should be set to match the
copper area, heat can be conducted away from the
trace impedance. If the 6-dB attenuation of a
package into either a ground plane or other heat
doubly terminated transmission line is
dissipating device.
unacceptable, a long trace can be
series-terminated at the source end only. Treat
the trace as a capacitive load in this case, and
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