High-Speed Amplifier PCB Layout Tips
6
SLOU483 – August 2017
Copyright © 2017, Texas Instruments Incorporated
THS3491RGT Evaluation Module
4
High-Speed Amplifier PCB Layout Tips
The THS3491 EVM layout, designed for use with high-speed signals, can be used as an example when
designing PCBs incorporating the THS3491. Careful attention has been given to component selection,
grounding, power supply bypassing, and signal path layout. Disregarding these basic design
considerations could result in less than optimum performance of the THS3491 high-speed operational
amplifier. Surface-mount components were selected because of the extremely low lead inductance
associated with this technology. This helps minimize both stray inductance and capacitance. Also,
because surface-mount components are physically small, the layout can be very compact.
Tantalum power supply bypass capacitors at the power input pads help filter switching transients from the
laboratory power supply. Power supply bypass capacitors are placed as close as possible to the IC power
input pins to minimize the return path impedance. This improves high-frequency bypassing and reduces
harmonic distortion. The GND side of these capacitors should be located close to each other, minimizing
the differential current loops associated with differential output currents. If poor high-frequency
performance is observed, replace the 0.1-
μ
F capacitors with microwave capacitors with a self-resonance
at the frequency that produces trouble. A proper ground plane on both sides of the PCB should be used
with high-speed circuit design. This provides low-inductive ground connections for return current paths.
In the area of the amplifier input pins, the ground plane has been removed to minimize stray capacitance
and to reduce ground plane noise coupling into these pins. This is especially important for the inverting
input pin. A capacitance as low as 1 pF at the inverting input can significantly affect the response of the
amplifier or even cause oscillation.
In general, it is best to keep signal lines as short and as straight as possible. Incorporation of microstrip or
stripline techniques is also recommended when signal lines are greater than 1 inch in length. These traces
must be designed with a characteristic impedance of either 50
Ω
or 75
Ω
, as required by the application.
Such a signal line must also be properly terminated with an appropriate resistor.
The PCB that is used with PowerPAD™ packages must have features included in the design to remove
the heat from the package efficiently. As a minimum, there must be an area of solder-tinned-copper
underneath the PowerPAD package. This area is called the thermal land. The thermal land varies in size
depending on the PowerPAD package being used, the PCB construction, and the amount of heat to be
removed. In addition, this thermal land may or may not contain thermal vias, depending on PCB
construction. The requirements for thermal lands and thermal vias are detailed in
PowerPAD™Thermally
Enhanced Package
(
) and
PowerPAD™ Made Easy
).
Finally, all inputs and outputs must be properly terminated, either in the layout or in the load
instrumentation. Unterminated lines, such as coaxial cable, can appear to be a reactive load to the
amplifier. By terminating a transmission line with its characteristic impedance, the load of the amplifier
then appears to be purely resistive, and reflections are absorbed at each end of the line. Another
advantage of using an output termination resistor is that capacitive loads are isolated from the amplifier
output. This isolation helps minimize the reduction in the phase-margin of the amplifier and improves the
amplifier stability, resulting in reduced peaking and settling times.
On boards operated from dual power supplies, it is helpful to place a capacitor directly across the positive
and negative power supplies. This helps the fully differential drive.
