Texas Instruments THS4503EVM manual Chapter, High-SpeedAmplifier PCB Layout Tips

Chapter 4

The THS4503EVM layout has been designed for use with high-speed signals and can be used as an example when designing PCBs incorporating the THS4503. 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 THS4503 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 in order 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-µF 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, however, the ground plane has been removed to minimize stray capacitance and reduce ground plane noise coupling into these pins. This is especially important for the inverting input pin. As low as 1 pF capacitance at the inverting input can significantly affect the response of the amplifier or even 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.

High-Speed Amplifier PCB Layout Tips 4-1