Texas Instruments THS3001 General Information, General High-SpeedAmplifier Design Considerations

General High-Speed Amplifier Design Considerations

1.6 General High-Speed Amplifier Design Considerations

The THS3001 EVM layout has been designed and optimized for use with high-speed signals and can be used as an example when designing THS3001 applications. Careful attention has been given to component selection, grounding, power supply bypassing, and signal path layout. Disregard of these basic design considerations could result in less than optimum performance of the THS3001 high-speed, current-feedback video operational amplifier.

Surface-mount components were selected because of the extremely low lead inductance associated with this technology. Also, because surface-mount components are physically small, the layout can be very compact. This helps minimize both stray inductance and capacitance.

Tantalum power supply bypass capacitors (C1 and C3 for original and C1 and C2 for Rev. A) at the power input pads help supply currents for rapid, large signal changes at the amplifier output. The 0.1 μF power supply bypass capacitors (C2 and C4 for original version and C3 and C4 for Rev. A) were placed as close as possible to the IC power input pins in order to keep the PCB trace inductance to a minimum. This improves high-frequency bypassing and reduces harmonic distortion.

A proper ground plane on both sides of the PCB should always be used with high-speed circuit design. This provides low-inductive ground connections for return current paths. In the area of the amplifier IC input pins, however, the ground plane was removed to minimize stray capacitance and reduce ground plane noise coupling into these pins. This is especially important for the inverting pin while the amplifier is operating in the noninverting mode. Because the voltage at this pin swings directly with the noninverting input voltage, any stray capacitance would allow currents to flow into the ground plane, causing possible gain error and/or oscillation. Capacitance variations at the amplifier IC inverting input pin of less than 1 pF can significantly affect the response of the amplifier.

In general, it is always best to keep signal lines as short and as straight as possible. Sharp 90_corners should be avoided Ð round corners or a series of 45_bends should be used, instead. Stripline techniques should also be incorporated when signal lines are greater than three inches in length. These traces should be designed with a characteristic impedance of either 50 Ω or 75 Ω, as required by the application. Such signal lines should also be properly terminated with an appropriate resistor.

Finally, proper termination of all inputs and outputs should be incorporated into the layout. Unterminated lines, such as coaxial cable, can appear to be a reactive load to the amplifier IC. By terminating a transmission line with its characteristic impedance, the amplifier's load 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 amplifier phase-margin and improves the amplifier stability for improved performance such as reduced peaking and settling times.

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