Common Mode Floating

Common Mode Terminated to

Ground at High Frequencies

BAD

GOOD

Common Mode Floating

Common Mode Terminated to

Ground at all Frequencies

BAD

BETTER

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Conclusion

As shown in Figure 7 the LMH6522 DVGA has excellent CMRR at modest frequencies, but as frequencies

increase, the CMRR gets worse. Fortunately, the board layout can help here as well. As shown in the

design of the filter. Creative use of the filter elements and the termination resistors can help reduce

common mode noise with very small changes in component configurations. Using capacitors to ground or

providing resistive termination to ground directs the common-mode energy to ground instead of

conducting it to the next circuit element.

Figure 8. Filters With and Without Common Mode Termination

Figure 9. Filters With and Without Common Mode High-Frequency Bypassing

2

Conclusion

High-performance circuit elements require support from a carefully planned printed circuit-board layout. By

following the following key guidelines, high-speed, high-performance circuits will perform to their highest

potential.

•

Place bypass capacitors close to supply pins

•

Locate termination resistors close to output pins

•

Remove metal from beneath input and output pins

•

Terminate transmission lines

•

Keep filters close to the amplifier

•

Provide both differential and common mode termination

5

SNOA869A – April 2012 – Revised May 2013

AN-2235 Circuit Board Design for LMH6517/21/22 and Other High-Speed

IF/RF Feedback Amplifiers

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