How many ground connections do I need to make when using flying lead probes with a logic analyzer?
Generally speaking, the higher your data rate, the more ground connections you will need. We recommend that you use a minimum of one ground connection for every two signals.
Flying lead probes allow the user to select the number of grounds. You can use only the pod ground (one ground lead for each 16 signals), or you can ground any number of the individual signal leads (up to one ground per signal).
To understand the problem of insufficient grounding, consider a 16-channel flying lead probe that only has one ground connection. The return current for all 16 signals must travel through the single ground connection. When the current from only one or two signals must return through the single ground connection, the self-inductance of the ground lead is low enough to prevent development of a significant voltage across it. But with 16 signals, the current can become large enough to make the induced voltage problematic.
Solving this problem requires increasing the number of grounds. Ideally, there will be one ground for each signal. The number of grounds required is proportional to frequency. We recommend that you use at least one ground connection per every two signals whenever possible.
If you experience problems capturing data correctly with a logic analyzer, insufficient grounding might be the reason.