Logic analysis is a powerful tool. However, the most powerful logic analyzer is useless without a sound probing connection to a system under test. If you approach the problem carefully, you'll find that you can even probe those "forgotten" signals. From eeProductCenter's Test and Measurement section, Agilent Technologies' engineers offer these pointers.
Logic analyzer probing can be classified into two groups. The first group consists of the signals that were "designed in." This means that signals that are to be observed were intentionally routed to a logic analyzer connector. These signals are then easily probed using a standard logic analyzer probe.
The second group of signals consists of the "forgotten signals." These are signals that weren't brought out to a connector---but still need to be probed for a successful logic analyzer measurement. Until recently, these forgotten signals were a headache.
In most cases, a messy rework was needed to gain accessibility to them. If that was impractical, elaborate work-arounds were created to make measurements without these signals.
In either case, compromises had to be made in either bandwidth or triggering, and the measurement that was ultimately taken wasn't necessarily what the designer wanted.
There Is Another Way
Today, differential flying leads make probing forgotten signals easy. Not only can signals be probed effortlessly, but full-bandwidth measurements can be made.
The key is to use flying leads such as Agilent's E5381A leads. Let's see how they can contribute to ease-of-connection to seemingly impossible-to-probe signals, looking at real world examples of electrical loading and performance.
A differential flying lead set normally consists of 17 channels, with both differential and singled-ended capability. The probe has 17 individual cables, each containing the probe tip network.
This topology provides the maximum probing flexibility while still achieving full bandwidth measurements. This type of probe exists for use with today's modern logic analyzers.
Each of the 17 probe tips contains a P and an N node. For a differential measurement, each side of a signal pair is connected to the appropriate node. A standard differential measurement can then be performed by the probe (i.e., P-N).
The probe is capable of measuring a differential signal swing of 200-mV at the tip network (P=100-mVpp, N=100-mVpp). Also, a typical probe tip has a DC loading of about 20-kohms. Capacitive loading depends on which accessory is used to connect to your target, but it can be as low as 0.9-pF.
Single-ended measurements can also be performed with these flying leads. You only need to connect the N node of the probe to ground. At this point, a user-defined threshold in the analyzer can be set to the middle of the signal swing, and a single-ended measurement can be taken.
The probe is capable of measuring a 250-mVpp single-ended signal. It should be noted that the N node of the probe tip must be connected to ground when using the P node for a measurement.
This is different from previous flying lead sets in which one ground could be used across all 17 channels. The reason for this constraint is to enhance the performance of the probe. The figure above shows an example of a differential flying lead probe.
Hands-Free Probing of BGAs
One of the most common problems you can run into is trying to probe ball grid array (BGA) packages. As systems become denser and circuit board layer-counts increase, more and more signals are only accessible at a breakout via pattern of a BGA package.
The conventional approach to probing these signals is to manually hold the probe tip on the via. This is very difficult, and not well suited for logic analyzer measurements that tend to be run multiple times. Using a damped wire solder-down accessory is the solution to this problem.
The damped wire accessory provides the most connectivity flexibility of all other flying lead accessories. It can easily provide a probing connection to extremely small features on the target.
The damped wire consists of a damping resistor at the tip of the accessory. This isolates the target from the capacitance of the 1-in. wire. It also damps any reflections that occur from adding additional trace length to the logic analyzer probe.
The damped wire is flexible so that it can be easily handled during positioning and soldering. It's capable of probing highly dense signals, such as soldering to vias and exposed traces. In addition, it can take measurements at 1500-Mt/s with loading as low as 1.3-pF.
The figure above shows a damped wire accessory soldered to the breakout vias of a 1-mm-pitch BGA. The accessory (red) simply slides into the receiving socket of the probe tip (blue).