Agilent Technologies 41A, 54621D, 24A, 42A, 54621A, 22A manual Best Probing Practices

MegaZoom Concepts and Oscilloscope Operation

Using Digital Channels to Probe Circuits

In addition to the common mode voltage, longer ground returns also degrade the pulse fidelity of the probe system. Rise time is increased, and ringing, due to the undamped LC circuit at the input of the probe, is also increased. Because the digital channels display reconstructed waveforms, they do not show ringing and perturbations. You will not find ground problems through examination of the waveform display. In fact, it is likely you will discover the problem through random glitches or inconsistent data measurements. Use the analog channels to view ringing and perturbations.

Best Probing Practices

Because of the variables L, di, and dt, you may be unsure how much margin is available in your measurement setup. The following are guidelines for good probing practices:

•The ground lead from each digital channel group (D15–D8 and D7–D0) should be attached to the ground of the circuit under test if any channel within the group is being used for data capture.

•When capturing data in a noisy environment, every third digital channel probe’s ground should be used in addition to the channel group’s ground.

•High-speed timing measurements (rise time < 3 ns) should make use of each digital channel probe’s own ground.

When designing a high-speed digital system, you should consider designing dedicated test ports that interface directly to the instrument’s probe system. This will ease measurement setup and ensure a repeatable method for obtaining test data. The 10085A 16-channel logic probe and termination adapter is designed to make it easy to connect to industry-standard, 20-pin board connectors. This probe consists of a 2 m logic analyzer probe cable and a 01650-63203 termination adapter that provides the proper RC networks in a very convenient package. Three 20-pin, low-profile, straight board connectors are included. Additional board connectors can be ordered from Agilent Technologies.

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