Agilent Technologies 54622A User Manual page 140

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.
4- 40