HP 54645A, User'S And Service Manual

Page: 125 / 301

In addition to the common mode voltage— ground bounce —longer ground
returns also degrade the pulse fidelity of the probe system. Risetime is
increased, and ringing, due to the undamped LC circuit at the input of the
probe, is also increased. Because the HP 54645D 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 (0-7 and 8-15) 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 (T
R
< 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.
Run/Stop/Single/Autostore/Erase Operation
Using Digital Channels to Probe Circuits
4-42