Probe Grounding
A probe ground is the low-impedance path for current to return to the source
from the probe. Increased length in this path will, at high frequencies, create
large common mode voltages at the probe input. The voltage generated
behaves according to the equation:
V = L
di
dt
Increasing the ground inductance (L), increasing the current (di) or
decreasing the transition time (dt), will all result in increasing the voltage
(V). When this voltage exceeds the threshold voltage defined in the logic
analyzer, a false data measurement will occur.
Sharing one probe ground with many probes forces all the current that flows
into each probe to return through the same common ground inductance of
the probe whose ground return is used. See figure 45. The result is increased
current (di) in the above equation, and, depending on the transition time
(dt), the common mode voltage may increase to a level that causes false data
generation.
Common Mode Input Voltage Model
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 logic analyzers 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.
Figure 45
Ensuring Accurate Measurements
Probing the Circuit Under Test
141
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