• Probe (connect to the SUT)
• Setup (clock mode and triggering)
• Acquire
• Analyze and display
When to Use a Logic Analyzer vs. a DSO
A DSO is an ideal test instrument
when you need to measure:
• You need to determine transient
states, such as runt signals, glitches,
and metastable states.
Each block in the simple logic
analyzer block diagram shown in Figure
4 symbolizes several hardware and/or
software elements. The block numbers
correspond to the four steps just listed.
The acquisition probes connect to the
SUT. The probe’s internal comparator is
where input voltage comparison occurs
against the threshold voltage and the
signal’s logic state (l or 0) is determined. You set the threshold value,
ranging from TTL levels to CMOS, ECL,
or your own user-definable ones.
Probe impedance (capacitance,
resistance, and inductance) becomes
part of the overall load on the SUT. All
probes exhibit loading characteristics.
The logic analyzer probe should
introduce minimal loading on the
SUT, and provide an accurate signal to
the logic analyzer.
Probe capacitance tends to “roll
off” the edges of signal transitions
(see Figure 5). This roll-off slows down
the edge transition by an amount of
time represented as “t” in the figure.
Remember, slower edges cross the
logic threshold of the circuit later,
introducing timing errors in the SUT.
This problem becomes more severe as
clock rates increase.
In high speed systems, excessive
probe capacitance can potentially
prevent the SUT from working. It is
always critical to choose a probe with the
lowest possible total capacitance. It’s
also important to note that probe clips
and lead sets increase capacitive loading
on the SUT. Use a properly compensated
adapter whenever possible. The impedance of the logic analyzer’s probe affects
signal rise times and timing relationships.
Logic analyzers capture data from
multi-pin devices and buses. The term
“capture rate” refers to how often the
logic analyzer samples the inputs. It is
the same function as the time base in
an oscilloscope. Logic analyzer literature interchangeably uses the terms
“sample,” “acquire,” and “capture.”
Timing acquisition captures signal
timing information. In this mode, an
internal clock samples data. The faster it
• A few analog characteristics of no
more than four signals at a time.
A logic analyzer is best when you
need to:
• You need to know analog characteristics of a signal, such as rise and
fall times, phase, power, current,
amplitude, and edge relationships.
• Debug and verify digital system
operations.
• You need to determine a signal’s
stability (jitter).
• Trace and correlate many digital
signals simultaneously.
• You need to measure timing margins
such as setup/hold and propagation
delays.
• Detect and analyze timing relationships for many digital signals.
• Trace embedded software operations.
FIGURE 4. A greatly
simplified functional
block diagram of a
logic analyzer.
FIGURE 5. An
illustration of how
the impedance of
the logic analyzer’s
probe can affect
a signal’s rise
times and
measured timing
relationships.
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