- the key points to note when choosing and buying a logic analyzer.
There are many specifications used to defined the
performance of a logic analyser. Fortunately it is possible to focus on some
of the more important or key specifications when investigating the purchase
of a logic analyzer to narrow down the choice of the best analyzer for the
purpose. This can make the task of choosing a logic analyser to purchase far
simpler.
Key points
The specification for a logic analyzer covers many areas of its performance,
but there are a number of parameters that are key to ensuring that it will
meet the majority of its operational requirements and be fit for the purpose
for which it was intended. It is a fine balance between buying a logic
analyzer that will meet the current and reasonable future requirements and
one that will not cost too much, and may not be able to meet all the future
requirements. Although it is not possible to define all the current and
future needs, the major parameters for a logic analyzer that are of
importance can usually be defined. Some are included below:
These three areas are normally the key logic analyzer
specifications for most applications. Once the requirements in these areas
have been determined, then it is possible to reduce the field of possible
analysers. These can then be compared, and any other requirements and
specifications can be compared and contrasted.
Logic analyzer speed
One of the major requirements for any logic analyzer is the speed of the
instrument. It is obviously necessary for the instrument to be able to cater
for the highest speeds in the circuits being analyzed. This will not just be
the clock speed, and it is necessary to look at the fastest events that can
happen anywhere on the circuit under test. With today's high speed circuits
it will be necessary to the logic analyzer to typically deliver sub
nano-second resolution.
The speed of the logic analyser is chiefly governed by
the timing resolution. This is the smallest time element that the analyser
can see. If the resolution is too coarse then it will not be possible to see
many of the fast occurrences happening in the circuits.
The speed of the analyzer must be such that it is able to
capture and display a variety of scenarios ranging from transient glitches,
any variety of software instructions which may lead to problems occurring,
timing violations, or set-up conditions. It is often within these areas that
the difficult problems can be found, and it is here that the capability of
the logic analyzer is needed. Without sufficient speed many of these
elements will not be seen.
Logic analyzer channels
Today's digital circuits are becoming more complicated, and they are
normally software driven. This means that it is important to ensure that
sufficient channels are available within the logical analyzer. Often high
end processor designs require between 50 and 150 channels to cover all the
lines that are relevant to the testing. If a logic analyzer with
insufficient number of inputs is purchased, then this will considerably
hamper testing.
Unfortunately, increasing the number of channels in a
logic analyzer considerably increases the complexity of the instrument. Not
only is the capture circuitry required, but the mechanical considerations
including the probe adaptors become more complex. Furthermore a greater
number of channels require increased memory, and processing within the
instrument. Thus increasing the number of channels considerably increases
the cost and complexity.
To help reduce this problem, most logic analyzers have
only a proportion of their channels that support the full specification in
terms of speed in resolution. As it is un-necessary for all the channels to
be able operate at the maximum spec, reducing the performance of some
simplifies the circuitry and reduces the requirements for memory and
processing.
The number that support the top resolution speed is of
importance because it is necessary to ensure that there are sufficient to be
able to perform the actions that will be required. To few at the top speed,
then it will not be possible to undertake the more exacting investigations.
Too many top specification channels and the cost increases.
Logic analyzer memory
In order that the logic analyzer can display the information it retrieves,
it must store it in memory. If only a small amount of memory is available
then it will only be able to store short sequences, and this may be
insufficient to analyze all the events occurring. Additionally a greater the
number if inputs, longer sequences that need to be stored, and greater
levels of resolution increase the requirement for memory. As memory can be
expensive, it is necessary to gain a sensible balance between memory
requirements and cost for the logic analyzer.
When looking at the memory requirements for different
analyzers it is necessary to ensure that the additional resolution,
sequences and inputs are not included at the cost of removing some of the
essential features that are required. Often memory requirements may be
reduced by turning off some functions such as time stamping, etc that may be
essential for some debugging. It is worth checking that functions that may
be essential for some jobs are not removed under some circumstances to save
memory as this can become a false economy.
Further requirements
Although speed and resolution, the number of channels, and the memory with
their associated specifications, are the main requirements for a logic
analyzer, it is also necessary to look at the other elements of the
specification for the instrument. Many logic analyzers come with debug
software and this enables a far greater level of visibility of what is
happening within the circuit.
Additionally other parameters such as the screen size and
ease of reading are important. While they may not be as important as the
basic elements of the logic analyzer, they are nevertheless very important.
A poor screen can be a significant disadvantage when someone is working with
an instrument over an extended period.
Summary
It is not easy to balance the requirements for a logic analyzer against its
cost. However by carefully considering the requirements against the budget
available, a reasoned decision can be taken, and an instrument that is fit
for purpose can be bought.
