Like all other forms of test instrumentation there is a number of key specifications associated with logic analyzers.
When looking to buy a logic analyzer it is necessary to gain at least a basic understanding of the key specifications so that an informed decision can be made about the particular logic analyzer to buy or rent.
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.
Logic analyzer speed
One of the major requirements for any logic analyzer is the speed of the test instrument. It is obviously necessary for the analyzer 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. Many lower end logic analyzers have four or eight. This may be sufficient for many basic applications, but not for the higher end professional requirements. A choice will need to be made dependent upon performance and cost.
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.
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.
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.
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