This is a continuation of an article which first ran in Dataweek 6 August 2008. It is intended to help users decide which logic analyser to buy and to avoid some common pitfalls.
5. Number of channels
The number of channels determines how many signals can be captured at the same time. When buying a logic analyser, do not be overly impressed if a large number of channels are offered at low price. The quality of the channels, whether they are backed up by an adequate buffer and have a quality input stage is more important. It is better to buy a logic analyser with a few good channels than one with many poor channels.
Many channels combined with a very small buffer are of value only to view short data sequences, such as a single read/write to a RAM chip, but if you wish to zoom out to get the bigger picture, you will be left disappointed and wishing that you bought a more professional instrument.
With the many I/O lines available on today's PLDs, it is quite easy to provide many channels at low cost. All you need is a DAC to create a threshold reference for all channels, and then a few small surface mount resistors and capacitors per channel. The PLD provides a small amount of RAM which can be used as a data buffer. In this way you can make a 'logic analyser' that costs very little. Whether you provide 16 or 128 channels does not add much to the price.
To create a top quality logic analyser, the cost per channel is much more than just adding a few capacitors and resistors and putting on a bigger external connector. This is the reason why a good 8-channel logic analyser would most likely cost more than a 64-channel product as described above and why it makes sense to rather buy a quality 8-channel product.
It is surprising how far you can go with just a few logic analyser channels, once you became 'wise' in the usage of logic analysers. Personally I think that 16 channels are more or less the optimum required for debugging most circuits. This includes debugging boards with processors with many address and data lines. If you have gone through the pains of connecting 128 test clips of a large logic analyser to your 32-bit processor with many address lines, you would most likely never do it again and rather start using your logic analyser in a 'smart way'. You will be surprised what you can do with an 8-channel logic analyser.
6. Sufficient input bandwidth
This input bandwidth specification is an indication of the maximum frequency that can be measured.
A logic analyser input acts as a low-pass filter and the 'bandwidth' normally indicates the -3 dB point where the input signal size has been reduced to half of its low frequency amplitude. The logic analyser thresholds can be adjusted to cope with the signals being attenuated. Independent threshold voltages on different input ports allow different thresholds for different signals.
The maximum sampling rate should always be at least four times higher than the maximum input bandwidth. This factor of four is needed for a reasonable representation of the signals after capture.
If the input bandwidth is too high compared to the sampling rate, external switching noise may be introduced to the logic analyser circuitry, by signal frequency components which are in any case too high for effective capturing. Such noise only serves to degrade the capture integrity.
7. Input impedance
The ideal measuring instrument can pick up information from the device under test (DUT) without influencing the functioning of the DUT under test at all. The input impedance should be as high as possible (high resistance and low capacitance), such that the instrument would not add excessive load to the DUT.
8. Variable threshold voltage
A variable input threshold is needed to measure signals of different amplitudes. An independently variable threshold voltage is preferable for every eight inputs.
Independent thresholds also allow measuring of different technology types at the same time. It is becoming increasingly common to have components operating on 5 V, 3,3 V, 2,5 V etc., all on the same PCB. If you buy a logic analyser with many channels and only one threshold for all of these channels, you are likely to have problems measuring on mixed technology boards, especially when the signal frequencies becomes high.
When input signal frequencies are relatively low, the actual threshold voltage is not all that important, because a perfect 2,5 V logic signal will be a square wave varying between 0 V and 2,5 V and a 5 V signal will vary between 0 V and 5 V. This means that a threshold of say 2 V will display both logic types correctly.
In the real world though, the square waves will look more like sine waves for relatively high frequencies, will have a DC offset and will diminish in size as a result of input bandwidth limitations. This means that especially for high frequencies, independent threshold adjustments become more important.
9. Versatile trigger options
This feature enables you to capture the exact data that you want to see. You need to be able to trigger on edges, patterns, and sequences, eg, edge condition, then pattern. Deep sequencing is seldom necessary and is limited to two to three stages by some manufacturers to keep the user interface simple.
10. External clock input
Synchronous capture is usually used to capture clocked data, using a clock signal from the hardware under test. For example you could capture data read by a processor by using the processor read signal as clock input to the logic analyser. As every sample is of a distinctly clocked moment it is usually best displayed as a text listing.
11. Easy-to-use software, manual, help
A logic analyser is an instrument that you may use intensively for a while and then you may not need it again for six months or a year. It is therefore very important that the software is easy to use and you would not need to go through a heavy learning curve each time you use it.
It is easy for manufacturers to pack a lot of features into the onboard programmable chips, but the problem is that this could lead to overcomplicated user interfaces, in which the user has trouble finding the commonly used features between the many often completely unnecessary features that clutter the setup dialog boxes.
For manufacturers the secret of balance is to provide the important features clearly and in an easily understandable manner, and not to allow features that users would never use to overcomplicate the user interface.
12. Nice extras
How about an onboard pattern generator to provide signals to your DUT, while the logic analyser simultaneously captures the response? You can use this to set up communication protocols to send to your DUT, or to simply create a controlled clock to your circuit.
Some manufacturers also provide assembler features. This may add quite a bit to the price of the logic analyser and should only be considered if you really need it.
13. After sales support from dealer and manufacturer
Both the manufacturer and dealer need to provide good before- and especially after-sales service.
Capable sales personnel should know the basics of working with a logic analyser. Difficult technical question regarding the usage and details of the specific instrument should be referred to the manufacturer, which should be answered promptly.
Most PC-based logic analyser manufacturers supply the latest software updates on their websites for free.
14. CE or other compliance standard
This indicates that the product meets certain electromagnetic emissions and susceptibility and safety standards.
15. Complete package
If everything you need is not included in the package, eg, test clips, you will waste time finding them and probably pay more for them. Quality test clips are expensive. Not including them in the package is a way by which some products appear much cheaper than what they really are. Good test clips are important, without which the usability of the instrument is severely limited.
Conclusion
In summary, certain important factors need to be seriously considered when evaluating which logic analyser to purchase.
Try to match what you need with quality and affordability.
To capture data with a logic analyser you need the following:
* Sampling speed.
* Buffer depth (go for real memory, not a minute memory with compression).
* Number of channels.
* Sufficient input bandwidth.
If your logic analyser does not provide enough capability on each of these specifications, you will have problems capturing and viewing the data for effective debugging.
Other specifications such as input impedance, threshold settings, etc are also important and should also be taken into consideration.
A logic analyser should be easy to use. The basic principles to which even the most sophisticated logic analyser hardware adhere are actually quite simple. The software interface can sometimes be overcomplicated by the implementation of seldomly used functionality. It seems that with the huge capability available in modern PLDs, some developers have difficulty with where to draw the line between usability and overcomplicating the software.
Do not buy the cheapest logic analyser that you can find on the market - you are sure to regret it eventually.
A short biography on the author
Eduard Theron graduated in 1984 from the University of Stellenbosch with a B.Eng degree in electronic engineering. He has been active in data capturing systems since 1989. Initially, he designed high reliability multiprocessor (transputer) designs for inertial navigation systems in military guidance systems. Later, he established his company, Janatek Electronic Designs. Currently, he is still actively involved in the designing of new hardware for logic analysers.
For more information contact Janatek Electronic Designs, +27 (0)21 887 0993, [email protected], www.janatek.com
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