Logic Analyzer Main Parameters

There are three important parameters of a logic analyzer: threshold voltage, sampling rate, and sampling depth.

Threshold voltage: (the wider the better)

Distinguish the interval between high and low levels. Logic analyzers and microcontrollers are both digital circuits. When reading external signals, they have certain limits on how high a voltage is recognized as a high level and how high a voltage is recognized as a low level. For example, a logic analyzer has a threshold voltage of 0.7~1.4V. When it collects external digital circuit signals, it recognizes a voltage above 1.4V as a high level and a voltage below 0.7V as a low level.

Sampling rate: (the higher the better)

The number of times a signal is collected per second. For example, if the maximum sampling rate of a logic analyzer is 100M, it means that it can collect 100M samples per second, that is, one sample is collected every 10ns, and the samples above the threshold voltage are considered high level, and the samples below the threshold voltage are considered low level. When we learned UART communication before, we learned that each bit will be read 16 times, and the principle of the logic analyzer is similar, that is, overclocking reading. If your signal has a frequency of 1M, I use a sampling rate of 100M to collect, then I can collect 100 times in one signal cycle, and finally use the point drawing method we learned in elementary school to connect the collected samples, and the signal will be restored. Of course, the pulse width error of 100 times the sampling rate is about one percent. According to Nyquist's theorem, the sampling rate must be more than twice the signal frequency to restore the signal. Because the logic analyzer is a digital system with a simple algorithm, the minimum sampling rate is 4 times the signal. Generally, choosing about 10 times will have a better effect. For example, if your signal frequency is 10M, then the sampling rate of your logic analyzer must be at least 40M, and it is best to reach 100M to improve accuracy.

Storage Depth: (The larger the better)

We just talked about the sampling rate. We need a memory to store the collected high-level or low-level signals. For example, if we use a 100M sampling rate, 100M state samples will be generated in 1 second. How many samples a logic analyzer can store is a very important indicator of the logic analyzer. If our sampling rate is very high, but the amount of data stored is very small, it doesn't make much sense. The maximum number of samples a logic analyzer can save is the storage depth of a logic analyzer. Usually, data acquisition time = storage depth/sampling rate.

In addition, the logic analyzer has several simple parameters such as input impedance and withstand voltage. All channels of the logic analyzer have equivalent resistance and capacitance. Since the analyzer channel is connected in parallel to the channel when measuring the signal, if the input impedance of the analyzer is too small and the capacitance is too large, it will interfere with the signal on our line. In theory, the larger the impedance, the better, and the smaller the capacitance, the better. Normally, the impedance of the logic analyzer is above 100K, and the capacitance is around 10pf. The so-called withstand voltage means that if you measure a signal exceeding this voltage value, the analyzer may be burned out, so you must pay attention to this issue when measuring.