When your MSO needs help: understand when a logic analyser can really add value to your MSO

November 04, 2014 // By Byte Paradigm
Mixed-Signal Oscilloscopes (MSO) have become everyone's 'Engineering Swiss Army Knife': why would you need an additional logic analyser? In fact this could be the only instrument that most electronic engineers will ever have to (or want to?) use for 90% of their lab time. But does this mean that you will not need a logic analyser (LA)? Read on...

Oscilloscopes vs. logic analysers – the basics, revised

Digital oscilloscopes and logic analysers are based on sampling techniques. Electrical signal (usually voltage) measurements are transformed into digital values by a high-speed analogue-to-digital converter (ADC) and stored into memory at regular intervals defined by the instrument's sampling clock.

A logic analyser can be thought of as an oscilloscope with 1-bit vertical resolution on all channels. It displays signals as logic (binary) values, according to whether the measured voltage is above or below a conventional voltage level called 'threshold value'. That is the first fundamental difference between a oscilloscope and a logic analyser.

Figure 1; The logic analyser's samples are displayed as either above or below a threshold value.

The other fundamental difference between an oscilloscope and a logic analyser is how the sampled values are displayed. In its most common mode of operation, an oscilloscope is essentially a device that repeatedly captures a “window” of events of a given length (defined by its total memory) and refreshes the display of a portion of that window on a screen. Many oscilloscopes simulate 'persistence' by superimposing multiple captured windows on the display and by modulating the screen pixel intensity.

A logic analyser is mostly used for single-shot captures (no superimposition of successive captures) and to analyse the sequence of events of – sometimes – over more than 100 digital signals before and after an event called trigger.

Figure 2 Oscilloscope vs. logic analyser acquisition and display

It was the appearance of microcontroller-based systems that required the creation of tools such as logic analysers. First, there was a need to observe digital busses – and hence to have more than two or four channels. Second, there was a need to see the signals the way logic circuit does, i.e. at the sampling events of the circuit, in the form of binary values. Over time, logic analysers have turned into less 'pure’ instruments with an ability to perform some analogue measurements – especially for checking threshold levels, detecting glitches and verifying the compliance of signals to specific I/O standards.

'Real-time', really?

It is very common to hear that the 'real-time display' capability is the difference between a scope and a LA. In fact, the automatic display refresh may mislead users into thinking that the user sees data 'as it occurs'. But note that the scope display isn't really refreshed faster than what the eye is able to see (that is about 30 times per second). In most cases, a LA is used by first capturing a bunch of data and then spending 'some time' to analyse it. The LA's repetitive trigger capability also enables refreshing the display based on the repetitive occurrence of a trigger event. It is true that data is displayed and presented differently in digital scopes and LAs, but fundamentally, in the two cases, the tools proceed by sampling signals and storing the samples into memory.

next; a scope plus a logic analyser makes an MSO?