Design Con 2015

MSOs probe analog and digital

-March 24, 2014

Many microcontroller-based systems have both analog and digital signals. Even those that appear to be entirely digital aren't because of analog effects such as ringing and crosstalk. Thus, you often need both analog and digital views of your system's signals. That's where an MSO (mixed-signal oscilloscope can help.

MSOs incorporate the functionality of both an oscilloscope and a subset of capabilities found in logic analyzer. The most common MSO configuration has four analog channels and 16 digital channels. MSOs find their greatest applicability in troubleshooting embedded microprocessor boards.

The block diagram of a processor board shown in Figure 1 contains analog signals such as power, clocks, ADC (analog-to-digital converter) inputs, and DAC (digital-to-analog converter) outputs. There are both parallel and serial digital signals. Parallel digital signals include the data and address lines of the CPU and the GPIO interface. Both high and low speed serial data signal are present in the form of Ethernet, SATA, PCIe, SPI, I²C, and UART. An MSO lets you view each of these signals in either the analog or digital domains simultaneously. Displays in either domain are time synchronous, which aids in finding problems. Diagnosis is also aided by being able to trigger from analog, digital, or a combination of both sources. These acquisition resources are complemented by a set of measurement-and-analysis tools that operate on data from either domain. There are also easy to use search capabilities to locate either serial or parallel digital data patterns.

Figure 1. An example of an embedded microprocessor board with analog (green), digital (red), and serial data (Blue) signals. Mixed signal oscilloscopes offer a single instrument capable of measuring and troubleshooting all of these signal types.


Comparing Analog to Digital
Analog waveforms in a digital oscilloscope represent the acquired signal as a series of samples taken at the oscilloscope's sampling rate and digitized with an amplitude resolution set by the number of bits in the scope’s analog to digital converter (ADC). Modern high frequency scopes have ADC resolutions from 8 bits (256 levels) to 12 bits (4096 levels).

Digital traces in an MSO represent a single bit, sampled at the digital sampling rate. The amplitude basically varies from 0 to 1 based on whether is it lower or higher than a preset logic threshold (many MSOs come with preset logic levels for several families of logic devices) and represents the state of the digital input. Figure 2 shows a comparison of an analog (bottom) and a digital trace (top).

Figure 2. The comparison of a digital trace (top) and an analog waveform. The digital trace amplitude is represented either by a 1 or a 0 based upon whether the voltage on the digital input was above or below a user set, logic threshold. The analog trace is resolved into any of 4096 (12 bit) amplitude levels.

Analog traces can display small changes in voltage that occur with time. You can see things like the pulse overshoot and ringing. Cursor amplitude readouts, visible in the C1 descriptor box, read amplitudes down to millivolts. The cursor readouts for the digital trace (in the Digital 1 descriptor box), reports amplitudes of 0 and 1. Remember that the digital traces only show the state of the digital lines and can have either one of two values, 0 or 1.

When multiple digital lines are displayed you generally get the choice of viewing them individually in a line view, bundled as a bus view or viewing both. This is shown in Figure 3. In this figure 8 digital lines (D0 to D7) are displayed along with the bus view (bottom trace) which shows the summed value of all the digital lines in a hexadecimal readout. Note that D7 is the most significant bit (MSB) and D0 is the least significant bit (LSB).

Figure 3. Multiple digital lines, D0 to D7, being displayed in both line and bus view. The bus view shows the sum of all eight lines in a hexadecimal readout. D0 is the LSB and D7 is the MSB. Typical measurement tools including cursors and timing parameters with digital lines as sources, are shown.

You can apply the oscilloscope's parameter measurements to either signal type, but measurements of the digital trace are restricted to time-related measurements such as period, width, duty cycle, and delay. These parameters, like the more common analog waveform parameters, can serve as the basis for analysis tools such as a trend (plot of parameter values in the order taken), track (plot of parameter values synchronous in time to the source trace) , and histograms. Eight parameters (P1 – P8), based on the displayed digital lines, are shown in Figure 3.


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