What does a hybrid oscilloscope do to prepare for digital acquisition?

 

    There are two basic tasks when preparing an MSO for digital acquisition. First, as with a logic analyzer, the MSO digital channel thresholds need to be configured for the logic family being tested to ensure that the correct logic levels are acquired. Second, the analog channel offsets need to be adjusted to achieve accurate time correlation between the analog and digital channels. The analog channels of the MSO can be used to quickly check the logic swing of a digital signal. In Figure 5, the MSO uses measurement statistics from multiple acquisitions to automatically measure the amplitude of a 5VCMOS signal. For logic families with symmetrical voltage swings, such as CMOS, the threshold is half the signal amplitude. In Figure 6, the digital channel threshold is set to 2.5V, which is half the amplitude of a 5VCMOS signal. However, for logic families with asymmetrical logic swings, such as TTL, it is generally necessary to consult the component data sheet and use half of the maximum low-level input voltage value (TTLVIL=0.8V) and the minimum high-level input voltage value (TTLVIH=2.0V) of the logic device as the threshold (TTLVthreshold=1.4V).

 

 

    Figure 6 shows the time offset between the rising edges of the analog and digital waveforms of the same signal. The analog waveform is ahead of the digital waveform. To make accurate measurements, the analog-to-digital time offset must be removed to achieve better time correlation between the analog and digital waveforms. The Tektronix MSO Series provides an adjustable analog probe deskew function to align the analog channels relative to each other and to align the analog channels with the digital channels. The analog channel deskew setting compensates for the propagation delay of different analog probes. Each oscilloscope in the Tektronix MSO Series comes with a logic probe. To simplify digital measurements, the oscilloscope compensates for the propagation delay of the logic probe, so there is no digital channel probe deskew adjustment.

 

 

    To align the analog channels with the digital channels, the 2.5V position on the CMOS analog waveform needs to be aligned in time with the CMOS logic transition that occurs on the 2.5V threshold. As shown in Figure 7, we use the -1.60ns deskew function to align the analog channels with the digital channels. Repeat this deskew process for the other analog channels. The analog channel skew should be checked when the analog probe is changed, and the digital threshold should be checked when measuring different logic families. By configuring the thresholds and offsets, the MSO can verify and debug the digital circuits at any time.