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Test & measurement


multiple point-of-load regulators for other ICs in a design, you can get up to seven or eight power rails in a hurry. Using a four-channel oscilloscope to verify


power rail timing in an embedded system can be time-consuming, but this is how most engineers do it currently. In this article, we will first look at why this can be challenging with a four-channel scope for this purpose, and then we will show a few examples using an eight-channel scope, which have becoming increasingly more common across the industry.


TradiTional four-channel oscilloscope


One approach when using a four-channel oscilloscope is to analyse the power system in blocks and using multiple acquisitions to check the timing block by block. To compare between blocks, one of the rails or a power good/fail signal can be used as a trigger and multiple captures can be taken, determining the star tup and shutdown timing relative to the reference signal. Since acquisitions are taken over multiple power cycles, variations in the relative timing of supplies will be difficult to characterise. However, the range of variation of each supply from cycle-to-cycle can be determined by measuring over multiple power cycles using infinite persistence on the oscilloscope. Another common approach is to


“cascade” multiple scopes. This is usually done by triggering the scopes on one of the power supplies or on a common power good/fail signal. Both of these approaches are time-


consuming and require special attention to synchronisation: Dealing with synchronisation and time


uncer tainty requires care Aggregating data to develop a system


timing diagram is possible, but time-consuming Complexity increases with the number of


power rails to be observed Setups must be perfectly consistent One measurement channel must be used


to provide synchronisation A mixed signal oscilloscope can provide


additional channels for power supply sequencing. For this to work, the MSO must have suitable voltage range on the digital inputs and independently adjustable thresholds. For example, a traditional MSO option offers 16 digital inputs with independent thresholds for each channel making it suitable for most of the voltage levels you would find in a typical design. Note that this approach works well if the objective is strictly to measure timing relationships, but it does not allow for measurement of rise/fall times or the shape (monotonicity) of the power-on/off. Using an oscilloscope with eight analogue


channels cuts the time and hassle significantly versus any of the methods covered so far. With an eight-channel oscilloscope, power supplies with up to eight power rails can be


Instrumentation Monthly April 2019 Figure 2


characterised using analogue probes. To measure turn-on and turn-off timing relationships on power supplies with more than eight power rails, a mixed signal oscilloscope with digital signal inputs and independently adjustable thresholds can also be used.


Turn-on delay


The switching power supply under test in Figure 1 produces a high-current, regulated 12 VDC output. This power supply is remotely controlled with a switch on the front panel of the instrument. Shor tly after the switch is


Figure 1


Figure 3


Continued on page 20... 19


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