Data Acquisition

Figure 5.22Time skew between channelsIn Figure 5.22, channel 1 is sampled properly since it is deemed the reference channel.Channel 2 exhibits time skew as samples 1 and 4 show significant errors relative to theiractual values at the time channel 1 was sampled.Where the time relationship between each channel sampled is unimportant, or the skew isnegligible compared to the speed of the channel scan rate, such delays are not significant. Inmany applications, however, such as those dealing with accurate phase measurements orhigh-speed transient analysis, time skew between channels is unacceptable, since it is crucialto determine the output of several signals on different channels, at precisely the same time.To avoid the timing errors introduced when continuously sampling from one input channelto the next, special applications require A/D boards capable of simultaneous sampling. TheseA/D boards are fitted with so-called simultaneous sample and hold devices on all inputchannels. The sample and hold device on each input channel holds the sampled data until theA/D converter can scan each channel.The maximum possible difference in sampling time between the channels, usuallyintroduced as a result of variations in the aperture time of the individual sample and holddevices, is the time variable known as the aperture matching, or sometimes known as apertureuncertainty. This measurement reflects the maximum possible difference in sampling timebetween channels. The aperture uncertainty can be calculated from the maximum inputfrequency of the signal to be sampled. For an error of less than 1-bit on a 12-bit A/D board,this is about 800 ns at 100 kHz, 1.6 ns at 50 kHz, and so on. Boards with aperture matchingof the order of 0.4 ns (±0.2 ns) are available.Dedicated plug-in boards that perform the function of simultaneous sampling, when it is notavailable on the main A/D board, can be interfaced easily to the A/D board with the necessaryconversion strobe signals.5.6.3 Block mode operationsWhere channel-gain arrays are available on an A/D board, an additional method by which agroup of channels can be sampled almost simultaneously becomes available. Block modetriggering, sometimes known as burst mode triggering or interval scanning, creates the effectof simultaneous sampling, while maintaining the lower cost benefits of continuous channelscanning.