
Chapter 2
Theory of Operation
2-16
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National Instruments Corporation
Multirate Scanning Using Ghost
If the ghost option in the configuration memory is set, that conversion
occurs but the data is not stored in the analog input FIFO. In other words,
a conversion is performed and the data is thrown away. By using this
option, multirate scanning with ratios such as x:y are possible, within the
limits imposed by the size of the configuration memory.
Figures 2-14 and 2-16 illustrate the advantages of using the ghost feature.
Figure 2-12 shows example 3 timing, and Figure 2-16 shows the same
example using ghost.
Example 3: channel 1: channel 0 = 2:3 (without ghost).
Figure 2-14. Multirate Scanning without Ghost
In example 3, channel 0 is sampled correctly. Although channel 1 is
sampled twice, it does not yield a 50% duty cycle. This type of acquisition
will result in imprecise rates. Figure 2-15 shows the relative occurrences of
convert pulses in Figure 2-14.
Figure 2-15. Occurrences of Conversion on Channel 1 in Example 3.
0
0
0
1
1
Sampling Rates
Channel 0: Channel 1 = 3:2
Start
Convert
t
t
Channel 1, Example 3
Convert
t
2t
t
Summary of Contents for AT-MIO E Series
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