User`s guide

Operation Theory • 29
4.1.4 Programmable scan acquisition mode
4.1.4.1 Scan Timing and Procedure
It's recommended that this mode be used if your applications need a fixed
and precise A/D sampling rate. You can accurately program the period
between conversions of individual channels. There are at least 4 counters,
which need to be specified:
SI_counter (24 bit): Specify the Scan Interval = SI_counter / Timebase
SI2_counter (16 bit): Specify the data Sampling Interval =
SI2_counter/Timebase
PSC_counter (24 bit):Specify Post Scan Counts after a trigger event
NumChan_counter (9 bit):Specify the Number of samples per scan
The acquisition timing and the meaning of the 4 counters are illustrated in
figure 11
Timebase Clock Source
In scan acquisition mode, all the A/D conversions start on the output of
counters, which use Timebase as the clock source. By software you can
specify the Timebase to be either an internal clock source (on-board
40MHz) or an external clock input (EXTTIMEBASE) on CN2. The external
clock is useful when you want to acquire data at rates not available with the
internal A/D sample clock. The external clock source should generate
TTL-compatible continuous clocks, and the maximum frequency is 40MHz
while the minimum is 1MHz.
Figure 11: Scan Timing
There are 4 trigger modes to start the scan acquisition, please refer to
4.1.4.3 for the details. The data transfer mode will be discussed in 4.1.4.4.
Note:
1. The maximum A/D sampling rate is 3MHz for DAQ/PXI-2204, 500kHz
for DAQ/PXI-2205 and 250kHz for DAQ/PXI-2206. Therefore, the
minimum setting for SI2_counter is 14 for DAQ/PXI-2204, 80 for
DAQ/PXI-2205 and 160 for DAQ/PXI-2206 while using the internal
Timebase.