User`s guide

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Operation Theory
4.1.3 Programmable scan acquisition mode
4.1.3.1 Scan Timing and Procedure
This mode is recommended for applications that need a fixed and precise
A/D sampling rate. Users can accurately program the period between
conversions of individual channels. There are at least four counters which
need to be specified:
SI_counter (24-bit): Specify the
S
can
I
nterval = SI_counter/Timebase
SI2_counter (16-bit): Specify the data
S
ampling
I
nterval =
SI2_counter/Timebase
PSC_counter (24-bit): Specify
P
ost
S
can
C
ounts after a trigger event
NumChan_counter (9 bit): Specify the Number of samples per scan
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. With software users
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.