Data Translation DT300 Series, User Manual

Page: 79 / 164

Principles of Operation
79
The sample rate is determined by the frequency of the A/D sample clock divided by the
number of entries in the channel list; refer to
page 76 for more information on the A/D sample
clock. The conversion rate of each scan is determined by the frequency of the Triggered Scan
Counter on the board; the Triggered Scan Counter is a 24-bit counter with a 20 MHz clock.
Using software, specify the frequency of the software retrigger. The minimum retrigger
frequency is 1.2 Hz (1.2 Samples/s).
Table 8 lists the maximum retrigger frequency supported
by the DT300 Series boards.
The appropriate retrigger frequency depends on a number of factors, determined by the
following equations:
Min. Retrigger = # of CGL entries x # of CGLs per t 2
μ
s
Period A/D sample clock frequency
Max. Retrigger = 1
Frequency
Min. Retrigger Period
For example, if you are using 16 channels in the channel list (CGL), scanning the channel list
256 times every trigger or retrigger, and using an A/D sample clock with a frequency of
100 kHz, set the maximum retrigger frequency to 24.41 Hz, since
24.41 Hz = 1
(16
* 256) +2
μ
s
100 kHz
To select software-retriggered scan mode, use software to specify the following parameters:
• The dataflow as Continuous, ContinuousPreTrigger, or ContinuousPrePostTrigger
(about-trigger).
• Triggered scan mode as enabled
• The retrigger source as software
• The number of times to scan per trigger or retrigger (also called the multiscan count)
• The retrigger frequency
The initial trigger source depends on the trigger acquisition mode you use; refer to page 80 for
more information on the supported and trigger sources and trigger acquisition modes.
Table 8: Maximum Retrigger Frequency
Board
Maximum
Retrigger Frequency
DT301/302 155 kHz
DT303/304 219 kHz
DT321/322 165 kHz