When the multi-function timer is used as a pre-divider, the frequency of the external clock
input may be varied from DC to 10 MHz as long as the high portion of the clock is at least
30ns and the low portion is at least 50ns. Except for the first period, the sampling rate of the
DAQ-16 will be the external clock frequency divided by the count value written to the
multi-function timer. Since one clock pulse is required to load the initial count value into the
timer, the first sampling interval will be one clock cycle longer than the rest. The valid range
of count values for the multi-function timer is 2 < count < 65,535 but the resulting sampling
rate must be less than 100KHz to assure proper operation of the A/D converter circuitry.
(See Figure 2-8).
100 nsec
min
30 nsec min 50 nsec min
Figure 2-8. Pre-Divider External Clock Pulses
2.6
Trigger Selection
The DAQ-16 is capable of accepting an internal software trigger or an external hardware
trigger. The trigger selection and trigger level bits in the DAQ-16 control word register select
the trigger source and level. Upon reset, the trigger selection and trigger level bits default to
the internal software trigger. When the internal trigger is used, an output to the
start-of-conversion register will trigger the DAQ-16 to begin sampling the input. For
triggering off an external event, the DAQ-16 accepts a level sensitive, TTL compatible trigger
input from the D-62 connector. The trigger level bit in the DAQ-16 control word register
determines which TTL level is used to trigger the A/D converter to begin sampling.
When an internal clock source is used, a delay of not more than 225ns will occur between the
trigger and the first data sample. When an external clock is used, the delay will be dependent
on the frequency and duty cycle of the clock input. If these delays are unacceptable, the clock
and trigger circuitry can be bypassed and a start of conversion pulse can be input directly into
the A/D circuitry with a maximum delay of 25ns. If the user controls the start of conversion
pulse directly, the sample will be taken on the low to high transition of the pulse, the pulse
must have a duration of at least 10 us, and the duty cycle must be between 5 and 80 percent.
Jumper J1, shown in Figure 2-9, configures start of conversion control.
1 2 3
J1
Start of Conversion:
DAQ-16 controlled: connect 1-2*
User controlled: connect 2-3
(* indicates factory default)
Figure 2-9. Jumper J1 Configuration
DAQ-16 Users Manual 20
