National Instruments 320682C

Chapter 10 Easy I/O for DAQ Library

For example, if the A/D conversions (represented by x's) on the waveform shown below are

placed side-by-side, they represent one cycle of the waveform.

_ _ _ x _ _ _

/ \ / \ x \ / \ / x / \ / \

/ \ x \ / \ / \ / \ / x / \

x \_/ \_/ \_/ \_/ \_/ \_/ x_/

x

x x

Equivalent Time Sampling is accomplished in this function as follows:

1. Set a hardware analog trigger condition for measuring your waveform using the Edge/Slope,

Trigger Level, and Trigger Source parameters of this function.

2. Whenever a hardware analog trigger occurs, the internal ATCOUT signal is strobed.

3. The ATCOUT signal is internally routed to the gate of GPCTR0, which is configured to

generate a pulse each time it receives a rising edge at it's gate input.

4. The output of GPCTR0 is internally routed to the data acquisition sample clock to control the

A/D conversion rate.

5. The very high effective scan rate is achieved through a pre-pulse delay that is programmed

into GPCTR0. This delay automatically increments before each GPCTR0 pulse so that the

A/D conversions occur at slightly larger intervals from the trigger condition as trigger

conditions occur over time.

6. Because the waveform being measured is periodic, A/D conversions that are at particular

intervals from trigger conditions over time can look the same as A/D conversions at

particular intervals from one unique trigger point in time.

In the following figure:

tn => the nth trigger condition

dn => delay between the nth trigger and the nth conversion

x=> an A/D conversion

- - - => the trigger level

_ _ _ x _ _ _

/ \ / \ x \ / \ / x / \ / \

/ \ x \ / \ / \ / \ / x / \

x- - -\-/- - -\-/- - -\-/- - -\-/- - -\-/- - -\-/- - -x-/-

________________________________________________________

t0 t1 t2 t3 t4 t5 t6

|| |-| |--| |---| |----| |-----| |------|

d0 d1 d2 d3 d4 d5 d6