Sample and Hold Application.

Question

I am acquiring data with the NI ELVIS on an analog input channel A0.
I need to sample and hold the data according to a 480 Hz (2.08 ms) clock as follows:
1. Acquire data for the 0.69 ms (1st third of the cycle) and place the data into a new waveform.
2. Acquire data from 0.69 ms to 1.39 ms (the 2nd third of the cycle) and place the data into a new waveform.
3. Acquire data from 1.39 ms to 2.08 ms (last third of the cycle) and place the data into a new waveform.
Currently, I have built a VI (attached) to generate two clock signals. The first clock signal turns on a red LED for 0.69ms then turns off for the rest of the cycle. The second clock signal turns on when the first clock signal transititions to a low state for the next 0.69 ms and remains off for the rest of the cycle. Both LED illumination signals are picked up by a photodetector, so this is why I need the sample and hold application explained above.
If you could help that would be great. My current VI is attached. Thanks so much!
-David
Attachments:
SaO2.vi ‏146 KB

Answer 1

Correct me if I'm wrong, you are turning on two LEDs in a pattern and reading the light level. You want to have three separate waveforms for the light reading, based on when the lights are turned on and off.
If this is the case, here is what I would try:
Trigger your analog input reading off of the turning on of the first LED. You might have to put an initial delay into your clock pulse (that turns on the first LED). This initial delay will insure that the LED is not turned on before the analog input has been started. Data collection will start when the first LED is turned on (the trigger) and will end after the appropriate amount of time. (You could end it in many ways; user presses a stop button, configure collection for a finite number of data points, final pulse has ended,..). After data collection is finished take the single waveform that is storing all of the light data and use the waveform functions in the Waveform palette to split it into your three waveform subsets.
I suppose another way to produce the three waveforms would be to wire your light sensor into three analog inputs, then configure three separate voltage inputs with appropriate triggers based on the turning on and off of the LEDs. CH 1 triggers when LED 1 is turned on, CH 2 triggers when the LED 1 is turned off, CH 3 is triggered when LED 2 is turned on.
I hope this is helpful in some way,
Sam