Temporal inconsistency between sample clock and data point acquisition time

[hexagram]

This might be a bit of a simple question, so I'll just illustrate the problem. The waveforms that I want to sample are shown in Data.png.

I'm collecting data from two photodiodes of the intensity of a pulsed laser. This is shown in the waveform Data.png, which is collected using the internal sample clock of my DAQ card and no trigger. (Sample clock is a PCI 6281)

It can be seen that I have 2 waveforms, each of them having peaks that coincide with each other at a rate of roughly 400 peaks per second (2.5ms between peaks). I am only interested in the value of the peaks, not the rest of the data. In order to speed up acquisition time, I am using a TTL signal as a sample clock that is synchronized with the peaks. So, the DAQ card should *only* acquire data when a peak occurs, and so should not be acquiring data at a sampling rate higher than 400Hz. (assuming I understand sample clocks correctly)

However, when I use my TTL signal as a sample clock, the data I get out is clearly acquired at a higher sampling rate than 400Hz (there is more than 1 point per 2.5ms, shown in DatawithSampleClock.png). Why is this happening? Should the DAQ card not only acquire data when the sample clock amplitude is at 4V? Also, if I have a sampling clock set, then why does the DAQ assistant still ask for a Rate in Hz alongside the samples to read? Should the sampling rate not be set by the sampling clock itself?

Note: The TTL sample clock is shown in magenta via oscilloscope in TTL signal.jpg, yellow is the peak itself, temporal synchronization between the sample clock and signal can be seen, the voltage scale is units of 1V and the temporal scale is units of 100us. The TTL signal also occurs at a rate of 400Hz

[DiegoH214]

Hello hexagram

I suggest reviewing this documents.

DAQmx Timing and Sample Rates

http://digital.ni.com/public.nsf/allkb/3E3D74E26B8A5B83862575CA0053E4B5

When Would I Use An External Sample Clock?

http://www.ni.com/white-paper/4825/en/

Timing and Synchronization Features of NI-DAQmx

http://www.ni.com/product-documentation/4322/en/

Tips and Techniques in Data Acquisition Triggering - NI-DAQmx

http://www.ni.com/tutorial/4329/en/

Reference:Analog Window:Trigger When Property

http://zone.ni.com/reference/en-XX/help/370469AH-01/daqmxprop/attr1427/

I do not understand why you are using an external clock for sampling. If what you need is just to acquire when the measurements are above 4V, then use the analog window triggering method.

Diego H

National Instruments.