DAQmx trigger level ignored?

[mitulatbati]

Hello all,

I have a very clear analog signal (See attached screenshot), which I want to record. The following settings will be used:

...(task is already created)...

DIGI_stat = DAQmxSetChanAttribute (DIGtask, "", DAQmx_AI_Max, 10.0);
DIGI_stat = DAQmxSetChanAttribute (DIGtask, "", DAQmx_AI_Min, -10.0);
DIGI_stat = DAQmxCfgSampClkTiming (DIGtask, "", 10e6, DAQmx_Val_Rising,DAQmx_Val_FiniteSamps, MAXDIGITARRAY);
DIGI_stat = DAQmxCfgAnlgEdgeRefTrig(DIGtask, "PXI1Slot3/ai0", triggerSlope, triggerLevel, 10000);
DIGI_stat = DAQmxSetTrigAttribute (DIGtask, DAQmx_AnlgEdge_StartTrig_Coupling, DAQmx_Val_DC);
DIGI_stat = DAQmxStartTask(DIGtask);

And my problem: 

if the triggerLevel=3.0 or 4.0, the rising edge will be found perfect. Look just the screenshot!

if the triggerLevel=2.0 or 1.0, there will be no trigger.

The simple question is, why?  

MB

[PeterAdelhardt]

Hi Mitulatbati,

the rising slope you measure is extremely steep; very few samples are collected during the rising edge. In order to avoid the issue you are observing, you can try the following:

- use the APFI line for triggering instead of the AI line. This will increase the triggering speed but decrease the resolution. 

- increase the AI sample rate to make sure more samples are converted during the steep slope. 

As for the actual issue you are seeing (triggering at 4V but not at 1V), I will gather more information and get back to you.

Regards,

Peter

[mitulatbati]

Hi Peter,

thanks for your reply.

- I use a PXI 6115 DAQ card. This has a maximum sample rate of 10MS/s. So I can´t set it faster.

- the voltage peak lasts about 18us. So I could use 1MS/s as well to catch the signal.

- the internal bandwidth of the PXI 6115 is 5MHz. This is fast enough to my signal.

- the minimum signal width could be ignored, since when the trigger level is set to 1V, the voltage remains over 1V after triggering.

I am trying on my tester, maybe I find the solution. But I am waiting for your reply.

regards (Grüße)

Mitulatbati

[mitulatbati]

Hi again,

I have set the presamples to 1000, trigger level to 4.0V, and I left each settings as mentioned above. I experience something stange, I don´t know if this has the same rootcause.

I expect that the first value over 4.0V in the result array has the index [1000]. But the first value over 4.0V ist the 1032th value. What happens during the time from sample 1000 to sample 1032?

(Sample rate = 10MS/s; bandwidth=5MHz)

regards