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. 

See the following KBs: 

Analog Trigger (DevZone)

APFI minimum signal width

APFI / AI trigger signal bandwidth

APFI signal deviation

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