period measurement with analog trigger NIDAQmx C/C++

[gavril]

I would like to acquire an analog signal and simultaneously measure the time from the beginning of the task

until the signal crosses some voltage threshold.

My idea was to start an AI and CIPeriod tasks simultaneously and to use an analog threshold

to stop the CIPeriod task (see below) but I am not sure about how to use the analog threshold circuitry

and connect the input signal .

I am using USB 6211 and NIDAQmx C.

Thank you for your help,

Gabriel

// AI Tasks

DAQmxCreateTask("Input Task",&InputTaskHandle)
DAQmxCreateAIVoltageChan(InputTaskHandle,"/Dev1/ai0","",DAQmx_Val_Diff,-10.0,10.0,DAQmx_Val_Volts,NULL);
DAQmxCfgSampClkTiming(InputTaskHandle,"",InputSamplingRate,DAQmx_Val_Rising,DAQmx_Val_FiniteSamps,InputLength);

// Timing task

DAQmxCreateTask("Timing Task",&TimingTaskHandle);
DAQmxCreateCIPeriodChan(TimingTaskHandle,"/Dev1/Ctr0","",minVal,maxVal,
        DAQmx_Val_Seconds,DAQmx_Val_Rising,DAQmx_Val_LowFreq1Ctr,0.000001,4,"");

// Set start trigger of timing task

DAQmxCfgDigEdgeStartTrig( TimingTaskHandle,"/Dev1/ai/StartTrigger, DAQmx_Val_Rising );

// Set Analog Threshold to stop timing task ?????

// Start tasks

DAQmxStartTask(TimingTaskHandle);
DAQmxStartTask(InputTaskHandle);

// Read results

DAQmxReadAnalogF64(InputTaskHandle, InputSamples, Timeout,
        DAQmx_Val_GroupByScanNumber, InputData, InputLength, &SamplesRead, NULL);
DAQmxReadCounterScalarF64(TimingTaskHandle,Timeout, &ResponseTime,NULL);

[gavril]

I found out that Analog Triggering is not supported by USB 6211.

1) Is it possible to use the properly amplified analog signal as a digital trigger to stop the timing task? 

2) How should I proceed to modify the above code)

3) My signal varies from 0-5 V and I would like to stop the timing task when it is about 0.5V .Can somebody suggest a circuit?

Thank you,

Gabriel

[bronzacuerta]

Hi Gabriel

what about using Schmitt trigger circuit < http://en.wikipedia.org/wiki/Schmitt_trigger > to transform the analog signal into a TTL signal (setting your custom thereshold) and a Semiperiod CI task? With this settings the time measure you need correspond with the low signal time!!!

Unfortunately i use LabVIEW and not C, but the idea behind is exactly the same...what do you think about it?

Best