Hello, I'm using CVI 9.0 in order to acquire continuosly an analogic voltage signal using USB6229 in une analog channel.
In the acquisition program, settings and start signal are executed in a determinate funcion, while reading and stop procedure in an other one.
Start function is:
START ADC ACQUISITION */
void StartAdc(void){
short status = 0;
DAQmxCreateTask ("", &P.Power.Adc.taskmodpower_diode);
status = DAQmxCreateAIVoltageChan (P.Power.Adc.taskmodpower_diode,
"Dev1/ai0", "", DAQmx_Val_RSE, -0.4, 0.0, DAQmx_Val_Volts, NULL);
if(status!=0) Failure(GetDAQErrorString(status));
status = DAQmxCfgSampClkTiming (P.Power.Adc.taskmodpower_diode,
"OnboardClock", 5000, DAQmx_Val_Rising,DAQmx_Val_ContSamps,
40000*P.Spc.TimeM);
if(status!=0) Failure(GetDAQErrorString(status));
status=DAQmxStartTask (P.Power.Adc.taskmodpower_diode);
if(status!=0) Failure(GetDAQErrorString(status));
}
P structures are defined into a librarie (so are global variables), and P.Spc.TimeM is the time measure (of about 1s).
Reading function, called approximetely 1s after start, is:
/* STOP ADC ACQUISITION */
void StopAdc(void){
short status;
double stdev,value;
status = DAQmxReadAnalogF64 (P.Power.Adc.taskmodpower_diode, -1, 0,
DAQmx_Val_GroupByChannel, P.Power.Adc.Data, 40000*P.Spc.TimeM,
&num_read, 0);
status = DAQmxStopTask (P.Power.Adc.taskmodpower_diode);
if(status!=0) Failure(GetDAQErrorString(status));
StdDev (P.Power.Adc.Data, num_read, &value, &stdev);
P.Step[P.Power.Step].Actual_mean_pow=(long)(value*ADC_FACTOR);
P.Step[P.Power.Step].Actual_std=(long)(stdev*ADC_FACTOR);
status = DAQmxClearTask (P.Power.Adc.taskmodpower_diode);
if(status!=0) Failure(GetDAQErrorString(status));
}
My questions are:
1)FUNCTION DAQmxCfgSampClkTiming: with a clock rate, for exempre, equal to 100, when I call the reading function the value num_read=0 (the function doesn't read any value); the same happen if I use clock rates inferior to abount 4000Hz: is there a minimin frequence with wich I can set the clock?
2)FUNCTION DAQmxCfgSampClkTiming: variyng rate of cloc, for exempre from 4000 to 7000, number of read sample num_read is always equal to 16384 and is fixed: there are discrete values of clock rate that I can choose? If I put clock rate at 10000 num_read is equal to the double of the previus value, ie 32768: it is somerelationship with the power of 2?
When I performe a measure the effective time is about 1 second while, for example, with a rate of 4000 I acquire 16384 sample and the time of measure shoud behigher than 4 seconds.
3)What happen if I put DAQmxStopTask before DAQmxReadAnalogF64?The program will stop acquisition and read only samples acquired until the stop signal?If I do this the program doesn't read anything and num_read=0.
My aim is to monitor the power of a laser while a certain measure is done: I call the Start function when the measure began and I call the Stop function when the measure finish, reading exactly the number of sample acquired during measure time. What happen realness with the function I wrote?
I thank you for the courtesy.
Daniele
