External Trigger for Reading Voltage (comedi)

[Thomas Pi.]

Dear all,

I have the following situation: I need to read a pulsed analog signal using the analog input of a DAQCard 6036E. I realise this by using an external trigger, going to the corresponding PFI. Both signals can be seen on the picture I attached (the red one is the trigger signal, the height of the blue one is the voltage, I'm interested in, the width is indicated as well) - so far so good!

Now comes my problem: The value of the voltage I read depends on the position of the trigger signal! If I move it (using a hardware-delay) to the left but still inside the blue rectangle, the voltage decreases! I would expect the voltage to be constant as long as the trigger signal is inside the blue rectangle and going to zero as soon the trigger signal is outside the blue rectangle! Is there any time dependence of the card or somewhere, that causes this effect? (The same occurs with a PCI-MIO-16XE-50.)

I would be extremely grateful for an answer!

Cheers, 

Thomas 

[AndreasStark]

What sampling rate are you using? It could be that you are sampling faster than settling time of your card is. Does using a lower samping rate solve the problem?

 From E-Series user manual, pages 2-9ff:

"In multichannel scanning applications, accuracy is affected by settling
time. When your E Series device switches from one AI channel to another
AI channel, the device configures the NI-PGIA with the input range of the
new channel. The NI-PGIA then amplifies the input signal with the gain
and polarity for the new input range. Settling time refers to the time it takes
the NI-PGIA to amplify the input signal to the desired accuracy before it is
sampled by the ADC. The specification document for your DAQ device
shows its settling time."