Missing some signals when using NI6210 and labview

[bigben542]

I am using NI6210 and labview to sample the output voltage and current. The output voltage going though the voltage sensor board  is connected to the NI6210. When I use labview to observe the voltage waveform,some sampling signals are missing. The PWM frequency of the original voltage signal is 10KHz. My sampling frequency range is from 20KHz to 60KHz.  

I am pretty confused with the missing signals I detected, do you have some suggestions? Thank you very much.

The figure above shows original voltage（top）and sensed voltage(bottom).

The figure above shows the voltage signal in labview.

I used the standard high frequency (20KHz) squarewave to test the setup and it works well. However, once I sample the output voltage, there are some signals missing. Is that because of the inaccurate sensed voltage I get from the voltage sensor board I made by myself? 

[James.M]

This really looks like a signal issue, rather than a LabVIEW issue. Is it repeatable? Is it happening at a consistant frequency?

Can you share your LV code to eliminate that as an issue? Please include a Snippet of your code, or attach your VIs, so that we can better assist you.

[James.M]

Edit: Oops, didn't mean to respond twice.

[JÞB]

I see a definate impedance mismatch between the signals.  I suspect you are filtering the signal improperly however the sample rates sound correct.

[Henrik_Volkers]

For a 10kHz PWM Signal I would recommend at least 20 to 30 times higher samplerates ....  if you want to see more than a coarse guess of the waveform 😉

So 250kSPS is worth a try

Or add a lowpass filter .....   depends what you want to measure

How does your sense board look like?  schematic and build up?  

[bigben542]

Thank you very much for your reply. Could you please explain more about impedance mismatch you mentioned? I will show you more information in my experiment. The sensor board I used is based on LV25P voltage sensor. The basic diagram is listed below.

The R1 is 40K ohm, the Rm is 120 ohm. The measured voltage is 400V, the calculated output is 3V.

I also checked more information related to " Troubleshooting Unexpected Voltages, Floating, or Crosstalk on Analog Input Channels " :

Problem:
I am getting crosstalk, floating, or some other kind of unexpected voltage on my analog input channel, and I would like some help troubleshooting this problem. What are some possible reasons for this behavior?

Solution:
There are a number of reasons that you may be seeing unexpected voltages or crosstalk on an analog channel. The following are some of the more common reasons:  

1. High Source Impedance
   If multiplexed systems are driven by sources with high source impedance compared to the input impedance of the NI card, you may see voltages from one scanned channel reflected on another scanned channel. In order to eliminate these reflections, the source impedance and input impedance should be matched.

Is this related to what you mentioned?

The VI in labview is listed below:

I did not use any filter before sending signal to NI6210 from sensor board at this time.

[bigben542]

Thank you very much for you reply. I tried higher frequency as you mentioned and it did help improve the existed waveform, however, the missing part doesn't change. The highest sampling frequency of NI6210 is around 150kHz for one channel.  My sensor board information is provided in my reply to Jeff·Þ·Bohrer. I did not add any filter between sensor board and NI6210.  Do you suggest I should add some lowpass filters? 

[bigben542]

Thank you very much for your reply. 

Is it repeatable?

Yes, it is.

Is it happening at a consistant frequency?

No, it is quite random

The VI is listed in the reply to Jeff·Þ·Bohrer

[bigben542]

I figured out the problem. This is because I did not connect one of the analog output to the ground port in 6210 like the figure below. Thank you very much for you help.

[bigben542]

I figured out the problem. The reason is replied above. Thank you very much.