DC offset in NI USB 7856R

[pratikpade]

Hi all,

I have developed a labview project to acquire data continuously from my FPGA device (NI USB 7856R) and later plot it in excel. After a series of debugging attempts, I am still left with one. I am applying a test signal: sine wave (through signal generator device) of 100Khz 1Vpp at input and expect the same at output. However I do not get a perfect sine wave but it experiences a certain DC offset. (Please find the attached waveform) I tested my signal generator itself and it works fine.

Also I face another issue. When I apply absolutely no input, the waveform plots a straight line of -10.5 V. I don't understand the reason behind it.

Please find the attached project and respective VIs. Any help is appreciated.

Thank You !

P.S: Please refer to attached VIs only. I apologize for the excess VIs and messy organisation.

Regards

Pratik

[kartiknattar]

Hello pratikpade,

If you are getting offset in your input signal then it means there is a impedance mismatch between your signal generator and fpga inpur channel. Secondly what do you mean by no input i.e. input channel is connected and no signal is applied or input channel is disconnected. Because if your input channel is disconnected then it will give you a floating value (minimum or maximum input signal value) or random value so make sure that your input channel is connected even if you are not applying any signal.

[pratikpade]

Hello Kartiknattar,

Thank You for helping me and sorry for the late response. 

I tried measuring the impedance. The connector pin of my fpga device has an impedance about 420k ohm (measured using DMM) and output impedance of my signal generator is 50 ohm(refered from  datasheet). I tried connecting a pot between my signal generator and connector pin to reduce the impedance mismatch but it has no significant effect on the sine wave.

However I found the problem, it is in my HOST VI. I used "Sample Compression" express VI, to reduce the sampling rate. Which causes the problem in the sine wave. Please find the attached image comparing the effect of plotting the sine wave without and with the express VI. 

I have a followup question though. I want to plot this wave on excel, thus I am using "Write to Measurement File" however the sampling rate is too high for excel to process and thus it cuts shorts the later data points. Please can you suggest a way to reduce the sampling rate and still be able to plot the complete wave in excel. Or else can you suggest any different approach to plot the wave in excel.

Thank You for your help

Regards

Pratik