I would like to know how to change the settings on 6361 card so it can measure

[AjmalG]

I would like to know how to change the settings on 6361 card so it can measure differential voltage across a sense resistor which is in series with output inductor on a buck regulator when output current is being supplied?  So Iload=V_diff/Rsense. My voltage measurement (V_diff) is varying all over the place since the current is trapezoidal and the frequency is 4Mhz and pk-pk current is +/-2A.  The irony is that I get the right current value when I use a Fluke DMM to measure DC voltage in mV across the sense resistor.  I am guessing that the Fluke meter has an input filter before it takes the voltage measurement. Unfortunately I don't have place holder for low pass filter on my PCB so it would be much easier if I can do some correction in SW. Any help on this topic would be greatly appreciated.   

[JayJayHella]

Hi AjmalG, 

In LabVIEW, you could add a low-pass filter in software with the Filter Express VI or the FIR Windowed Filter VI.

Filter Express VI

http://zone.ni.com/reference/en-XX/help/371361M-01/lvexpress/signal_filter/

FIR Windowed Filter VI

http://zone.ni.com/reference/en-XX/help/371361M-01/lvanls/fir_windowed_filter/

Have you tried implementing one of these VIs into your program? Do you think that theywill work for you application?

[AjmalG]

Thanks Jake for your quick response. Can I use Filter express VI and add LPF before I take a voltage measurement with 6361 DAQ card?

Regards,

Ajmal

[JayJayHella]

Hi Ajmal,

You would have to read the signal in for a voltage measurement, then you could filter it. Can you send a screenshot or attach the the code you have so far?  

[AjmalG]

Hi Jake,

On my board after the switching supply output capacitor, I have a 5mohm sense resistor with a 1k and 0.1uF low pass diff filter before going to the DAQ card. My sense resistor is after I close the feedback loop on the switching regulator so the current across the 5mohm sense resistor is DC. I am doing a diff voltage measurement and dividing it by 0.005 to get current. The value from LAB view fuctuates from 842mA to 870mA versus the real measurement is 800mA.  What can I do to improve my measurement accuracy. I have the DAQ signal set to 2V scale since my voltage signal is from 1.4V to 1.98V. I don't have a problem when I use a fluke DMM and measure voltage diff in mV scale.

[2Shanez]

Hi AjmalG,

Is this after implementing the filters Jake suggested earlier? Would it be possible to attach a screenshot of your VI so far? This would be helpful to make sure we are on the same page. 

Thanks!

ShaneK

Applications Engineering