How to do a current acquisition using NI-6251

[shawn1]

There is a project that needs acquisize one line current of circuit , accuracy requirement is 1uA level. before the Design is: in Ni-6251 difference points at both ends of the AI welding a 0.5 ohm resistor (accuracy is 1%), then Using DQAmx voltage acquisition function to measure the voltage between the two ends of the resistor ,finally convert the volatge to current, but the actual precision of current is 10uA level, since the resistance is 1% accuracy. so there are two questions i wanna ask , hope to hearing: 1,Measure the differential voltage to do a current conversion , if i want to get the precision is 1uA level, what method to improve?? (hardware / software) 2, I see the DAQmx has a current function, Can be directly through the built-in resistanceof NI-6251 to get the current value, But i wonder if NI 6251 has a built-in resistance? If there is no resistence in instruments NI 6251? Which NI instrument has built-in resistence .thank you

[crossrulz]

What is the range of currents you are trying to measure?

What I have learned is that you really want to use an external amplifier.  There are some out there made specifically for current shunts.  I have found the TSC103IPT to work really well.  It has a selectable gain of 20, 25, 50, or 100.  So you can do all kinds of combinations with it depending on what current range you actually need.  For instance, if your upper current limit is 1A, then the voltage drop across your 0.5 Ohm resistor will be 0.5V.  If you use a gain of 20, then your upper input voltage will be 10V (the make range of your DAQ).  This amplifier also gives you a single ended output, so you do not need to do the differential input on your DAQ.  You will likely need to make the custom DAQmx scale to do the conversion of your reading into current.

[shawn1]

Thanks for your answer, I would also like to question. I measured current range is 1uA~100mA, The accuratecy is 1uA . For example, I measure the one line circiut current using TSC103IPT, This line current acutally is 2uA , the TSC103IPT output voltage will be the 2uV level instead of 10uV? TSC103IPT input resistance accuracy is required 1‰? thank you

[shawn1]

sorry Suppose that the AMP gain is 1(perphase the gain is 20/50/100 made by the AMP sel pin)，How do you think about the resistence selection? Thank you

[crossrulz]

I would got with a 1 Ohm resistor with that low of current.  Then use the 100 gain on the amplifier.  That will give you 10V at 100mA, 100uV at 1uA.

[shawn1]

If I wouldn't use AMP,what affects will be have in the process of current measure? （small voltage can not be indentified by NI-6251? then the current measure is not accurate? This is my guess）Thanks

[crossrulz · Accepted solution]

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@shawn1 wrote:
If I wouldn't use AMP,what affects will be have in the process of current measure? （small voltage can not be indentified by NI-6251? then the current measure is not accurate? This is my guess）Thanks

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You basically hit is.  You are trying to measure a level that is basically in the noise floor of the DAQ.

[shawn1]

Got it ,Thanks for your help

[shawn1]

Hi kinight : I am ready to use the Amplifier TSC103IPT,but I didnt find the absolute accuary about input range,So can you help me to find the min voltage absolute accuary of the TSC1031PT？ 1uA can be Identified? My measure current Expect: 1uA~100mA