Measuring low dc current with USB 6009

[tripper]

Hi all,

I am trying to measure low dc current(0 -2A) with USB 6009.I am using 200m Ohm as a shunt but still getting 15mA error.

My question is should i use hall effect sensor, feedback ammeter method or the simple method what I am using ?

Thanks

[SnowMule]

A 2.5Ω resistor would give you 5V at 2 amps... maybe a larger shunt resistor would work better.  Isotek makes shunt resistors with Kelvin leads making the connections and measurements real easy.

In some cases I've also needed to use a current amplifier to condition the small signals before passing them into the DAQ, but that's to measure down into the micro-/milli-amp range.

[johnsold]

15 mA is less than 1% of 2 A.  What is the resistance tolerance of your shunt resistor?  The absolute accuracy of the USB-6009 is 1.53 mV on the +/-1 V range.  That is half of your error.  System noise can add another half mV.  Have you accounted for all other possible errors? What is the power rating and temperature coefficient of your shunt resistor? at 2 A your shunt will be dissipating 0.8 watt.  What is the error of the other method you are using to determine the current (in other words, how did you decide you have a 15 mA error)?

Lynn

[Coq_Rouge]

You will also always see some bit toggling on the two least significant bits on the ADC result. Which sample rate do you use. Depending on your system noise type. You may gain some by sampling 10 to 20 times faster than needed, and then averaging X samples. And present that as result rather than presenting each sample as a single result

[tripper]

First of all thank you everyone for participating

Secondly, I am sorry poor explanation. When i measure real time dc motor current(approx 80-110mA) and compare it with Tektronix 3014 scope its fine.

I am trying to build a dedicated mechatronic system tester need to calculate mean,Max current and energy in one complete cycle of system.And also total time for one cycle.See VI for details.

Now the waveshapes are exactly the same with Tektronix 3014 scope . All measurements are close but not very close.for instance time difference is 20mSec,mean current diff is 15mA and Max current diff is 20mA.I am using the same sampling rate 5K and recording same length of samples 10K.

Please advise.

[johnsold · Accepted solution]

Most scopes have 8-bit digitzers, so their resolution and accuracy are limited.  What vertical sensitivity range is set on the scope?

15 mA out of 100 mA seems like a lot but as I pointed out above, you have not accounted for the possible errors in the system, so we cannot tell you if you have a problem or are reaching the limits of your equipment.

Lynn