USB-1608G Error Trying to Read Current with Differential Analog Input

[liamsul]

Hello,

I am using a USB-1608G to measure current using an analog differential input with the universal library for Python. I am trying to measure the  the system current load by sending 4.0V across a 0.1 ohm resistor and measuring the voltage drop across it. When changing the system load's current I am unable to get accurate readings when the current is above 200 mA. However when I start to get into the lower counts, such as 20 mA, I am unable to get any accurate readings. At 4V with a 20mA load I am getting a reading of about 15mA and as the current goes lower it gets more and more inaccurate. See table below.
          

Also during testing this I noticed that the ADC counts are higher than the expected 32,768 (2^15), with the differential input ADC count at 4.0V at 20mA being 32818 which is higher than the ADC should be able to read. I was under the impression that each the MSB was used for positive or negative with the remaining 15 being the count.

With ~20ma Load:

Any help on this would be greatly appreciated!

[DAQman]

Let's assume your current numbers are accurate. 200mA through a 0.1 ohm shunt resistor will generate 0.02 vdc. (I x R) If the 1608G is set to the one volt range, the A/D volts per count is 30.5uV (2 volts ÷ 2^16). The A/D counts range from 0 up to 65535 where zero is -range (-1 volt) and 65535 is +range (+1 volt). Zero is 32767. For 0.02 vdc, the A/D will return 32767 + (0.02 ÷ 30.5uV) or 33423. Because the voltage at 200mA is only 0.02 vdc, consider a larger shunt resistor or a measuring device with a lower voltage range. Also, when measuring current using a shunt resistor, the best place to put it in the negative power lead to keep common mode voltage to a minimum. If your system is isolated from ground, you may need to use a reference resistor as outlined on page 11 in the user manual.