I'm trying to measure small currents in a wireless sensor application using a USB6009.
I expect 2uA in normal operation from the device, and have sucessfully measured these on the low side of the device with a 250 ohm resistor.
This was with a USB6008. Now, we would like to use a calibrated standard shunt (.001ohm) and a USB6009 to get data closer to the actual use case being able to see
150 - 350mS transitions of the field detector circuit cycling on and off (needed the higher sample rate) and hopefully seeing real activity not having several millaohm draw of a 250 ohm shunt.
Granted, the noise of the 6009 is .5mV in the 1V range. I was thinking, (perhaps wrongly) I could accurately see microvolt level voltage data with a differental system.
This may be my issue to begin with. My experiement is below.
I have a .001 ohm shunt resistor standard from http://www.rc-electronics-usa.com/current-shunt.html (A series)
In my first experiement, I had the device (powered by a 3.3V CR232) with the current shunt bridging the low side of the battery, the terminals attached to AL0(+-)
The dynamic range of the measurment was set to 800nA to 10uA
I was then seeing millavolt noise on the line weather the shunt was connected or not, basically no data.
I then tried this with 1.5Kohm resistors pulling the AL0(+-) lines to a GND contact on the 6009. Same result.
Then, I took a known lab voltage supply at 5V and attached a 1.5Kohm resistor in series with the shunt. AL0(+-) was accross the shunt.
I calculated that I should see 3.33... uA on the shunt in this configuration, yet saw just the .300 mV ripple on the line which is a much higher current than should be there.
Any recomendations on doing these small current measruments? I'm thinking buying a SOIC from TI or someone might be a good solution / or just a plain old op amp to get this voltage.