USB Data Logging

[Dani]

Hi,
I am new to CurrentRanger device usage.
I connected the CR to my laptop (running from battery only) and I can see the CR readings on the terminal.
CR is not connected to any device or power source, so no current should be measured, but on the terminal I see readings values of (CR in mA):
-0.81e-3
-0.40e-3
-0.81e-3
-0.81e-3
and if I change CR to uA:
-1.21e-6
-1.21e-6
-0.81e-6
-0.81e-6
-0.81e-6
-0.81e-6
while changing CR to nA shows:
-1.62e-9
-1.21e-9
-0.40e-9
-0.40e-9

Why do I get those reading values? shouldn't it all be 0.00 mA/uA/nA ?
Why the values are negative? (only a few readings are 0.00 or positive value)

While testing my device (in sleep mode) I also have a negative readings for the majority of the values displayed on the terminal - am I doing something wrong?

Thank you,
Dani

[Felix]

The lower the range the more sensitive, so it could be several reasons, some of which are mentioned in the guide, but does that still happen if you have the inputs SHORTED?
Does it happen if you power ON the unit with OLED but have the USB disconnected?

[Dani]

Hi Felix,

It still happens if the inputs are shorted.
While USB is disconnected the OLED measurements are 0.0 in all ranges (nA/uA/mA).

[Felix]

It may be that you are somehow picking up more noise but I see about the same thing, it's not unusual. The minus sign or non-zero is because of an ADC offset. You can adjust that with "*" and "/" if you want to get closer to 0 (as you adjust the offset look at the upper left number that changes - that's the ADC reading and you should be within [-1, 1] range). After doing that I can adjust mine to give out readings closer to 0:

0.00e-3
0.00e-3
0.40e-3
0.00e-3
0.40e-3
0.00e-3
0.40e-3

But the raw opamp output is what really matters if you're looking for precision, a true RMS DMM should show you a 0.0V in all ranges. The 12bit ADC of the CR is simply reading that output and will generally not be as good/fast as a DMM, and it needs calibration - a best effort is made here after manufacturing them to make the gain linear but it's not a perfect science.
I hope this all makes sense and helps explain it a little better what you're observing.

That's not to say the ADC sampling could not be improved, I am sure it would by using mode advanced averaging algorithms at the cost of perhaps slower speed, however the stock firmware does simple averaging.

[Dani]

I'll check this out later.

Thanks.