180mA range with nA load reads 10mA

I just dragged out my Joulescope to test out some new hardware for comparison to old. I’m seeing one anomaly that has me puzzled. When my device is in sleep mode, it is very very low current. And, in most ranges, it is reading nominally 0mA/0uA or close to it. So far so good. However, when I switch off auto-range; set to fixed 180mA range to test general (non-sleep) mode I see strange things. When not sleeping, I expect to see 40-100mA nominal currents (plus switching regulator spikes), This does seem normal and gives me reasonable values. However, when going back to sleep, the Joulescope is claiming I have around 11 mA and 37mW when it should be in the nW range.

Is it broken? Or what setting is wrong?

Hi @skaterdad,

Your Joulescope should be measuring correctly, so the question is, what is going on with your setup? I think you say that your Joulescope works well in autoranging mode, but you want to set it to a fixed range. I suspect that 180 mA is not enough for the maximum current on your target device, and you are partially browning out the target. What happens if you set Joulescope to the 2 A range?

Here are the Joulescope current measurement specifications from the User’s Guide:

If you are still having an issue, could you post a short JLS file (about 0.25 seconds) capturing the sleep mode?

20200318_165253.zip (1.3 MB)

At 180mA range and the device in sleep mode, I would be expecting uA. In all other ranges it indicates “approximately zero” but when in the 180mA mode it is indicating around 11mA.

Based upon the file that you sent, Joulescope appears to really be measuring that 11.6 mA current, so the question is why does the target draw 11.6 mA with Joulescope’s 0.11 Ohm resistor (180 mA range) in series?

How long is the sleep duration? If it is long enough, what about using “auto” until the target goes to sleep, then switch to the 180 mA range? This will allow you to sanity check that the 180 mA range is still working. Alternatively, you can disconnect the target while in 180 mA range and should see approximately 0 current.

I was not able to open that screen capture. If you want to share, could you capture in a format that does not require Adobe Flash? ScreenToGif is great and free.

well, therein lies the rub: disconnect both input and output and still have 11mA reading… though mW just dropped down.

I’m not sure how to capture .25 second; the files are huge even as fast as I can stop/start it. Can you describe the right procedure to get a quick sample? Also I just had Joulescope go ‘not responding’… had to terminate it.

I changed the sampling rate to a much lower (200k vs I think it was set to 2m) and dropped the buffer to 15 seconds. 20200318_191154.zip (1.6 MB)

Note that NOTHING is attached, yet it’s reading 11-12mA.

I’ll try to post videos again in a bit.

The easiest way to capture a short JLS file is to use dual markers. You can just live-stream USB data to the buffer, and then click the play button on the Control widget to stop live-streaming data. Add dual markers by right-clicking on the x-axis, AnnotationsDual Markers.
Move the dual markers to the time range of interest. Then right-click on the dual markers and select Export data.

So, you have fully detached IN and OUT? Do all ranges other than 180 mA read close to zero amps? If so, that is very, very strange. I will contact you through email.

I am following up to close out this issue. The unit was replaced under warrenty, and we received the misbehaving unit for analysis. The MOSFET which selects current range 2 was damaged and bleading current from its gate into the source-drain. The exact cause of the MOSFET damage was not determined. After replacing the MOSFET, the unit is operational and meets specifications.

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