Been loving my Joulescope! Has been working great and has been very useful!
However last night I started seeing unexpected average current values. The device I am testing pulls a few mA on startup and then settles into about 5uA average current draw after a few seconds. Power supply is 3V.
The Joulescope consistently shows average current of about -15uA when the actual current draw is about +5uA.
If I disconnect the power supply or the device, the Joulsescope correctly reads low nA noise. If I connect a 1M ohm resistor in place of the DUT, the Joulescope correctly reads a steady +3uA @3V.
I’ve double checked by putting a multi-meter in series with the Joulescope and it reads the expected average current. Also tried taking the Joulescope out of the loop and again I see the expected average current values.
I’ve also tried restarting the Jouscope and the UI software.
Any ideas why I am seeing these incorrect negative current values?
Hi @bigjosh, Great to hear that you have been loving your Joulescope, and sorry to hear something is not right.
This seems to indicate that your Joulescope is still functioning correctly, at least for this static case. In the Joulescope UI picture you posted, the current min/max is very troubling: -114 to +30 mA. If your device is really drawing 5 µA, we are picking up some major noise somewhere. With this much noise, measuring µA will be a major problem.
I have a few things to try:
Measure the current with -3V (swap power supply + and - to Joulescope IN) with the 1 MΩ load. Verify that you measure -3 µA and that p2p is small.
Measure the current with +3V with the 1 MΩ load. Verify that you measure +3 µA and that p2p is small.
Configure your system for the troublesome 5 µA mode. In the Joulescope UI, click Current Range → 180 µA. Is the value now correct?
If all the above work, I would like to take a look at the captured data (if possible) in that troublesome 5 µA mode. In the Joulescope UI, open the oscilloscope view. Right-click on the x-axis, select Annotations → Dual Markers. Drag the markers to select just over 0.2 seconds of data. Right-click on one of the dual markers, click Export Data and save a “.jls” file. Then post that file here.
Thank you for the great response with all the data! I took a look at the data you captured, and I noticed a few things. First, your Joulescope seems to be working as designed and is not broken. whew
Fixed current range capture
The image that you posted with your Joulescope in the 180 µA range is saturating. Notice how the max value clips at about 200 µA? The 180 µA value is the guaranteed range, but your Joulescope is actually designed to measure up to 195 µA (typical). Could you try increasing the range until it does not clip? Could you then post that JLS file? I would like to get a clean capture of the existing target current draw. This capture will help me confirm my guess at what is happening - read on
Autoranging current mode capture
In the JLS file you posted, it looks like your device is consuming very short bursts of current. Joulescope uses the rate of change (among other things) to determine the automatic range switching, and it appears that you have some very fast transients that cause Joulescope to switch all the way to the 10A range:
In the case of your target, it appears that this spike disappears in less than 4 µs. How much capacitance do you have on your target? Based upon the data, I suspect that the load capacitance is quite low. Joulescope’s autoranging feature does require some load capacitance to limit bandwidth if you have very short pulses. Are you able to add some additional load capacitance? If so, go ahead and add 10 µF, but you should event see a great improvement with just 1 µF. Hopefully this fixes the issue. Either way, I would appreciate if you could capture and post the Joulescope JLS captures both in fixed range and in autoranging.
Hope this helps! Let me know what you find, and thanks!
The circuit is intentionally very low capacitance and I do not want to add extra capacitance to it.
As long as I am manually selecting the lowest range that captures the max and min current limits, can I expect to get accurate readings to within the resolution of the selected range?
Understood. With very low capacitance and high current bandwidth, Joulescope’s autoranging is not going to be able to keep up. The autoranging feature will still try its best which will cause it to introduce measurement error that you observed.
Yes. However, you do need to keep in mind Joulescope’s measurement bandwidth. Joulescope is rated for 250 kHz bandwidth at 2 MSPS. Signals above 250 kHz will be attenuated.
