Power Profiler showing negative current for external DUT

[_armin]

Hi,

I am using the Power Profiler Kit (Rev. 1.1.1.) with Power Profiler app (v2.3.1) to measure power consumption on an external DUT. The issue I am facing is negative current values shown by the Power Profiler app when I set the PPK output (regulated) voltage to around 2 V or less. Negative values are shown when nRF chip on my DUT is in low-power mode, and when it is not in low power mode (e.g. advertising) it seems to give logical measurements. If I set the output voltage to more than 2 V, I get currents of 4-5 mA or more while my DUT is in low-power mode, not sure what is the cause of this, but I would like to at least resolve the issue of negative idle currents with low output voltage.

My test setup is as follows, taken from the PPK User Guide:

- PPK stacked on top of a nRFDK52840,

- The nRFDK52840 connected to the PC with a USB cable,

- Power Supply switch on the PPK set to Regulator position,

- Power Select switch on the PPK set to External position.

Note that there is no battery or another power source connected to the DUT, it is powered solely from the PPK.

I have found a similar issue on the DevZone, and the answer here is that PPK cannot be stacked on top of the DK when using the voltage regulator (if I understood it correctly):

https://devzone.nordicsemi.com/f/nordic-q-a/59474/power-profiler-showing-negative-current

Besides this, an answer to another questions suggests that PPK has to be powered through the DK, because USB supply on the PPK is faulty:

https://devzone.nordicsemi.com/f/nordic-q-a/36162/power-profiler-graph-is-in-negative-region-and-average-current-also-appears-to-be-negative

Please advise on how to proceed with measuring the consumption of an external DUT.

Thanks,

Armin

[Jared]

Hi,

How much negative current are you measuring? Is the external DUT connected to anything? Can you measure on the PPK output and verify that it's actually 2V? Also, could you upload the schematic of the DUT? 

_armin said:

I have found a similar issue on the DevZone, and the answer here is that PPK cannot be stacked on top of the DK when using the voltage regulator (if I understood it correctly):

The DK and the PPK can be stacked when you use the regulator but you should not use the regulator if you're measuring current on the DK.

regards

Jared 

[_armin]

The current I measure changes quickly with output voltage, it goes below zero when the voltage is around 2.05 V, and it goes down to -1.5 mA when I lower the voltage to 1.95 V. As the voltage is raised above 2.05 V, it reaches a few mA (positive).

External DUT is not connected to anything, only to the PPK, and I have verified that I get the output voltage which I set in Power Profiler app.

The board that I am testing is Acconeer's XB122 breakout board with XM122 module connected to it. I am not sure if I can upload the schematic here (legally speaking), but you can probably gain access to it easily on the Acconeer's website.

One more thing, not sure if this is expected or can in any way help - if I disconnect the external DUT from the PPK, when I increase the voltage, I get a current of around 65 uA, and when I lower the voltage below 2 V, I get around -230 uA. If I reset the PPK and lower the voltage then, it shows it go down to -0.8 nA, and if I increase the voltage it goes up to 300 uA.

Thanks,

Armin

[Jared]

_armin said:

One more thing, not sure if this is expected or can in any way help - if I disconnect the external DUT from the PPK, when I increase the voltage, I get a current of around 65 uA, and when I lower the voltage below 2 V, I get around -230 uA. If I reset the PPK and lower the voltage then, it shows it go down to -0.8 nA, and if I increase the voltage it goes up to 300 uA.

How do you measure the current after you've disconnect the PPK? What do you measure if you flash the board with the power profiling application?(Remember to replace the board file)

[_armin]

I meant that I am measuring these values by the PPK itself and the Power Profiling app, without an external device connected at all.

I tried the power profiling application, I am getting the same results.

Could this be an issue with PPK or the Power Profiling app, or the setup I am using? Getting constant reverse current of 1 - 2 mA with just PPK connected to the external DUT seems to suggest this (no other power sources are connected).

[Jared]

Hi,

I understand, this is as expected. Measuring with just the PPK ontop of the DK with the power select being set to REG will result in invalid results. 

Could you try to connect it as shown in this figure, with the J-link being the debugger onboard the DK. Use a 10 pin-connector to connect the DK to the PPK.

regards

Jared 

[_armin]

Hi, I have tried the one where PPK uses USB for power supply, and I am getting different, but still bad results. Now the current is always shown to be in the range of 5 to 10 mA while in low-power mode, instead of going below zero like before (when I lower the voltage).

The link I posted earlier seems to suggest that USB supply is faulty on the PPK, so not sure if trying an external power supply would help? I could try that if there is reason to believe it can work, but it is a little inconvenient so would like to avoid doing it if it cannot be helpful.

Here is the link regarding the PPK USB power supply again:
devzone.nordicsemi.com/.../power-profiler-graph-is-in-negative-region-and-average-current-also-appears-to-be-negative

Any other ideas on what else to try?

[Jared]

Hi,

The thing is that if you're going to use the PPK on top of the DK then the PPK voltage should match the DK voltage. If it doesn't, you risk current leakage and erroneous current readings. I suggested configuring the connection as in the figure in my previous reply since the voltage of the external DUT didn't match the DK voltage. It seems that you don't longer have negative current readings but still measure a higher current then you expected. Now, what application are you running on the DUT when you measure? And what do you mean by low power? System OFF or System ON IDLE?