Power Profiler Kit II measurement error

[MartinZ]

I made some tests with PPK-II in source meter mode by connecting a load resistor in series with a Fluke 8846A ampere meter to the PPK output. A also hooked a GW Instek GDM-8351 voltmeter in parallel to the PPK output.

Here are the results:

The PPK is out of spec around 500uA. From time to time, while changing the output voltage up and down, those red values suddenly got much better, in accordance with the +/-10% specified max. error. But I couldn't figure out what exactly makes the PPK suddenly jump in/out of spec. However, most of the time I observed the large errors from the table above.

Given the described behavior, I suspect a FW error in range changing logic. Are you aware of this? Can this be fixed in a future FW upgrade?

[Audun]

Hi,

thanks for the detailed results and setup!

As you mention, it could indeed be an issue with the measurement ranges. We will investigate internally with the same setup as you have used and see if we can make adjustments to improve this behavior.

As a side note, the updated PPK2 firmware has a secondary COM port used for command line access. To determine if it is actually an issue with the ranges, you can open this COM port and execute the "sampling metrics" shell command after sampling has stopped. The command line interface is non-responsive while sampling, to avoid interfering with the primary function of PPK2.

See this screenshot as a reference:

If the "samples dropped" metric is high, it indicates that there is an issue with the measurement ranges.

Finally, you can use the shell command "sampling discard_switch_samples 0" to never discard any samples. I would be very interested to know if this improves the behavior or not.

Best regards,

Audun

[Audun]

Hi again MartinZ ,

I do have a new version of the firmware you can try if you want. I don't expect this to drastically improve accuracy, but hopefully it will be a bit more predictable. Some of the measurement range detection timings have been tightened, and the VDD output accuracy might be a bit improved.

You can program the new firmware using the nRF Connect for Desktop Programmer app. Power Profiler will complain that there is a version mismatch, but you can just click "No" in the prompt.

During testing on my PPK2, I observe that measurement range 3 (0.5 mA - 5 mA) shows an offset of about 100 µA. For a load in the 500 µA range, the accuracy depends on whether PPK2 uses range 2 or range 3. Note that the range selection is done purely in hardware, and cannot be directly controlled from firmware.

I can compensate by changing the gain for this particular range under "Advanced configuration":

It might be possible to improve the calculation done on the Power Profiler side, but I have no timeline for when we might look into this.

Best regards,

Audun

pca63100_ppk2_0.0.0_5bdd04e.hex