Incorrect current measurements with PPK2 when using an nPM2100

[David]

Hello,

I am experiencing issues with the current measurements taken by the PPK2 when using the nPM2100-EK to power a device. The current drawn from the 2.7V power source (PPK2) is lower when using the nPM2100 (configured to output 3.0V) compared to a direct connection without the nPM. This is clearly incorrect. How should the PPK2 be configured to reliably measure our system, including the nPM2100?

The system is a Bluetooth-enabled measurement device that uses an nRF52840 chip. When connected to a Bluetooth master, the system draws an average current of around 530µA at 2.7V, which is quite high for a CRxxxx coin cell primary battery. Therefore, I would like to use the nPM2100 to boost the voltage to a constant value of 3.0V. However, when using the nPM2100-EK for this purpose, the PPK2's current measurement shows large spikes and an average value of only 320µA. As the nPM2100 does not generate energy, this measurement must be incorrect. 

I am using Power Profiler Version 4.2.2 and even a second PPK2 produces similarly incorrect results. I have read about the phantom spikes occurring at the highest sampling rate. I therefore selected a sampling rate of 10kHz, but the values are still incorrect.

Has anyone else experienced this issue? What is wrong with the PPK2 measurements?

[Kenneth]

Hi David, 

Unfortunately Asbjørn is away this week (and previous week), he is back on Monday. This case is slightly lengthy and I do believe he has been in direct contact with the team that made the hardware, so I suggest we wait until he is back before we can follow up.

Kenneth

[David]

Dear Kenneth, 
Thank you for the information. I am OK with your suggestion — I will wait for Asbjørn to return from holiday.
Best regards

David

[Asbjørn]

Hello David,

Thank you for waiting here, what surprises me a bit in your reply is that you are seeing about 0.5% discards and only with the capacitance on VINT it changes. Based on the conversation here I think this might be a good reason for us to try and look into the discard algorithm and see if we can find ways of improving it. The PPK2 setup was created for measuring directly on the nRF devices, not the nPM family. The usage and behaviour of nPM is different and I'd say there's room for improvement on our PPK2 offerings when used on nPM products. The resources allocated to the development of PPK2 are limited and the performance on nPM have not been the highest of priorities.

"Having said that, I'm slightly confused because the nPM2100-EK User Guide suggests using the PPK2 to measure the nPM2100's currents. Have you tried this method?" 

• The honest answer here is we've done some initial work, but the development for use on nPM and especially for nPM2100 is still very much ongoing. We haven't had this sort of feedback for the nPM2100 with PPK2, but I expect that we will see more as we go forward. We do intend to have the PPK2 as a tool and resource for the nPM2100EK for our customers, but we need to update the documentation to reflect setup of capacitance on supply, FW improvements and to some extend explain some limitations for the PPK2. I wish I could give you a timeline for these things, but that's beyond my scope and I'd have to refer you to our sales organisation to assist with timelines and feedback on those.

Now for your situation, with the series ammeter, do you then also have additional capacitance on the supply net? What numbers are you reading from PPK2 with this setup?

Best regards

Asbjørn

[David]

Dear Asbjørn,

Thank you for your honest response and evaluation of the situation. We really appreciate the PPK2 and have several in use. Hopefully, an update will be available soon so that we can achieve better results with the nPM2100 as well. 

My simplified test with the PPK2, the nPM2100-EK, and a 7.5kΩ resistor does not include any additional capacitors. The VBATIN/VSUPPLY net has a 10µF capacitor and VINT a 10µF; that's it. With an ammeter in series, the PPK2 measures an average current of 546µA. The ammeter confirms this result. In this case, approximately 2.4% of the samples are discarded.

Furthermore, I have found that accuracy increases when no samples are discarded. Without the ammeter in series, 3.8% of the measurements are discarded, resulting in an average value of 13µA. Using all the measurements (discard_switch_samples 0) results in an average value of 223µA, which is closer to the correct value of about 540µA, but still does not correspond to reality.

I hope this information helps improve the accuracy of the PPK2 when used with the nPM2100.

Best regards
David