Power Profiler Kit Lowest Range Doesn't Work

[FUEN]

When using the PPK to measure the current of our custom hardware the lowest range doesn't give logical results. (Either the PPK python GUI or nRF connect) When the nRF52 on our custom hardware goes to sleep the current reads as 84mA constant. However while the unit is awake, LEDs are flashing or the unit is transmitting the current value is as expected. I see peaks of ~28mA when transmitting and a constant draw of 10mA when our LEDs are on as expected. 

From my understanding of the PPK hardware, it will automatically switch between ranges by reading the voltage drop over different resistors. (R2 (510ohms), R3 (33 ohms) and R4 (1.8 ohms) on the PPK)

I measured their values with the unit switched off and both R3 and R4 read close to the values indicated in the schematic, however R2 read as 430 ohms whereas it is listed as 510 ohms in the schematic. The calibration value in the PPK software reads 503.26 ohms.

The reading of 84mA with a voltage of 3V suggests a load of ~36 ohms, which is possibly close to the value of R3?

What other values or readings should I test to find the cause of this fault?

[wlgrd]

Hi FUEN, this does indeed sound strange. We have had some issues the other way around (higher currents are too high), which has been due to ESD breaking the switching circuitry.

The measured values for the resistors are unfortunately not straight forward to measure, this is why it is calibrated during production. The path of the power line passes a some switches and other analog components that themselves introduces some series resistance, so only using a multimeter across the resistors will not give a proper result. Your understanding of the switching circuitry is correct.

When in highest range, all switches are latched in, causing a parallell connection between the three resistors. When middle range, only R3 and R2 is active, and lowest range, the R2 is the only on in. What you are explaining, seems to be an overflow value of what the ADC is able to handle, hence I thought that a switch was broken. We see this overflow when the system thinks it has latched in all resistors, but for some reason hasn't, and the calculations are wrong because the voltage drop is way to high. 

So it seems like yes, R2 is not "active" when the system thinks it is (all analog switches are open), which makes me wonder if R2 is not properly attached to the board, or we have a connection break there. It is not activated by any switch, it is always used, so it is hard to believe that any components are broken since the rest of the measurement system is working properly. 

If you have the schematic, and you seem to know how to find your way through it, I would suggest you to measure continuity in and out from R2 to the appropriate connections, and maybe also swap it out with another 510R in the rare case it is broken somehow.

Sorry for the long post, but I'm trying to make sure you have enough info to be able to fix this yourself :) 

[FUEN]

Hi wlgrd, thanks for taking the time to give an in-depth reply. Unfortunately I am out of the office for awhile but when I am next in I will look at replacing R2 as it would be an easy fix. Am I correctly understanding that the PPK re-calibrates itself on start-up or with the press of a button in the PC software? It looks as if Q1A/B and Q2A form a current mirror that allows recalibration in software?

I have had a bit more of a look over the schematic and think I am able to follow most of it. If replacing R2 yields no results I will check that the other switches are working correctly. Unless something has gone wrong with one of the amplifiers or references there is not really much else to fail, and given that the other ranges give sensible values it is extremely unlikely.

[wlgrd]

I agree. Yes, your keen eye has spotted a current mirror quite correctly ;) Unfortunately, this is not in use at the moment, but we are planning on using it in the near future for calibration. Right now, the hardware does an offset calibration on every startup (for the 2.0 firmware), and resistor calibration only in production.

Since this problem intrigues me, I really am looking forward to your reply. Please do not hesitate to ask further questions if there is anything I can help you with. 

[FUEN]

Sorry for the extremely slow reply. For anyone who comes across this in future it turned out to be an issue with the hardware I was measuring. This would have been apparent much sooner had I just tested the PPK using a resistor value that would be on the low range of the PPK.

[wlgrd]

That's great to hear! This has confused me for some time now ;D