nRF9160 DK Current Measurement

RE: nRF9160 DK Current Measurement

Stian Røed Hafskjold — Tue, 24 Sep 2019 11:20:46 GMT

From the schematics: https://www.nordicsemi.com/Software-and-Tools/Development-Kits/nRF9160-DK/Download#infotabs

VDD_nRF ' is the middle pin and connected to the supply.

VDD_nRF is the uppermost pin and connected to the chip.

VDD_nRF is expected to drop below VDD_nRF '.

RE: nRF9160 DK Current Measurement

ERIK — Tue, 24 Sep 2019 02:19:42 GMT

Thanks for the response.

Interesting because I have Ch1 connected to the uppermost pin on P24, which I thought was the power supply side. But I see it dropping below the middle pin of P24. That is one of the observations I was questioning.

RE: nRF9160 DK Current Measurement

Stian Røed Hafskjold — Mon, 23 Sep 2019 10:39:21 GMT

I'm currently out of office, but will be back on Wednesday this week. Then I can do some measurements on the scope and compare them with the power analyzer so we can see how reliable this scope setup is.

I mixed up channel 1 and 2 in my previous answer. So ch2 is on the power supply side of the measurement resistor, and ch1 is on the nRF side (VDD_nRF). Ch1 voltage is expected to drop even further than ch2 when current flows through the resistor. And the resulting current going through the resistor is (V_ch2 - V_ch1) / R. (So you need to invert the math function)

RE: nRF9160 DK Current Measurement

ERIK — Sun, 22 Sep 2019 00:08:46 GMT

Also, Stian, you mention the voltage drop over the resistor being small compared to the voltage drop from the source to the measurement resistor. Channel 1 is read at that topmost pin of P24, which is labeled VDD_nRF. Even if there is a voltage drop from the external power supply applied to P28, it still seems like that topmost pin of P24 would remain pretty steady. Before putting more effort into measuring currents, I want to be sure I'm doing so correctly.

RE: nRF9160 DK Current Measurement

ERIK — Sat, 21 Sep 2019 19:27:57 GMT

Thanks Stian, for explaining why both voltages drop when transmitting. But to confirm things, Channel 1 is dropping below Channel 2 when transmitting. Is that right?

Thanks,

Erik

RE: nRF9160 DK Current Measurement

Stian Røed Hafskjold — Thu, 19 Sep 2019 10:50:55 GMT

This looks similar to what I get here. Since the measurement resistor is only 0.5 Ohm, the voltage drop over the resistor is very small, smaller than the voltage drop in the path from the source to the measurement resistor (channel 1). Be aware that it's very hard to get accurate measurements on low currents (idle currents in the uA range) using this technique. You would have to increase the resistor to about 10-20 Ohm, but then you would get too high voltage drop during the radio current spikes. So I think it's better to use an amperemeter for lower currents. Or a power analyzer.