Difficulty determining current consumption with oscilloscope

[ericoneill]

Hi, I'm currently trying to determine the power consumption of my nrf51DK during different states of my program using an oscilloscope. To do so, I am using an external power supply in series with a resistor on the external power supply pins, and then using an oscilloscope to measure the voltage across the resistor. Here is a picture of my setup:

Regardless of whether I turn on Bluetooth advertising or not or use the ADC or not, the voltage on the oscilloscope looks like the following picture

I am not sure why there is a periodic signal with a period of ~2ms, despite there being nothing in the code that executes at that frequency (That I set up, at least). Is there any issue of the way I am trying to measure power?

Any help would be greatly appreciated! Thanks

[Marco-Luis]

Hi, just two questions:

• What's is the value of the shunt resistor (should be a very low value)?
• Have you measured the Power Supply ripple without the nRF module?

In any case, I'm not sure if this suggestion can be useful to you, but I have read somewhere about placing two capacitors (10uF electrolytic and 0.1uF ceramic) near the nRF module can help smoothing the power ripple.

Just my two cents.

[awneil]

It looks like your series resistor is way too large!

[Stefan Birnir Sverrisson]

Hi

This is not the recommended way of measuring current consumption of the nRF51422 chip on the nRF51-DK board. Look at the method described on page 18 in the nRF51-DK user guide. When you measure current consumption on the external supply pins, you are measuring current consumption of the Segger chip as well. If you measure on the "Current measurement" pins, then you are measuring on the nRF51422 only.

However, there is a problem on the nRF51-DK board when measuring current consumption, at least on v1.0.0 boards, that there appears a current spike every 1 ms or 2 ms, when the board is powered with USB, and perhaps also when powered by external supply. However, it should not be present if you power the the board from battery.

[awneil]

Note that the cited document says, "The resistor should not be larger than 10 Ω".

Note also that it recommends using differential mode - remember that the scope's "earth" clip does, in fact, connect to ground!

Probably best, also, to try to avoid x10 probes - you don't want to attenuate the (already small) signal, and you don't need high impedance.

[ericoneill]

Thanks for the simple solution, powering from the battery and using the guide worked!