Does ADC input have impact to results measured with the main terminal input?


#1

Hi,

I’ve been measuring current consumption of nRF52840-DK with the Otii. I measured with both inputs, the main terminal (Arc Main Current) and the ADC input (Arch ADC Current) with 3.9 ohm shunt resistor at same time, i.e. I used both inputs to measure current of the whole board.

When both inputs were connected I got about 10uA lower figures with ADC compared to main input. This one I understand since the ADC has lower sampling rate and it might miss some current spikes compared to main input. Is this right conclusion?

But I also notice that the figures from the main input were about 8uA (avg) bigger when the ADC± inputs where connected compared to figures when ADC wires were removed (I would say that these were closer to true values). Any ideas what could explain this phenomenon?

Best Regards,
Ari


#2

Hi Ari,

The main measurement is much more accurate than the ADC measurement.
You are right in that the ADC current measurement has lower sampling rate that the main current measurement but this is not the difference you see.

To explain what you see:
There is some leakage into ADC+/- and 10uA sound reasonable. This leakage will give you a current measurement error for the main current measurement.
There might also be an offset error in the ADC measurement that can be positive or negative. This offset error has been improved recently in FW updates.

So, the best way for you to measure is to only use the main current measurement and leave ADC unconnected. This will give you the best accuracy.

Best regards,
Björn


#3

Thanks Björn for your quick response.

This case can be closed.

Regards,
Ari