Sara-N2 power measurement


#1

Hello,

We have got Otii Arc to measure the currrent in u-blox Sara-N2 but we could it set it up. I have seen Otii demo on Sara-N4 on the net : https://www.qoitech.com/blog/otii-a-part-of-energy-efficient-iot-demo-at-ericsson-garage but still it is not detailed. Could you please show me how to measure current in Sara-N2? Also, what is the maximum voltage and current if we connect Otii Arc with external adopter rather than USB?

BR. Anas


#2

Hi Anas,

Do you have the Sara-N2 module or is it a development board of some kind?

Generally, you can measure current by using Otii Arc ADC+ and ADC -, you then need to have an in-line sense resistor on the power rail. Connect the ADC+ to the high side of the resistor and ADC- to the low side and AGND to your IoT device GND. The size of the resistor depends on your system peak current. The voltage over your resistor should be between -81.9175 mV and 81.2 mV.

If you have the EVK-N2 evaluation kit, then you can use J103 jumper with a suitable sense resistor and connect to ADC+ and ADC-, together with AGND on Otii Arc expansion port.

By reading the user guide for EVK-N2 it seems like you can power up the evaluation board by connecting J103 (one of the pins, can’t see which one in the user guide) to Otii Arc red main output and the evaluation board GND to Otii Arc black main, and use 3.8V output voltage. But, since I do not have the board here, nor the schematic, it is very hard to give you exact instructions. If you have schematic I can help you further.

Regarding maximum voltage and current, if you have an external DC adaptor, then maximum voltage is 5V and maximum current is 5A peak and 2.5A continuously.

Best regards,
Björn


#3

Hello Bjorn,

I have EVK-N2 evaluation kit and I use J103 jumper to measure current with UT61D multimeter. I think it is suitable to use ADC+ and ADC- together with AGND. However, I do not know what is the peak current for SARA-N2. If we assume that 250-350 mA as the peak current. What is the suitable sense resistor? I think it is similar to SARA-R4.

BR. Anas


#4

Hi Anas,

The ADC shunt input has a range of just over +/- 81mV, so a 0.2 Ohm shunt will give you a maximum reading of 0.081 / 0.2 = 405mA. A 0.25 Ohm shunt will give you a maximum reading of +/- 324mA.

/wj


#5

I have contacted u-blox about the SARA-N2 peak current and I have been told up to 500mA. So I used 0.15 Ohm shunt resistor which worked Ok but still the current goes to some uA negative values. I have tried calibration but did not help much. We need to measure current with uA precision.


#6

Using a 0.15Ω shunt will result in ~16µA resolution on the expansion ADC input. When you need to measure current with µA precision but still deal with 500mA+ peaks I would suggest you power the N2 module from the main terminals of the Otii Arc instead. The dynamic range of the main terminal current measurement is more than four orders of magnitude better than the ADC input. Set the output voltage of the Arc to match the N2 module supply voltage (the voltage present on the other, now unconnected, side of the J103 jumper), enable recording of the main current channel (with auto-range selected) and you should be good to go.

This is the way we have connected some other devkits in the past (for example the U-blox Zoe).


#7

Connecting side of the J103 to Arc main red main and N2 GND to Arc main black will measure also the power consumed by N2 board. I just want to measure power consumption of SARA-N2 module.


#8

05%20PM
According to the EVK-N2 User Guide, by removing J103 you will only power the SARA-N2 VCC if you connect power to the “right” pin. From the documentation it’s not obvious how this jumper is laid out on the board (as in which physical pin corresponds to the right one in the picture above - this is the one to connect Arc Main + to). Arc Main - connects to system ground.