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I am using a EFM32GG990 microprocessor and I am trying to get it to read a voltage using it's on-board ADC. I have made a circuit that I wish it to measure, both the loaded and open circuit voltage for the battery and the readings I should be getting are around 2.5-3V or somewhere around there, but when I actually set the chip up with the circuit it reads the voltage to be around 1.6V open-circuit and 0.9V loaded which is well of the mark.

I am not entirely sure whether it is a software problem or a hardware problem but I have assumed hardware for now as when measured with a volt meter aswell while it is set up, it also reads a much lower voltage than expected.


simulate this circuit – Schematic created using CircuitLab

This is the circuit diagram of what I am doing and as you can see there a three different loads, one is always on and the other two are triggered intermittently depending on whatever I choose it to be. I personally can't see any issues with this but if anyone does and could help me out, that would be great!

EDIT: Something interesting is that it all works with use of a power supply instead of a CR123A battery...

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Where's the ADC reference voltage at? Is it the same as Vdd? – Lundin Aug 11 '14 at 10:11
up vote 2 down vote accepted

According to your circuit, the 3v supply is connected directly to your ADC - forget about all the transistors and loads - 3V and 0V goes to the ADC.

Then, according to your measurements, the 3v supply is "much lower voltage than expected". So, you have the ADC telling you the reading is low and the meter telling you the same - what on earth makes you think the voltage is 3v?

I reckon the CR123A battery is "drooping" under load conditions.

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If there is just raw battery voltage then indeed there will be no stable 3V. It isn't clear where the 3V is supposed to come from. Is there a step-up regulator somewhere on-board? – Lundin Aug 11 '14 at 10:14
@Lundin it's a CR123A 3V battery. – Andy aka Aug 11 '14 at 10:17
@Andyaka The 3V supply, V1, is coming from the battery and all the other supplies are coming from the EFM32GG990, I got that the voltage should be around 3V because that is what the battery supplies when not connected in this circuit. I am expecting the voltage to 'droop' under load conditions but not for both states. – MrPhooky Aug 11 '14 at 10:33

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