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I noticed that my Arduino Mega performs differently in how it presents values from say analog input A0, depending on whether it is being powered by USB or by a fresh 9V battery wired and connected through the barrel connector.

There must be some sort of difference to what happens when both USB and the barrel connector are employed vs. just USB being used for power.

To be specific, when running just on USB power, the analog inputs seem to flicker more allowing for oversampling and measurements beyond the 10 bit accuracy of the DAC. In fact with analogReference(DEFAULT) it is possible with 64x oversampling to see 100uV level changes when only USB power is employed. Note that these changes are not just noise, they actually correspond to 100uV level changes in the input connected to A0 with high correlation.

As soon as I connect the barrel connector, the picture changes significantly. Values coming out of the ADC seem to pretty much snap and there is far less flicker. The ADC becomes, well, discrete and you get your 1,024 levels and that's about it. There may be a tiny bit of flicker as you increase or decrease voltage and approach the next increment, but this is not significant enough to actually increase the ADC resolution reliably.

Even going so far as setting the analogReference to EXTERNAL while the barrel connector is employed and connecting the AREF pin to the 5v connector directly, which by the way reads at 5.004 V (unlike the 4.992 V it reads when USB is powering the device) still reproduces the problem.

Summary: Effectively, when running on USB power only I can oversample the analog inputs and get significantly more than 10-bit resolution. While when running using the barrel connector, with the USB still attached, I can only get the 10-bit resolution natively provided by the ADC.

If anyone has any ideas on what is causing this difference in behavior, I am all ears. If I failed to explain anything or perhaps created some confusion, I would be more than happy to clarify.

Note: This is an authentic Arduino Mega board.

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Looking at the schematic the barrel plug has a couple of linear voltage regulators (top left of schematic), while the USB input just has a couple of MOSFETs (bottom right of schematic) which are basically just there to switch off the USB power if there is something plugged into the barrel jack (and also maybe provide a little input protection). So the Arduino is relying on the USB power to be pre-regulated. For the most part this is correct, but USB voltage can vary widely depending on the cable length (voltage drop), the power source it is connected to (no two computers or wall warts are the same), and how many devices are the same hub (if connected to a computer or hub). I would not rely on the two power sources being matched at all.

The flicker you are talking about sounds like the power coming from USB could have significant ripple. I doubt that the USB power is allowing you to "oversample". The way ADCs in AVRs work is they have a reference voltage and when sampling, will give you a reading of where the voltage is between 0 and the voltage reference. With a 10-bit ADC there are ~1000 steps, so to measure 100uV changes, you would have to have a 100 mV reference voltage, which according to the datasheet (p.379), the minimum is 1.0 V. Basically, it is physically impossible for the Arduino Mega to measure those kinds of changes.

What is more likely to be happening is that you are probably getting some weird aliasing between the ripple frequency of your USB voltage and the sampling frequency of your code. The best way to debug this kind of thing would be to pull out an oscilloscope and check if and how much ripple is coming off the USB power.

If accuracy is important to your project, and being able to accept both power inputs is a requirement, you might look into maybe getting your own external voltage reference and piping it into VREF. Otherwise, I would stick to only using the barrel plug as it is regulated and will be consistent.

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  • Or use one of the internal voltage references (1.1V or 2.56V) in the ATmega1280. – Gerben Aug 9 '15 at 8:52
  • @Gerben Yeah I thought of that, the asker didn't make it clear what types of sensors he was using and what village ranges he was looking at, but vaguely implied it was 5 volts. – Jake C Aug 9 '15 at 14:40
  • @Jake, Analog Sensor A0, voltage range required = 0 - 5 V. analog reference DEFAULT(or EXTERNAL tied to 5V), same result. – Michael Goldshteyn Aug 10 '15 at 1:18

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