Operating at 8 Volt

Is there a workaround that allows me to have all the features of the Arc yet have a Mains of 8V?


You could use the ADC connector to power your device from sort of other power supply and then measure a shunt resistor with the Arc. There’s a section in the help under Connecting the Hardware that describes how to do this. It’s important that you measure the resistor you are using so you get accurate values from Arc. The accuracy of this method would depend on the resistor you choose.

Check out the ADC, Single Ended Analog/Digital Conversion pin ADC+ section in the Tech Spec for information on ranges and resolution.

Let us know if you need further assistance in this area.

I set up an ADC differential current test. Shunt resistor 50 milliohm. The resistor is on the return side (ground) as the external power supply is set for 7V. ARC main is off. See link for PDF sketch

Even with Aground connection to the resistor ground side, the noise masks all readings in the uA range.

Running the same setup but powered by the ARC at 4.55V works beautifully. But I need to measure currents in real life condition of 6-7.5V battery.

With a 50mOhm shunt the step size will be 49uA (for a maximum reading of +/-1.62A (+/-81mV)). If you can increase the resistance of the shunt resistor (and update the resistor setting in Project Settings accordingly) the noise level will decrease, but that will only work of you don’t need to measure the full 1.62A.

And yes, the ADC will read out independent of the main output on/off state.

It seems that the ARC is defective. Without any wires connected to the unit, the screen is full of 60 Hz signal from rail to rail. See project file here:

Do I contact DigiKey for a replacement?

Notice that in the Main all is well; only when I look at the ADC current all hell breaks loose, even without wires connected to the expansion header.

I’m not really sure if we’re looking at the same thing here. The recording looks like I would expect it to given the selected 50mOhm shunt resistor (screenshot below). The ADC step size will be 49uA as mentioned above. Leaving the inputs floating will result in slightly higher noise levels than if they were connected to a sense resistor.

Note that the noise isn’t rail to rail, the lowest value is -400uA, the highest one is 200uA. The graph will normally auto-size to fit the highest and lowest leaks present, which in this case is the noise:

Given the 50mOhm resistor that was configured we have a full-scale readout of 1.62A. If we zoom out (using the ALT+scrollwheel action) to cover this full range, the screenshot will look like this:

As far as I can see from the recording the unit performs as specified. Is it possible for you to use a higher resistance shunt resistor and still manage to keep the voltage drop in the +/-81mV range of the input?

Werner – problem solved. With the ground connected to the Aground and the shunt connected to the ADC+ and ADC-, I am getting data that is acceptable, as far as baseline noise.

Thanks for the fast help.

Excellent, thanks for letting us know!