# How to solve ADC measurement issue?

For a project, I am using the following circuit to detect the load3 change. With an 16-bit coulomb counter ADC I am measuring the current I. Using onboard Arduino Uno ADC I am measuring votage, V at node A.

With the equation R = (OCV - V) / I; where OCV is the open circuit voltage at no load condition, R = 10 mΩ + Load3 + Source (3.7 V) internal resistance if it is a battery, I am measuring the change of load3 in the figure. By changing load3 I mean initially there is no load3 (shorted-indicated by blue line) and then I introduce a 10 Ω load3. But the problem is when there is no load3 the code reports it almost correctly (R = 14 mΩ) but with the 10 Ω load3 I found it reports only 80 mΩ instead of 10 Ω.

My questions are:

1. Why is the measurement mismatch so huge? If it is because of the ADC (both V and I measurements are from coulomb counter and Arduino ADC), how can I address that it is for the ADCs?
2. What are the possible ways to improve the measurements accuracy?

Thanks!

• You discuss 'R' but there is no 'R' in your schematic (which is, let's say, a little untidy). I've read your question three times and I'm still not sure what you're trying to do. Commented Sep 26, 2020 at 18:48
• @Transistor sorry the figure is messy! R= load3+10mohm+ source (3.7V) internal resistance (if it is a battery!). I'm editing the post also. Thanks!
– Shu
Commented Sep 26, 2020 at 18:54
• Coulomb counter will often have a long average. You will need to measure voltage during PWM ON/OFF times and account for PWM duty cycle. Commented Sep 26, 2020 at 18:57
• You can add a schematic in using the CircuitLab button on the editor toolbar. Double-click a component to edit its properties. 'R' = rotate, 'H' = horizontal flip. 'V' = vertical flip. Note that when you use the CircuitLab button on the editor toolbar and "Save and Insert" on the editor an editable schematic is saved in your post. That makes it easy for us to copy and edit in our answers. You don't need a CircuitLab account, no screengrabs, no image uploads, no background grid. Commented Sep 26, 2020 at 18:58
• @Shu My recommendation is that you take Transistor's suggestion right now and use the schematic editor as step 1 in clarifying your question here. Don't wait until "next time." That should be followed by a discussion you provide us about your approach and why you think it should produce what you think it produces. Pay special attention to describing node A as you see it operating under your PWM scenario. By the time you get done with those two things, I suspect you'll be a lot closer to your own answers. And so will we.
– jonk
Commented Sep 26, 2020 at 21:11