A Tale of Two ADCs: ESP32 versus INA219

I started looking at Home Assistant and ESPHome because I realized I did not have enough enthusiasm to write my own sensor data gathering and processing framework. Learning how to put an ESPHome node to sleep to save power was one of many steps I had to retrace, but I’m finally ready to pick up where I left off with an INA219 DC voltage and current sensor breakout board.

I had been using an ESP32 with its integrated ADC to monitor the DC voltage output of my MPS500 battery. I started with an ESP32 because they received factory calibration by Espressif whereas the ESP8266 did not. Sadly that wasn’t as useful as I had hoped because the ADC was only capable of measuring up to 2.45 volts. The MPS500 battery voltage range is consistent with three lithium chemistry battery cells in series (“3S”) meaning up to 12.6V, requiring a voltage divider built with a few resistors. These were cheap resistors that were several percent off the nominal value, so I had to use my voltage meter to recalibrate anyway.

While off-nominal resistors would affect the accuracy of my readings, I had expected the precision to be pretty good if I followed Espressif recommendation on reducing ADC noise with use of multisampling plus a bypass capacitor. And indeed the results were perfectly sufficient for me to log the change in battery voltage over time.

But once I had an INA219 up and running, it took over voltage monitoring duty in addition to current (and power) monitoring. After just one day, I can see the task-specific ADC circuit in an INA219 significantly outperformed the general-purpose ESP32 ADC. This graph covers two days: the day before switchover, and the day after.

The green line on the left were voltage fluctuations recorded by ESP32 ADC, the yellow line on the right reflected the same usage pattern but recorded by INA219. There is a very drastic difference in noise fluctuations between the two graphs! The ESP32 ADC plot was a little jagged and perfectly fine for my purpose, but it was a real treat to see INA219 values tracing out a smooth curve with no visible noise. At least, at the scale of my graph. This improvement should help as I move on to the next step of my project.

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