ESP8266 Analog to Digital Converter – How accurate?

2821-01

I was having a difficult time understanding how to interpret the results of the HUZZAH Feather ESP8266 ADC. I wanted to have a reliable way to measure the level/voltage of the battery attached to the feather. The popular guidance was to “keep the adc input voltage to no more than 1 volt,” which would lead you to believe that the reference voltage for this ADC is 1 volt, but I couldn’t get the results to match. I found some discussions that said the reference is 0.96 volts, which given my test, is close.

1083-00So I put an ADS1015 ADC on a proto board and connected it with I2C, and set up a test to read the ADS1015 and the ESP8266 ADC and send the results via wifi to a mysql database running on the raspberry pi.

The test code, csv file of results, and related files are posted at https://github.com/Maddox-zephyr/ESP8266_adc_test

Battery Monitor – Test Conditions:

Adafruit HUZZAH Feather ESP8266, fitted with a 500 mah lithium-ion battery and an Adafruit ADS1015 ADC breakout board

ADS1015 is sampled (column B)

Arduino code samples the ESP8266 10 times and provides an averaged result (column D)

ESP8266 sends the data to a Raspberry Pi2 that is running mysql and then goes to sleep for 5 minutes.  ESP.deepSleep(SLEEP_TIME * 1000000, WAKE_RF_DEFAULT);

Voltage Divider: 100K + 20K resistors (1%) measured values. 100K resistor is tied to the battery output and then connected to the 20K resistor and the other side of 20K resister tied to ground

Both ESP8266 and ADS1015 were measuring the voltage across the 20K resistor and ground

ADS1015 was set for a gain of four -> 1 bit = 0.5mV

Column C (ADS1015_v) is therefore the value in column B (ADS1015-raw) * 0.5 mV * 6 (six is the voltage divider ratio)

Column D (ESP8266_ADC) is the value returned by the 8266 adc.

Column E (ESP8266_v) is the column D value multiplied by (0.96v/1024), which is the 1 bit value, and then multiplied by 6 (voltage divider) Using 0.96v as the reference voltage for the adc.

The php file that I used on the raspberry pi is included. It needs to be in the /var/www/html directory of the pi after you set up apache. If you want to learn more about setting up mysqyl on the pi, I recommend you check out this ebook “Raspberry Pi: Measure, Record, Explore.” by Malcolm Maclean at http://leanpub.com/RPiMRE

Observations:

ESP8266 adc values are what they are. Question is, how do these readings map to the ads1015 readings?

Column G is the percentage error that the ESP9266 differs from the ADS1015 (scaled by the voltage divider ratio).

Error is between 0 % and 2%

The battery ran the ESP8266 for 2.5 days for this test. As documented by Adafruit, the Lithium-Ion batteries shut off at 3 volts, hence the data ends at 3 volts :-) It took about four hours to recharge the battery with the HUZZAH Feather ESP8266 plugged into
a USB port on the computer (Feather charges at 100 ma).

Here is an excerpt of the excel/csv file of results:

Table-Snapshot

 

Leave a Reply

Your email address will not be published. Required fields are marked *

You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <strike> <strong>