ADS1115 sixteen-bit ADC

Four analog inputs and a sixteen-bit converter on a blue I2C breakout. Aliases: 16-bit ADC, analog to digital converter, ADS1015.

It is what to reach for when the board's own ten-bit analogRead is not fine enough, when you want the difference between two voltages rather than each against ground, or when the board has no analog inputs to spare.

Pins

Pin	What it does
VDD	Supply, 2 to 5.5 V.
GND	Ground.
SCL	I2C clock. The module carries the pull-ups.
SDA	I2C data.
ADDR	Address select: to GND (or left alone) is 0x48, to VDD is 0x49.
ALRT	Comparator or conversion-ready output, open drain: needs a pull-up.
A0 to A3	The four inputs.

The gain is a full scale, not an amplifier

A result is 32768 × V ÷ FS, where FS is the gain's full scale: ±6.144, ±4.096, ±2.048, ±1.024, ±0.512 or ±0.256 V. The gain decides what range the counts cover; it does not let an input go past the supply. An input measured against GND can only be positive, so a single-ended reading uses fifteen of the sixteen bits, and a differential one between two inputs uses all of them.

The library

Adafruit_ADS1X15.h works as its tutorial uses it:

#include <Adafruit_ADS1X15.h>

Adafruit_ADS1115 ads;

void setup() {
  Serial.begin(9600);
  ads.setGain(GAIN_ONE);              // +/-4.096 V, 125 uV a count
  if (!ads.begin()) { Serial.println("No ADS1115"); while (1); }
}

void loop() {
  int16_t counts = ads.readADC_SingleEnded(0);
  Serial.print(counts);
  Serial.print("  ");
  Serial.print(ads.computeVolts(counts));
  Serial.println(" V");
  delay(500);
}

computeVolts() turns counts into volts at the gain in use, and tutorials that multiply the counts by 0.1875 themselves work as written.

What the model gets right

The four registers (conversion, config, and the two thresholds) and their reset values, the multiplexer's eight settings, the six gains and the eight data rates.

A conversion takes time: one over the data rate, 7.8 ms at the default 128 samples a second and 125 ms at 8. In single-shot mode the OS bit reads 0 until it is done, and reading the conversion register before then gets the previous result, which is the commonest ADS1115 bug. Continuous mode refreshes the result every period.

The comparator on ALRT: traditional or window, active low or high, latching or not, and asserting after one, two or four readings past a threshold. Set the high threshold's top bit and clear the low one's and ALRT becomes the conversion-ready signal instead.

What it does not model

Noise: a steady input gives the same count every time. The input impedance and the anti-aliasing filter. ADDR tied to SDA or SCL for 0x4A and 0x4B: only 0x48 and 0x49 are modeled.

Common mistakes

Reading the conversion register straight after starting a single-shot conversion. Expecting a higher gain to amplify: GAIN_SIXTEEN makes the full scale 0.256 V, and a 1 V input just reads as the top of the scale. Leaving ALRT without a pull-up and seeing nothing on it.