Understanding ADCs and Using the ADS1115 with Raspberry Pi

Learn how analog signals become digital data, and how to connect the ADS1115 ADC to a Raspberry Pi for high-precision sensor measurements.

Introduction

One of the first surprises many Raspberry Pi users encounter is that the Pi has no built-in Analog-to-Digital Converter (ADC).

This means that while the Raspberry Pi excels at digital communication, it cannot directly read analog sensors such as:

Potentiometers
Light sensors (LDRs)
Gas sensors
Analog temperature sensors
Soil moisture sensors

To bridge this gap, we use an external ADC such as the ADS1115, a high-precision 16-bit ADC that communicates with the Raspberry Pi over I²C. The ADS1115 provides four analog input channels, programmable gain amplification, and up to 860 samples per second. (Texas Instruments)

What is an ADC?

An Analog-to-Digital Converter (ADC) converts a continuously varying analog voltage into a digital number that software can process.

Consider a potentiometer:

Potentiometer
     │
     ▼
  0V ───────► 3.3V

The output voltage changes smoothly as you rotate the knob.

A computer, however, understands only numbers.

The ADC measures the voltage and converts it into a digital value.

Analog Voltage
      │
      ▼
+------------+
|    ADC     |
+------------+
      │
      ▼
Digital Value

The Principle Behind ADCs

ADC conversion involves three major stages.

1. Sampling

The ADC captures the voltage at a specific moment in time.

Voltage

5V |      /\
   |     /  \
   |    /    \
0V +---+------+----> Time
      ↑
   Sample

The sampled voltage is then processed.

2. Quantization

The ADC divides the voltage range into discrete levels.

For example, a 3-bit ADC can represent:

2³ = 8 levels

0V ───────────── 8V
│ │ │ │ │ │ │ │
0 1 2 3 4 5 6 7

Any input voltage is rounded to the nearest level.

3. Encoding

The selected level is represented in binary form.

Example:

Voltage	Level	Binary
5.3V	5	101

The ADC outputs the binary value to the processor.

ADC Resolution Explained

Resolution determines how many unique values an ADC can produce.

Formula:

Levels = 2^N

where N is the number of bits.

Resolution	Levels
8-bit	256
10-bit	1,024
12-bit	4,096
16-bit	65,536

Higher resolution means greater measurement precision.

Why the ADS1115 is Popular

The ADS1115 is a 16-bit delta-sigma ADC featuring:

16-bit resolution
Four analog input channels
I²C communication
Programmable Gain Amplifier (PGA)
Differential measurements
Up to 860 samples/sec
Supply voltage from 2.0V to 5.5V (Texas Instruments)

ADS1115 Module

Why Raspberry Pi Needs the ADS1115

Unlike many microcontrollers, the Raspberry Pi does not include ADC hardware.

Without an ADC:

# This will not work on Raspberry Pi
analogRead(sensor)

With the ADS1115:

Sensor
  │
  ▼
ADS1115
  │
  ▼
I²C Bus
  │
  ▼
Raspberry Pi

The Pi reads digital values from the ADS1115 through I²C. (Adafruit Learning System)

Hardware Connections

Connect the ADS1115 to the Raspberry Pi as follows:

ADS1115	Raspberry Pi
VDD	3.3V
GND	GND
SDA	GPIO2 (SDA)
SCL	GPIO3 (SCL)

Wiring Diagram

Enable I²C on Raspberry Pi

Open Raspberry Pi configuration:

sudo raspi-config

Navigate to:

Interface Options
 └── I2C
     └── Enable

Reboot:

sudo reboot

Verify the ADS1115 is detected:

sudo apt install -y i2c-tools

i2cdetect -y 1

Expected output:

The default I²C address of the ADS1115 is 0x48. (Instructables)

Installing Python Libraries

Install the required packages:

sudo apt update

pip install adafruit-circuitpython-ads1x15

The Adafruit ADS1x15 library provides Python support for both ADS1015 and ADS1115 devices. (CircuitPython Docs)

Reading Analog Voltages

Create a file:

nano ads1115_test.py

Paste:

import board
import busio
import adafruit_ads1x15.ads1115 as ADS

from adafruit_ads1x15.analog_in import AnalogIn

i2c = busio.I2C(board.SCL, board.SDA)

ads = ADS.ADS1115(i2c)

chan = AnalogIn(ads, ADS.P0)

while True:
    print(f"Voltage: {chan.voltage:.3f}V")

Run:

python ads1115_test.py

Example output:

Voltage: 1.245V
Voltage: 1.251V
Voltage: 1.260V

Reading a Potentiometer

Connect:

3.3V ─────┐
          │
      Potentiometer
          │
A0 ◄──────┘
          │
GND ──────┘

Code:

from adafruit_ads1x15.analog_in import AnalogIn

pot = AnalogIn(ads, ADS.P0)

while True:
    print(pot.value)

You should see values changing as the knob rotates.

Measuring Multiple Sensors

The ADS1115 provides four channels:

A0
A1
A2
A3

Example:

ch0 = AnalogIn(ads, ADS.P0)
ch1 = AnalogIn(ads, ADS.P1)

print(ch0.voltage)
print(ch1.voltage)

This allows one Raspberry Pi to monitor multiple analog sensors simultaneously.

Differential Measurements

The ADS1115 can measure the difference between two voltages.

Example:

diff = AnalogIn(ads, ADS.P0, ADS.P1)

print(diff.voltage)

Useful for:

Current sensing
Wheatstone bridges
Precision instrumentation

The ADS1115 supports both single-ended and differential input configurations. (Adafruit)

Understanding the Programmable Gain Amplifier (PGA)

One of the most powerful ADS1115 features is its built-in PGA.

Suppose your sensor outputs:

0V - 0.2V

Using the PGA:

ads.gain = 16

The signal is amplified internally, allowing much higher measurement precision. (Adafruit Learning System)

Real-World Applications

The ADS1115 is commonly used in:

Smart agriculture
Environmental monitoring
Battery management systems
Industrial automation
Embedded AI projects
Raspberry Pi sensor networks

Example workflow:

Temperature Sensor
       │
       ▼
    ADS1115
       │
       ▼
 Raspberry Pi
       │
       ▼
 Machine Learning Model
       │
       ▼
 Prediction

Common Troubleshooting

Device Not Found

Check wiring:

i2cdetect -y 1

If 0x48 is missing:

Verify SDA connection
Verify SCL connection
Verify power supply

Permission Errors

Add your user to the required groups:

sudo usermod -aG gpio,i2c $USER

Logout and login again.

Incorrect Voltage Readings

Verify:

Sensor voltage range
ADS1115 gain setting
Ground connections

Final Thoughts

The ADS1115 is one of the most useful expansion modules for Raspberry Pi projects. It solves the Pi's lack of analog inputs while providing significantly higher precision than many built-in microcontroller ADCs.

By understanding how ADCs work—through sampling, quantization, and encoding—you gain a deeper appreciation of how physical sensor data becomes digital information that software, machine learning models, and intelligent systems can use.

If you're building IoT devices, sensor networks, robotics systems, or Embedded AI projects, the ADS1115 is an excellent addition to your toolkit.

References

Happy building! 🚀

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