ADS1115 4-Channel 16-Bit ADC Module

Precision 4-channel 16-bit Analog-to-Digital converter with I2C interface.

DESCRIPTION

The ADS1115 is a precision 4-channel 16-bit Analog-to-Digital converter with I2C interface that greatly improves the measurement accuracy over the built-in ADC of Arduino and similar uCs.

PACKAGE INCLUDES:

• ADS1115 4-Channel, 16-Bit ADC Module
• Male header strip

KEY FEATURES OF ADS1115 4-CHANNEL 16-BIT ADC MODULE:

• 16-bit analog-to-digital conversion
• 4 input channels
• Input channels can be configured as 4 single-ended, 2 differential or  use in a comparator mode
• Single-shot or continuous conversion modes with conversion rates up to 860 samples per second
• Built-in programmable gain amplifier can boost small signals up to 16x
• 0.1875mV to 7.8125uV step resolution
• Built-in oscillator and reference voltage for high stability.
• 3.3V and 5V logic compatible.

Microcontrollers usually have built-in ADC capability, but it is generally limited to no more than 10-bit resolution.  That resolution is fine for monitoring basic analog voltages such as reading a potentiometer setting, but they fall far short for more demanding applications such as calculating current flow by measuring the small voltage drop across a shunt resistor or measuring the output of a high precision analog temperature sensor.

With its 16-bit resolution and programmable measurement ranges, it can measure voltage in resolution steps from 0.1875mV when measuring a 5V range to as small as 7.8125uV when measuring a 0.256V range.

These modules use the I2C interface and include the power conditioning and pull-up/pull-down resistors needed to basically make it plug-and-play to get up and running doing basic A-D conversions very easily, but they are also very flexible and powerful devices that have a wealth of features that can be explored as interests warrant.

I2C Interface

The module has an easy to use I2C interface that can be configured to use any one of four different I2C addresses if you want to use multiple modules in the same system or if you run into an address conflict with another device..

The ADDR address pin determines which I2C address to use by connecting it as follows:

• 0x48 = ADDR connected to Ground (Default)
• 0x49 = ADDR connected to VDD
• 0x4A = ADDR connected to SDA
• 0x4B = ADDR connected to SCL

The module includes a 10K pull-down resistor from ADDR to ground, so the module will default to 0x48 if no connection is made.

Input Modes

The 4 inputs can be configured to operate in 3 different modes.

Single-Ended Mode

In single-ended mode, the 4 inputs (A0-A3) can each be used as separate inputs with each referenced to ground.  This mode provides more inputs, but have less electrical noise rejection and provides 15-bits of resolution.

Differential Mode

In differential mode of operation, the inputs act in pairs to measure the difference in voltage between the two inputs.  The channel pairing is A0 – A1 and A2 – A3.  This mode provides differential noise immunity which may be important in some critical measurement situations when very small signals are being measured or there is a lot of electromagnetic noise pickup.

Since the measurement is relative to each other, the full 16-bits of resolution are available.

You will sometimes see it stated that the differential mode allows you to measure negative voltages, but this is somewhat misleading.  No inputs on this device can be negative relative to ground or the device will be damaged.  What it means is that one of the differential inputs can be negative relative to the other differential input, but they will both be positive voltages relative to ground.  See the section below about measuring negative voltages for more info on this.

Comparator Mode

In comparator mode, the input is constantly measured and compared against a value that has be set in the device.  When the signal exceeds the predefined value, the ALRT pin is activated LOW.  This pin can be polled by a MCU or it can be used to drive an interrupt.  The board includes a 10K pull-up on this line.

Measurement Range

The ADS1115 is a 16-bit device, but in single-ended mode the output is a signed integer and one of the bits is used for the sign so in practice it is providing 15-bits of resolution which means there are 32,767 steps of resolution.

The maximum voltage that can be measured and thus the size of the steps of resolution is controlled by the Programmable Gain Amplifier (PGA).  The PGA has 6 full-scale ranges that can be set as shown in the table below.

The default setting of the PGA is set for the maximum range of 6.144V.  This provides a resolution step size of 6.144V / 32767 steps = 187.5uV.  This is a much smaller step size than the standard Arduino ADC which provides about 4.88mV per step.

Keep in mind that the absolute maximum voltage that can be measured is VDD + 0.3V.  If the device is operating at 5V, the maximum voltage that can be safely measured is 5.3V.  The range above that that is basically wasted.  If higher voltages then the maximum are input, the device can be damaged.

If the voltages to be measured are smaller and it is desired to optimize the resolution of the reading, the PGA can be programmed to have a full range down to as small as 0.256V which gives a resolution of 7.8125uV per step.

Important Note:  If using the device to measure the voltage across a shunt resistor using differential mode, ensure that the 5.5V maximum voltage of the ADS1115 device is not being exceeded.  While the voltage being measured across the shunt may be very small, if the shunt is on the high side of the load in a 12V circuit for instance, the voltage relative to ground will far exceed the rating of the device.  Typically a shunt must be on the low side of a load for this device to be used.

Conversion Modes

Continuous Conversion Mode:

The module normally operates in continuous conversion mode.   In this mode, the ADS1115 is constantly performing conversions.  If a read is made, the last conversion that was completed will be reported.

Single-Shot Mode

The device draws little power, but if you are looking for minimum power draw, the ADS1115 can be put into single-shot mode.  In this mode, the conversion is only run when requested and then the device goes back into a low power mode.  The Alert/Ready pin can optionally be used to determine when the conversion has been completed.

Measuring Negative Voltages

A common point of confusion when using this device is that the spec sheet gives the full scale range in both positive and negative numbers such as ±6.144.  This makes it easy to think that the device can measure both positive and negative voltages relative to ground, but that is not the case.  Any negative voltage relative to ground > -0.3V will damage the device.

The ± is only applicable to the differential mode of operation.  With the A0/A1 differential pair for instance, the A0 input is considered the positive input and A1 is the negative input.  Both inputs must have positive voltages applied to them, but if the voltage on A0 is larger than A1, the difference between them is reported as a positive number while if A1 is larger than A0, the result is reported as a negative number.

For example, if you connect a battery across the two inputs with the positive lead connected to A0 and negative lead to A1, the voltage will be reported as +1.5V.  If you then reverse the battery, the voltage will be reported as -1.5V.  Similarly if A0 is connected to +2V and A1 is connected to +3V, the measurement will be -1V.

If you application requires the measurement of voltages that are negative relative to ground, you will need to provide an offset to make it a positive voltage for measurement.

Module Assembly

This module comes with the header loose.  This allows you to configure the module to meet your particular requirements such as which side of  the board you want the header on or if you want to solder on wires to make the connections.  For use with breadboards, we put the headers on the bottom so that the module can plug directly into the breadboard.

Module Connections

The connections to the module are straight forward.

1. Supply 3.3 or 5V power and ground
2. Connect I2C SCL and SDA lines to same on the MCU.  The ADDR and ALRT lines can be ignored for now.
3. Connect the voltage to be measured to the analog input(s).  Be sure to observe the requirement not to exceed the VDD voltage on the analog inputs.

1 x 10 Header

• VDD  = VDD (2.0V – 5.5V) typically connects to MCU 3.3V or 5V
• GND =  Ground connects to MCU ground
• SCL =   I2C SCL connects to MCU I2C SCL
• SDA =  I2C SDA connects to MCU I2C SDA
• ADDR = I2C Address.  Default if left unconnected is 0x48.
• ALRT= Alert / Ready.  Alert output for comparator or Ready output for conversion complete in single-shot mode
• A0 = Analog input 0
• A1 = Analog input 1
• A2 = Analog input 2
• A3 = Analog input 3

OUR EVALUATION RESULTS:

These modules have good build quality and are extremely useful in for making precision analog measurements.

Getting up and running with a basic setup is easy with these modules using the Adafruit ADS1X15 library as shown in the example below.  The library can be easily installed from within the IDE using the Library Manager.

If you are interested in some of the more advanced features, checkout both the datasheet below as well as the example files that are installed when the library is installed.

This test setup just requires the following connections.

• Vdd – Connect to 3.3V or 5V to match the MCU
• Gnd – Connects to ground on the MCU
• SCL – Connects to I2C SCL on MCU
• SDA – Connects to I2C SDA on MCU
• ADDR – leave disconnected for default address of 0x48
• A0-A3 – Connect to 3.3V power on MCU or other known voltage < 5V

An example of the program output is shown below.  The 3.3V is attached to the AIN0 input.   The small amount of variation in the readings could be from the ripple on the power supply or from noise pickup on our breadboard jumper wires.

ADS1115 4-Channel 16-Bit ADC Module Test Program

/*  ADS1115 4-Ch 16-bit ADC Test Program
 
    Basic code to test the ADS1115 ADC Module.  Takes a single-ended reading
    on module channel A0.  Uses default measurement range of of 6.144V 

    Module connections:
      VDD = 5V
      Gnd = Ground
      SCL = SCL / A5 on MCU
      SDA = SDA / A4 on MCU
      ADDR = Board has pull-down to ground (sets I2C address of 0x48)
      A0-A3 = 3.3V on MCU.  Voltage to be measured. Can be any voltage < 5V

    Uses Adafruit ADS1X15 library which can be downloaded via IDE     

    For 16-bit vs 12-bit verification, connect input to ground and verify the 
    minimum reading is less than 3mV.  Readings should change in approximately
    200uV (0.0002V) steps
*/
#include <Adafruit_ADS1X15.h>

Adafruit_ADS1115 ads;  
//===============================================================================
//  Initialization
//===============================================================================
void setup(void)
{
  Serial.begin(9600);

  Serial.println("Read single-ended readings from AIN0..3");
  Serial.println("ADC Range: +/- 6.144V (1 bit = 0.1875mV");

  if (!ads.begin()) {
    Serial.println("Failed to initialize ADS.");
    while (1);
  }
}
//===============================================================================
//  Main
//===============================================================================
void loop(void)
{
  int16_t adc0, adc1, adc2, adc3;
  float volts0, volts1, volts2, volts3, volts;

  adc0 = ads.readADC_SingleEnded(0);
  adc1 = ads.readADC_SingleEnded(1);
  adc2 = ads.readADC_SingleEnded(2);
  adc3 = ads.readADC_SingleEnded(3);

  volts0 = ads.computeVolts(adc0);
  volts1 = ads.computeVolts(adc1);
  volts2 = ads.computeVolts(adc2);
  volts3 = ads.computeVolts(adc3);

  Serial.println("-----------------------------------------------------------");

  Serial.print("AIN0: ");Serial.print(volts0, 4); Serial.println("V");
  Serial.print("AIN1: ");Serial.print(volts1, 4); Serial.println("V");
  Serial.print("AIN2: ");Serial.print(volts2, 4); Serial.println("V");  
  Serial.print("AIN3: ");Serial.print(volts3, 4); Serial.println("V");
  Serial.println();
  delay(1000);
}

BEFORE THEY ARE SHIPPED, THESE MODULES ARE:

• 100% inspected and tested per incoming shipment
• Repackaged in high quality resealable antistatic bag

Notes: 

1. None

TECHNICAL SPECIFICATIONS