LamaPLC: Texas Instruments ADCs: Delta-sigma multi-channel Analog Converters with SPI communication

Texas Instruments' ADCs are all delta-sigma converters that primarily differ in resolution, interface type, sample rate, and integrated features such as input channels and programmable gain amplifiers (PGAs). The ADS111x series is a 16-bit, I2C-compatible device, while the ADS12xx series is typically a 24-bit, SPI-compatible device with more advanced features for high-precision applications such as weighing scales and industrial process control.

The Texas Instruments ADS111x series is a set of 16-bit-precision ADCs designed for low-power, space-constrained sensor measurement tasks. Key features include a broad supply-voltage range (2.0V to 5.5V), low current consumption (about 150µA in continuous operation), and built-in components such as a low-drift voltage reference and an oscillator.

ADS111x General Specifications

• Resolution: 16 bits
• Supply Voltage Range: 2.0 V to 5.5 V
• Low Current Consumption: Typically 150 µA in continuous-conversion mode
• Internal Components: All devices include a low-drift voltage reference and an internal oscillator.
• Operating Temperature Range: -40°C to +125°C

Key Pin Notes

• DOUT/DRDY: This pin serves a dual purpose. It provides serial data output and serves as a data-ready indicator, pulling low when a new conversion result is ready. An internal pull-up resistor can be enabled on this pin via software.
• Analog Inputs (AIN0-AIN3): These pins can be configured as four single-ended inputs (measured against GND) or two differential pairs (AIN0-AIN1 and AIN2-AIN3).
• Input Limits: Voltages on any analog input pin must remain between GND - 0.3V and VDD + 0.3V to prevent damage to the internal ESD diodes.

To interface the ADS1118 16-bit ADC with an Arduino, the most common approach is to use the ADS1118 library by denkitronik.

Wiring Diagram

The ADS1118 communicates via SPI. Connect it to your Arduino as follows:

Example Arduino Code

This basic example initializes the ADC and reads the voltage from input AIN0 and the internal temperature sensor.

#include <SPI.h>
#include <ADS1118.h>
 
// Define the Chip Select (CS) pin
#define CS_PIN 10
 
ADS1118 ads1118(CS_PIN);
 
void setup() {
  Serial.begin(115200);
 
  // Initialize ADS1118 with default settings
  ads1118.begin(); 
 
  // Optional: Set sampling rate (e.g., 128 SPS)
  ads1118.setSamplingRate(ads1118.RATE_128SPS);
}
 
void loop() {
  // Read voltage from Single-Ended Input 0 (AIN0)
  // Returns value in millivolts (double)
  double voltage = ads1118.getMilliVolts(ads1118.AIN_0);
 
  // Read internal chip temperature
  double temp = ads1118.getTemperature();
 
  Serial.print("Voltage AIN0: ");
  Serial.print(voltage);
  Serial.print(" mV | ");
 
  Serial.print("Internal Temp: ");
  Serial.print(temp);
  Serial.println(" C");
 
  delay(1000); 
}

Key Library Methods

• ads1118.begin(): Initializes the SPI communication.
• getMilliVolts(input): Takes the ADC reading and converts it directly to mV. Inputs can be single-ended (AIN_0 to AIN_3) or differential (DIFF_0_1, DIFF_2_3, etc.).
• getTemperature(): Retrieves the temperature from the ADS1118's internal sensor.
• setSamplingRate(): Adjusts speed from 8 SPS up to 860 SPS.

The Texas Instruments ADS12xx series includes high-resolution, 24-bit delta-sigma ADCs designed mainly for precise measurement tasks such as weigh scales, strain gauges, and industrial process control. They come with built-in features such as onboard PGAs, internal references, and temperature sensors.

ADS12xx Series Specifications

Key Takeaways

• The ADS1232 and ADS1234 are unique in that they are controlled via simple pin-driven commands, eliminating the need for complex register programming. They are specifically designed as a complete front-end solution for bridge sensors, such as weigh scales.
• The ADS1256 offers a significantly higher maximum data rate of 30 kSPS compared to the 80 SPS of the ADS123x family, making it suitable for faster data acquisition while still maintaining high resolution.
• The ADS1220, ADS1261, and ADS1263 include highly integrated features such as internal temperature sensors and programmable current sources (IDACs), simplifying designs for applications such as RTD measurements.
• The ADS1263 is the most feature-rich, integrating a main 24-bit ADC and an additional auxiliary 24-bit ADC channel for background measurements.

The ADS1220 is a 24-bit precision ADC. It uses an SPI-compatible interface and includes specific pins for dual-matched current sources (IDACs) and a low-side power switch.

Key pins include:

• SPI interface pins such as SCLK, CS (active low), DOUT/DRDY, and DIN.
• Analog inputs like AIN0 through AIN3.
• Reference inputs REFP0, REFN0, and the dual-function pins AIN0/REFP1 and AIN3/REFN1.
• Supply pins DVDD, DGND, AVDD, and AVSS.
• The DRDY pin indicates when new data is available.
• An external clock input CLK is also available.

For a detailed pinout, please refer to the Olimex ADS1220 datasheet.

Critical hardware notes include

• AIN0 and AIN3 serve as the external reference inputs REFP1/REFN1.
• The DRDY pin or DOUT can signal data readiness.
• The AIN3/REFN1 pin connects to an internal low-side switch.
• Decoupling capacitors are advised between AVDD/AVSS and DVDD/DGND

To use the ADS1220 with an Arduino, the ADS1220_WE library by Wollewald or the Protocentral ADS1220 library is the most common option. Both are available via the Arduino Library Manager.

Wiring Diagram (Typical)

The ADS1220 uses the SPI protocol. Connect it to your Arduino (e.g., Uno) as follows:

Basic Example Code

This example uses the ADS1220_WE library to perform a simple differential measurement between AIN0 and AIN1.

#include <ADS1220_WE.h>
#include <SPI.h>
 
#define ADS1220_CS_PIN 7
#define ADS1220_DRDY_PIN 6
 
ADS1220_WE ads = ADS1220_WE(ADS1220_CS_PIN, ADS1220_DRDY_PIN);
 
void setup() {
  Serial.begin(9600);
 
  if (!ads.init()) {
    Serial.println("ADS1220 not connected!");
    while (1);
  }
 
  // Set to differential mode: AIN0 vs AIN1
  ads.setCompareChannels(ADS1220_MUX_0_1); 
 
  // Set Gain (1, 2, 4, 8, 16, 32, 64, 128)
  ads.setGain(ADS1220_GAIN_1); 
 
  // Optional: Set Data Rate (e.g., 20, 45, 90, 175, 330, 600, 1000 SPS)
  ads.setDataRate(ADS1220_DR_LVL_0); // 20 SPS (normal mode)
}
 
void loop() {
  // Read voltage in millivolts
  float voltage = ads.getVoltage_mV(); 
 
  Serial.print("Voltage AIN0-AIN1 (mV): ");
  Serial.println(voltage, 4);
 
  // Read internal temperature sensor
  float temp = ads.getTemperature();
  Serial.print("Internal Temp (°C): ");
  Serial.println(temp);
 
  delay(1000);
}

• Temperature Compensation: For thermocouples, you can use ads.enableTemperatureSensor(true) to read the ambient temperature for cold-junction compensation.
• Data Ready: The library handles the DRDY pin internally to ensure the ADC has finished its conversion before the Arduino attempts to read data.
• High Resolution: For raw 24-bit data (useful for weigh scales), use ads.getRawData() instead of voltage methods.

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