ADS1256: ADS1256 output is not correct for analog sine wave

lavanya r

Part Number: ADS1256

Hi,

we have brought ADS1256 ADC module, https://rarecomponents.com/store/1951 and using the library from this link https://github.com/adienakhmad/ADS1256 and the board in interfaced to Arduino UNO using SPI protocol.

I used ADS1256_Basic_Switching_Channel.ino code with some modifications to read from all 8 channels in single ended mode. I tested Channel 0 with reference to ACOM in singe ended mode. The first input is the potentiometer which is been powered by 5V and wiper terminal is connected to Ch-0 and the serial monitor printed 0.00 to 4.84V for wiper changes.

Secondly, I applied Sine wave with proper offset voltage in order to be measured in Single ended mode, ADC data rate is 7500sps, Gain 1. The serial plotter prints sine wave properly from 1Hz to 5Hz after that signals gets distorted with increasing frequency.

The same thing happens if the signal is applied between AIN0 and AIN1(Differential mode with no offset voltage), code used is different, The serial plotter prints sine wave with distortion but this time with 1Hz itself. data rate and gain remains same.

what is the problem... How can I verify the frequency of Signal which is being read by ADS1256? Is there any option for performing FFT of the signal read by ADS1256? Also how to verify the samples per sec read by ADS1256?

Im attaching code and screenshots of output signal.

Pls help me to rectify this problem..

//Single ended code

// Arudino Sample Code to use ADS1256 library
// Reads 4 differential channels using effiecient input cycling
// Written by Adien Akhmad, August 2015
// Modfified  Jan 2019 by Axel Sepulveda for ATMEGA328

#include <ADS1256.h>
#include <SPI.h>

float clockMHZ = 7.68; // crystal frequency used on ADS1256
float vRef = 2.5; // voltage reference
// Initialize ADS1256 object
ADS1256 adc(clockMHZ,vRef,false); // RESETPIN is permanently tied to 3.3v

/* Variable to store sensor reading
  sensor1 = connected on AIN0 - AIN1
  sensor2 = connected on AIN2 - AIN3
  sensor3 = connected on AIN4 - AIN5
  sensor4 = connected on AIN6 - AIN7
*/

float sensor1;


void setup()
{
  Serial.begin(9600);
  
  Serial.println("Starting ADC");

  // start the ADS1256 with data rate of 15 SPS
  // other data rates: 
  // ADS1256_DRATE_30000SPS
  // ADS1256_DRATE_15000SPS
  // ADS1256_DRATE_7500SPS
  // ADS1256_DRATE_3750SPS
  // ADS1256_DRATE_2000SPS
  // ADS1256_DRATE_1000SPS
  // ADS1256_DRATE_500SPS
  // ADS1256_DRATE_100SPS
  // ADS1256_DRATE_60SPS
  // ADS1256_DRATE_50SPS
  // ADS1256_DRATE_30SPS
  // ADS1256_DRATE_25SPS
  // ADS1256_DRATE_15SPS
  // ADS1256_DRATE_10SPS
  // ADS1256_DRATE_5SPS
  // ADS1256_DRATE_2_5SPS
  // 
  // NOTE : Data Rate vary depending on crystal frequency. Data rates listed below assumes the crystal frequency is 7.68Mhz
  //        for other frequency consult the datasheet.
  //Posible Gains 
  //ADS1256_GAIN_1 
  //ADS1256_GAIN_2 
  //ADS1256_GAIN_4 
  //ADS1256_GAIN_8 
  //ADS1256_GAIN_16 
  //ADS1256_GAIN_32 
  //ADS1256_GAIN_64 
  adc.begin(ADS1256_DRATE_7500SPS,ADS1256_GAIN_1,false); 

  Serial.println("ADC Started");
  
   // Set MUX Register to AINO and AIN1 so it start doing the ADC conversion
 
}

void loop()
{ 

 // for (int i = 0; i < 8; ++i)
 // {
    adc.waitDRDY();
    adc.setChannel(0);
     sensor1 = adc.readCurrentChannel();
    Serial.println(sensor1);
     // Read the multiplex register to see the current active channel
 // }

  // Efficient Input Cycling
  // to learn further, read on datasheet page 21, figure 19 : Cycling the ADS1256 Input Multiplexer
  
/*  adc.waitDRDY(); // wait for DRDY to go low before changing multiplexer register
  adc.setChannel(2,3);   // Set the MUX for differential between ch2 and 3 
  sensor1 = adc.readCurrentChannel(); // DOUT arriving here are from MUX AIN0 and AIN1

  adc.waitDRDY();
  adc.setChannel(4,5);
  sensor2 = adc.readCurrentChannel(); //// DOUT arriving here are from MUX AIN2 and AIN3

  adc.waitDRDY();
  adc.setChannel(6,7);
  sensor3 = adc.readCurrentChannel(); // DOUT arriving here are from MUX AIN4 and AIN5

  adc.waitDRDY();
  adc.setChannel(0,1); // switch back to MUX AIN0 and AIN1
  sensor4 = adc.readCurrentChannel(); // DOUT arriving here are from MUX AIN6 and AIN7

  //print the result.
  Serial.print(sensor1,10);
  Serial.print("\t");
  Serial.print(sensor2,10);
  Serial.print("\t");
  Serial.print(sensor3,10);
  Serial.print("\t");
  Serial.println(sensor4,10);*/
 delay(10);
}

//differential mode code

// Arudino Sample Code to use ADS1256 library
// Reads 4 differential channels using effiecient input cycling
// Written by Adien Akhmad, August 2015
// Modfified  Jan 2019 by Axel Sepulveda for ATMEGA328

#include <ADS1256.h>
#include <SPI.h>

float clockMHZ = 7.68; // crystal frequency used on ADS1256
float vRef = 2.5; // voltage reference
// Initialize ADS1256 object
ADS1256 adc(clockMHZ,vRef,false); // RESETPIN is permanently tied to 3.3v

/* Variable to store sensor reading
  sensor1 = connected on AIN0 - AIN1
  sensor2 = connected on AIN2 - AIN3
  sensor3 = connected on AIN4 - AIN5
  sensor4 = connected on AIN6 - AIN7
*/

float sensor1, sensor2, sensor3, sensor4;


void setup()
{
  Serial.begin(9600);
  
  Serial.println("Starting ADC");

  // start the ADS1256 with data rate of 15 SPS
  // other data rates: 
  // ADS1256_DRATE_30000SPS
  // ADS1256_DRATE_15000SPS
  // ADS1256_DRATE_7500SPS
  // ADS1256_DRATE_3750SPS
  // ADS1256_DRATE_2000SPS
  // ADS1256_DRATE_1000SPS
  // ADS1256_DRATE_500SPS
  // ADS1256_DRATE_100SPS
  // ADS1256_DRATE_60SPS
  // ADS1256_DRATE_50SPS
  // ADS1256_DRATE_30SPS
  // ADS1256_DRATE_25SPS
  // ADS1256_DRATE_15SPS
  // ADS1256_DRATE_10SPS
  // ADS1256_DRATE_5SPS
  // ADS1256_DRATE_2_5SPS
  // 
  // NOTE : Data Rate vary depending on crystal frequency. Data rates listed below assumes the crystal frequency is 7.68Mhz
  //        for other frequency consult the datasheet.
  //Posible Gains 
  //ADS1256_GAIN_1 
  //ADS1256_GAIN_2 
  //ADS1256_GAIN_4 
  //ADS1256_GAIN_8 
  //ADS1256_GAIN_16 
  //ADS1256_GAIN_32 
  //ADS1256_GAIN_64 
  adc.begin(ADS1256_DRATE_7500SPS,ADS1256_GAIN_1,false); 

  Serial.println("ADC Started");
  
   // Set MUX Register to AINO and AIN1 so it start doing the ADC conversion
  adc.setChannel(0,1);
}

void loop()
{ 

  // Efficient Input Cycling
  // to learn further, read on datasheet page 21, figure 19 : Cycling the ADS1256 Input Multiplexer
  
  adc.waitDRDY(); // wait for DRDY to go low before changing multiplexer register
  adc.setChannel(2,3);   // Set the MUX for differential between ch2 and 3 
  sensor1 = adc.readCurrentChannel(); // DOUT arriving here are from MUX AIN0 and AIN1

 adc.waitDRDY();
  adc.setChannel(4,5);
  sensor2 = adc.readCurrentChannel(); //// DOUT arriving here are from MUX AIN2 and AIN3

  adc.waitDRDY();
  adc.setChannel(6,7);
  sensor3 = adc.readCurrentChannel(); // DOUT arriving here are from MUX AIN4 and AIN5

  adc.waitDRDY();
  adc.setChannel(0,1); // switch back to MUX AIN0 and AIN1
  sensor4 = adc.readCurrentChannel(); // DOUT arriving here are from MUX AIN6 and AIN7

  //print the result.
  Serial.print(sensor1,10);
  Serial.print("\t");
  Serial.print(sensor2,10);
  Serial.print("\t");
  Serial.print(sensor3,10);
  Serial.print("\t");
  Serial.println(sensor4,10);
  
}

over 5 years ago

0 Bob Benjamin over 5 years ago

TI__Guru** 113765 points

Hi Lavanya,

Welcome to the E2E forum! I suspect a couple of possible things for you to check. You are attempting to capture the data at 7500sps. You serial output is at 9600 baud. Your capture speed and transmit speed are probably too close to the same value for you to capture every data point.

In one loop case there is delay of 10ms, and this will prevent every data point from being captured as about 75 results will be lost from just this delay. I would suggest using an oscilloscope and monitoring DRDY and DOUT to see if you are capturing every conversion. Most likely you are not and this will show as an error in your results.

Best regards,

Bob B

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ADS1256: ADS1256 output is not correct for analog sine wave