Ways to Improve ADC Measurement Accuracy and Resolution Part 1

In this video series we look at ways or tips to improve ADC measurement accuracy and resolution. In part 1 define what accuracy and resolution is and different types of error that can affect ADC measurements. We also look at the importance of using an accurate ADC reference voltage and why you want to scale the range of the signal you are measuring to the ADC's range.

//***************Arduino code used in video*******************************
/*This code demonstrates how to change the ADC voltage reference on an Arduino 
 * This was shown in an ADC tutorial on the ForceTronics YouTube Channel.
* This code is free and open for anybody to use at their own risk. 
*/

void setup() {
  Serial.begin(115200); //setup serial connection
  while(!Serial);
  analogReference(AR_EXTERNAL); //sets the ADC reference voltage to external (input is aRef pin)
  //analogReference(AR_DEFAULT); //set the ADC reference to default which is AVCC (uses power supply voltage or VCC as reference)
  //analogReference(AR_INTERNAL2V23); //uses 2.23V internal reference in SAMD21 uC
  analogReadResolution(12); //Set ADC to 12bit resolution, default is 10
  burn8Readings(); //make 8 readings but don't use them to ensure good reading after reference change
  delay(100);
  for(int i=0;i<500;i++) { //loop through a bunch of ADC readings and print them to serial plotter
    Serial.println(analogRead(A0)); //Make ADC measurement at A0
  }
  
}

void loop() {
  //don't need the loop
}

//This function makes 8 ADC measurements but does nothing with them
//Since after a reference change the ADC can return bad readings at first. This function is used to get rid of the first 
//8 readings to ensure an accurate one is displayed
void burn8Readings() {
  for(int i=0; i<8; i++) {
    analogRead(A0);
    delay(1);
  }
}

Tutorial on Digital to Analog Converters (DAC) and Example Using the MCP4728 Part 1

In this video we go over what a digital to analog converter (DAC) is and how it works. We then focus on the MCP4728 4 Channel DAC with some simple examples. In part two we do a deeper dive into the MCP4728.

Link to Analog Devices detailed tutorial on converters: https://www.analog.com/media/en/training-seminars/design-handbooks/Basic-Linear-Design/Chapter6.pdf

Kelvin Divider or String DAC Architecture

Utilizing Advanced ADC Capabilities on Arduino’s with the SAMD21 (Zero, MKR1000, etc) Part 1

We are all familiar with the Arduino "analogRead()" function, but there is a lot more to the SAMD21 ADC then just taking simple readings. In this video series we look at some of the more advanced ADC capabilities of the SAMD21 and how to use them. In part 1 we look at how to use the window monitoring capability of the ADC.

//*******************Arduino code from the video*********************
//This sketch is from a tutorial on the ForceTronics YouTube Channel called 
//Utilizing Advanced ADC Capabilities on Arduino’s with the SAMD21 (Zero, MKR1000, etc)
//This code is public domain and free to anyone to use or modify at your own risk

//declare const for window mode settings
const byte DISABLE = 0;
const byte MODE1 = 1;
const byte MODE2 = 2;
const byte MODE3 = 3;
const byte MODE4 = 4;

void setup() {
  //call this function to start the ADC in window and define the window parameters
  ADCWindowBegin(MODE1, 512, 750); //Do not use the Arduino analog functions until you call ADCWindowEnd()
  Serial.begin(57600);
}

void loop() {
  delay(1500);
  Serial.println(readADC()); //the "readADC()" function can be used to get ADC readings while in Window mode
  Serial.println();
}

//This is the interrupt service routine (ISR) that is called 
//if an ADC measurement falls out of the range of the window 
void ADC_Handler() {
    digitalWrite(LED_BUILTIN, HIGH); //turn LED off
    ADC->INTFLAG.reg = ADC_INTFLAG_WINMON; //Need to reset interrupt
}

//this function sets up the ADC window mode with interrupt
void ADCWindowBegin(byte mode, int upper, int lower) {
  setMeasPin(); //function sets up ADC pin A0 as input
  setGenClock(); //setup ADC clock, using internal 8MHz clock
  setUPADC(); //configure ADC
  setADCWindow(mode, upper, lower); //setup ADC window mode 
  setUpInterrupt(0); //setup window mode interrupt with highest priority
  enableADC(1); //enable ADC 
}

void ADCWindowEnd() {
  NVIC_DisableIRQ(ADC_IRQn); //turn off interrupt
  enableADC(0); //disable ADC 
}

//setup measurement pin, using Arduino ADC pin A3
void setMeasPin() {
  // Input pin for ADC Arduino A3/PA04
  REG_PORT_DIRCLR1 = PORT_PA04;

  // Enable multiplexing on PA04
  PORT->Group[0].PINCFG[4].bit.PMUXEN = 1;
  PORT->Group[0].PMUX[1].reg = PORT_PMUX_PMUXE_B | PORT_PMUX_PMUXO_B;
}

//Function sets up generic clock for ADC
//Uses built-in 8MHz clock
void setGenClock() {
   // Enable the APBC clock for the ADC
  REG_PM_APBCMASK |= PM_APBCMASK_ADC;

  configOSC8M(); //this function sets up the internal 8MHz clock that we will use for the ADC
  
  // Setup clock GCLK3 for no div factor
   GCLK->GENDIV.reg |= GCLK_GENDIV_ID(3)| GCLK_GENDIV_DIV(1);
   while (GCLK->STATUS.reg & GCLK_STATUS_SYNCBUSY);  

  //configure the generator of the generic clock, which is 8MHz clock
  GCLK->GENCTRL.reg |= GCLK_GENCTRL_GENEN | GCLK_GENCTRL_SRC_OSC8M | GCLK_GENCTRL_ID(3) | GCLK_GENCTRL_DIVSEL;
  while (GCLK->STATUS.reg & GCLK_STATUS_SYNCBUSY);
  
  //enable clock, set gen clock number, and ID to where the clock goes (30 is ADC)
  GCLK->CLKCTRL.reg |= GCLK_CLKCTRL_CLKEN | GCLK_CLKCTRL_GEN(3) | GCLK_CLKCTRL_ID(30);
  while (GCLK->STATUS.bit.SYNCBUSY);
}

//Function that does general settings for ADC
//sets it for a single sample
//Uses internal voltage reference
//sets gain factor to 1/2
void setUPADC() {
  // Select reference, internal VCC/2
  ADC->REFCTRL.reg |= ADC_REFCTRL_REFSEL_INTVCC1; // VDDANA/2, combine with gain DIV2 for full VCC range

  // Average control 1 sample, no right-shift
  ADC->AVGCTRL.reg |= ADC_AVGCTRL_ADJRES(0) | ADC_AVGCTRL_SAMPLENUM_1;

  // Sampling time, no extra sampling half clock-cycles
  REG_ADC_SAMPCTRL |= ADC_SAMPCTRL_SAMPLEN(0);

  // Input control: set gain to div by two so ADC has measurement range of VCC, no diff measurement so set neg to gnd, pos input set to pin 0 or A0
  ADC->INPUTCTRL.reg |= ADC_INPUTCTRL_GAIN_DIV2 | ADC_INPUTCTRL_MUXNEG_GND | ADC_INPUTCTRL_MUXPOS_PIN4;
  while (REG_ADC_STATUS & ADC_STATUS_SYNCBUSY);

  // PS16, 8 MHz, ADC_CLK = 500 kHz, ADC sampling rate, single eded, 12 bit, free running, DIV2 gain, 7 ADC_CLKs, 14 usec
  ADC->CTRLB.reg |= ADC_CTRLB_PRESCALER_DIV16 | ADC_CTRLB_RESSEL_10BIT | ADC_CTRLB_FREERUN; // Run ADC continously, 7 ADC_CLKs, 14 usec
  while (REG_ADC_STATUS & ADC_STATUS_SYNCBUSY);
}

//This function is used to setup the ADC windowing mode
//inputs are the mode, upper window value, and lower window value
//
void setADCWindow(byte mode, int upper, int lower) {
  ADC->WINCTRL.reg = mode; //set window mode
  while (ADC->STATUS.bit.SYNCBUSY);

   ADC->WINUT.reg = upper; //set upper threshold
   while (ADC->STATUS.bit.SYNCBUSY);

   ADC->WINLT.reg = lower; //set lower threshold
   while (ADC->STATUS.bit.SYNCBUSY);
}

//This function sets up an ADC interrupt that is triggered 
//when an ADC value is out of range of the window
//input argument is priority of interrupt (0 is highest priority)
void setUpInterrupt(byte priority) {
  
  ADC->INTENSET.reg |= ADC_INTENSET_WINMON; // enable ADC window monitor interrupt
   while (ADC->STATUS.bit.SYNCBUSY);

   NVIC_EnableIRQ(ADC_IRQn); // enable ADC interrupts
   NVIC_SetPriority(ADC_IRQn, priority); //set priority of the interrupt
}

//function allows you to enable or disable ADC
void enableADC(bool en) {
  if(en) ADC->CTRLA.reg = 2; //2 is binary 010 which is register bit to enable ADC
  else ADC->CTRLA.reg = 0; //0 disables ADC
}

//This function will return the latest ADC reading made during free run window mode
//must first start the ADC before calling this function
unsigned int readADC() {
  // Free running, wait for conversion to complete
  while (!(REG_ADC_INTFLAG & ADC_INTFLAG_RESRDY));
  // Wait for synchronization before reading RESULT
  while (REG_ADC_STATUS & ADC_STATUS_SYNCBUSY);
  
  return REG_ADC_RESULT;
}

//function enables the 8MHz clock used for the ADC
void configOSC8M() 
{
  SYSCTRL->OSC8M.reg |= SYSCTRL_OSC8M_ENABLE;
}

Utilizing the Arduino ADC Internal Reference

In this video we look at the ADC internal reference in an Arduino, why / when would you use it, how to ensure you get good measurement accuracy when using it, and how to use it to check your battery voltage.

**********************Arduino Code from the video****************************
float iREF = 1.08; //internal reference cal factor

void setup() {
  // put your setup code here, to run once:
  analogReference(EXTERNAL);
  //burn some ADC readings after reference change
  for(int i=0; i<8; i++) analogRead(A0);
  Serial.begin(57600);
}

void loop() {
  // put your main code here, to run repeatedly:
  delay(3000);
  Serial.print("Measured battery voltage is: ");
  Serial.println(fReadVcc());
  Serial.println();
}

//This function uses the known internal reference value of the 328p (~1.1V) to calculate the VCC value which comes from a battery
//This was leveraged from a great tutorial found at https://code.google.com/p/tinkerit/wiki/SecretVoltmeter?pageId=110412607001051797704
float fReadVcc() {
  ADMUX = _BV(REFS0) | _BV(MUX3) | _BV(MUX2) | _BV(MUX1);
  delay(3); //delay for 3 milliseconds
  ADCSRA |= _BV(ADSC); // Start ADC conversion
  while (bit_is_set(ADCSRA,ADSC)); //wait until conversion is complete
  int result = ADCL; //get first half of result
  result |= ADCH<<8; //get rest of the result
  float batVolt = (iREF / result)*1024; //Use the known iRef to calculate battery voltage
  return batVolt;
}

Maximizing Arduino’s ADC Resolution and Accuracy Part 3

In part three we look at the three main sources of error in an ADC measurement. We will discuss how to reduce total ADC error and how to quantify the total error of an ADC measurement.

Maximizing Arduino’s ADC Resolution and Accuracy Part 2

In part 2 we will look at how to increase the accuracy of our ADC measurements using the built-in Noise Reduction Mode

Arduino Code********************************************************************
/*This example sketch shows how to make ADC measurements via registers and how to use the low noise or noise cancellation ADC measurement capability. 
This was shown in an ADC tutorial on the ForceTronics YouTube Channel.
This code is free and open for anybody to use at their own risk. 1/9/15
*/
#include <avr/sleep.h>

int normADC[10]; //Create an arrray to hold the "normal" or non-low noise ADC measurements
int lowNoiseADC[10]; //Create an array to hold the low noise ADC measurements

int const aVCC = 0b01000000; //variable to set ADC to use VCC as reference
int const iREF = 0b11000000; //variable to set ADC to use internal 1.1V as reference
int const aREF = 0b00000000; //variable to set ADC to use VCC as reference
//bits 7 and 6 select the ADC reference source
//The four zeros (bits 3 thru 0) at the end of the above binary register values sets the analog channel to A0
//Bit 5 selects right adjust for the result and bit 4 is not used

//Setup code only run once
void setup() {
  ADMUX = aVCC; //Configure the ADC via the ADMUX register for VCC as reference, result right adjust, and source A0 pin
  ADCSRA |= 1<<ADEN; //Turn the ADC on by setting the ADEN bit to 1 in the ADCSRA register
  ADCSRA |= 1<<ADIE; //Setting the ADIE bit to 1 means when the ADC is done a measurement it generates an interrupt, the interrupt will wake the chip up from low noise sleep
  ADCSRA |= ((1<<ADPS2)|(1<<ADPS1)|(1<<ADPS0));    //Prescaler at 128 so we have an 125Khz clock source
  //The ADC clock must be between 50kHz and 200kHz, since the Uno clock is 16MHz we want to divide it by 128 to get 125kHz. This may need to be adjusted for other Arduino boards
  //We do not have to use registers to set the sleep mode since we are using the avr/sleep.h library
  sleep_enable(); //enable the sleep capability
  set_sleep_mode(SLEEP_MODE_ADC); //set the type of sleep mode. We are using the ADC noise reduction sleep mode
  //Note that in this sleep mode the chip automatically starts an ADC measurement once the chip enters sleep mode
  //Note that the reason for making the ADC measurements and storing in arrays was to avoid using the arduino serial functions with sleep mode since serial uses interrupts
  for(int i=0; i<10; i++) { //Loop ten times and make ten ADC measurements in low noise mode
    sei(); //enable interrupts
    sleep_cpu(); //enter low ADC noise sleep mode, this action turns off certain clocks and other modules in the chip so they do not generate noise that affects the accuracy of the ADC measurement
    //The chip remains in sleep mode until the ADC measurement is complete and executes an interrupt which wakes the chip from sleep
    lowNoiseADC[i] = ADC; //get ADC result and store it in low noise measurement array
  }
  
  for(int k=0; k<10; k++) { //loop executes 10 times and makes 10 normal ADC measurements
    ADCSRA |= 1<<ADSC; //this bit starts conversion. It will remain high until conversion is complete
    while(ADCSRA & (1<<ADSC)) { delay(1);} //When ADSC bit goes low conversion is done
    normADC[k] = ADC; //get ADC result and store it in normal ADC measurement array
  }
  
  Serial.begin(9600); //Start serial comm to print out results to serial monitor
  while(!Serial.available()) { delay(1); } //wait to print out results until serial monitor is open and any character is sent
  Serial.println("Noise reduction:"); //labels the readings being printed
  for(int j=0; j<10;j++) { //loop ten times to print out all the low noise measurement in array
    Serial.println(convertToVolt(lowNoiseADC[j]),3); //Convert each low noise measurement to a voltage level and send it over serial with three decimal places precision
  }
  Serial.println(); //print a line feed
  Serial.println("No noise reduction:"); //print heading for normal ADC measurements
  for(int l=0; l<10;l++) { //loop ten times and print each ADC reading
    Serial.println(convertToVolt(normADC[l]),3); //Convert each normal ADC measurement to a voltage level and send it over serial with three decimal places precision
  }
}

void loop() {
 //do nothing in the loop
}

//This function takes the ADC integer value and turns it into a voltage level. The input is the measured ADC value.
float convertToVolt(int aVAL) {
  float refVal = 5.01;  //hard coded in AVCC value, measured the real value for higher accuracy
  return (((float)aVAL/1023)*refVal); //formula to convert ADC value to voltage reading
}

//this is the interrupt service routine for the ADC interrupt. This must be in the code for the interrupt to work correctly
ISR(ADC_vect) {
}

Maximizing Arduino’s ADC Resolution and Accuracy Part 1

In part 1, of this 3 to 4 part series, we will look at what ADC measurement resolution is and how to maximize it on the Arduino. We will also look at a simple hint to increase Arduino's ADC measurement accuracy. In later parts we will get much deeper into accuracy and how to increase it.

Arduino Code using analogreference()*************************************************

/*This example sketch shows how to change the reference for Arduino's ADC to default or internal 1.1V.

This sketch was used to show the measurement resolution advantage of scaling Arduino's ADC reference to 

the voltage range or scale being measured. This was shown in an ADC tutorial on the ForceTronics YouTube Channel.

This code is free and open for anybody to use at their own risk. 1/6/15

*/

float const aVCC = 5.049; //Actual measured voltage value at AVCC pin. This is im

float const iREF = 1.084; //Actual measured voltage value of the internal 1.1V reference

//Note that these values can be measured at the AREF pin

void setup() {

  Serial.begin(9600); //setup serial connection

}

void loop() {

  

  delay(1000); //delay 1 second between each ADC measurement

  analogReference(DEFAULT); //set the ADC reference to default which is AVCC (uses power supply voltage or VCC as reference)

  burn8Readings(); //make 8 readings but don't use them to ensure good reading after reference change

  Serial.println("Default (AVCC) Ref:");

  Serial.println(convertToVolt(aVCC,analogRead(A0)),3); //Make ADC measurement at A0, convert it to a voltage value, write value to serial with 3 decimal places 

  Serial.println(" ");

  delay(1000); //delay 1 second between each ADC measurement

  analogReference(INTERNAL); //set the ADC reference to internal 1.1V reference

  burn8Readings(); //make 8 readings but don't use them to ensure good reading after reference change

  Serial.println("Internal 1.1V Ref:");

  Serial.println(convertToVolt(iREF,analogRead(A0)),3); //Make ADC measurement at A0, convert it to a voltage value, write value to serial with 3 decimal places

  Serial.println(" ");

}

//This function makes 8 ADC measurements but does nothing with them

//Since after a reference change the ADC can return bad readings at first. This function is used to get rid of the first 

//8 readings to ensure an accurate one is displayed

void burn8Readings() {

  for(int i=0; i<8; i++) {

    analogRead(A0);

    delay(1);

  }

}

//This function convers the ADC level integer value into a useful voltage value.

//The inputs are the measured ADC value and the ADC reference voltage level

//The formula used was obtained from the data sheet: (ADC value / 1024) x ref voltage

float convertToVolt(float refVal, int aVAL) {

  return (((float)aVAL/1024)*refVal);

}

Arduino Code using Registers*************************************************

/*This example sketch shows how to change the reference for Arduino's ADC to default or internal 1.1V using registers.

This sketch was used to show the measurement resolution advantage of scaling Arduino's ADC reference to 

the voltage range or scale being measured. This was shown in an ADC tutorial on the ForceTronics YouTube Channel.

This code is free and open for anybody to use at their own risk. 1/6/15

*/

int const aVCC = 0b01000000; //variable to set ADC to use VCC as reference

int const iREF = 0b11000000; //variable to set ADC to use internal 1.1V as reference

int const aREF = 0b00000000; //variable to set ADC to use VCC as reference

//The three zeros at the end of the above binary register values sets the analog channel to A0

void setup() {

  Serial.begin(9600);

  ADCSRA |= 1<<ADEN; //Turn on ADC

  ADCSRA |= ((1<<ADPS2)|(1<<ADPS1)|(1<<ADPS0));    //Prescaler at 128 so we have an 125Khz clock source

}

void loop() {

  delay(1000);

  Serial.println("Internal Ref:");

  burn8Readings(iREF);

  Serial.println(measADC(iREF),3);

  Serial.println(" ");

  delay(1000);

  Serial.println("AVCC Ref:");

  burn8Readings(aVCC);

  Serial.println(measADC(aVCC),3);

  Serial.println(" ");

}

//This function makes an ADC measurement using registers

float measADC(int ref) {

  ADMUX =  ref; //bits 7&6 select reference, bit 5 R or L adjust (0 R), bit 4 unused, 3:0 is channel select (0000 is ADC0)

  //ADMUX |=  0b00000000; //bits 7&6 select reference, bit 5 R or L adjust (0 R), bit 4 unused, 3:0 is channel select (0000 is ADC0)

  ADCSRA |= 1<<ADSC; //this bit starts conversion. It will remain high until conversion is complete

  

  while(ADCSRA & (1<<ADSC)) { delay(1);} //ADIF bit goes high when conversion is complete and result is in ADCL and ADCH

  return convertToVolt(ref, ADC);

}

//This function takes the ADC integer value and turns it into a voltage level. The inputs are

//the selected reference source and the measured ADC value.

float convertToVolt(int refSet, int aVAL) {

  float refVal;

  if (refSet == iREF) { refVal = 1.08; } //hard coded in reference value for internal

  else { refVal = 5.05; } //hard coded in AVCC value

  

  return (((float)aVAL/1023)*refVal);

}

//This function makes 8 ADC measurements but does nothing with them

//Since after a reference change the ADC can return bad readings at first. 

//This function is used to get rid of the first 

//8 readings to ensure an accurate one is displayed

void burn8Readings(int ref) {

 for(int i = 0; i<8; i++) {

  measADC(ref);

  delay(1);

 } 

}