Arduino^® Reference Voltage Source : REFDUINO

Shield "RENÉ"

This project is a Hack-together of an Arduino UNO (others may need modifications) and a Shield "RENÉ". It generates an ultra-stable Reference Voltage of ± 0.001 up to ± 10.000 V, adjusteable in 0.001 V steps.

✈ Circuit Description

The device consists mainly of an REF102, a 10V reference voltage source with a 2.5ppm/°C temperature drift. It mayst be calibrated by a potentiometer R6. This reference is fed into an AD5791 DAC (20 Bit, B version) which has an 1 ppm resolution and a 1 ppm INL. A buffer at the output shall eliminate pulling/pushing effects of the load. We know that this DAC is expensive, but you get what you pay for. And for a reference, only the best components shall be used.

✈ Downloads

✈ Arduino Sketch - The Code

Double click on code to select ...

/*
 *    REFERENCE VOLTAGE SOURCE
 *    https://www.changpuak.ch/electronics/Arduino-Shield-RENE.php
 *    Software Version 1.4, suiteable for AD5791
 *    28.10.2015, Alexander C. Frank
 */

#include <LiquidCrystal.h>

LiquidCrystal lcd(7,6,5,4,3,2);


// INPUT
int PotiA = A0 ;        // MSB
int PotiB = A1 ;
int PotiC = A2 ;
int PotiD = A3 ;        // LSB

// OUTPUT
int SYNC = 13 ;    
int SCLK = 12 ;   
int SDIN = 11 ;   
int SDO = 10 ; 
int CLR = 9 ;
int LDAC = 8 ;

int ActualValuePoti[] = { 0, 0, 0 , 0} ;
int LastValuePoti[] = { 1, 1, 1, 1} ;
bool change = true ;      // HAS THE VOLTAGE CHANGED ???
bool first = true ;
unsigned long Spannung ;
unsigned long Zeiger ;
float LCDSpannung ;

void UpdateOutput() 
{
  Zeiger = 0b00000000100000000000000000000000 ;
  if (first == false) { Spannung |= 0b000100000000000000000000 ; }
        
  digitalWrite(SYNC, LOW);                      // START THE WRITE CYCLE
  for (int i=0; i<24; i++)
  {
    if ((Spannung & Zeiger)>0) { digitalWrite(SDIN, HIGH); }
      else { digitalWrite(SDIN, LOW); }
    delay(1);
    digitalWrite(SCLK, HIGH); delay(1); digitalWrite(SCLK, LOW);
    Zeiger = Zeiger >> 1 ;
    delay(1);
  } 
  digitalWrite(SYNC, HIGH);  delay(1);
}

void setup() 
{
  pinMode(SYNC, OUTPUT);
  pinMode(SCLK, OUTPUT);
  pinMode(SDIN, OUTPUT);
  pinMode(SDO, INPUT_PULLUP);
  pinMode(CLR, OUTPUT);
  pinMode(LDAC, OUTPUT);
  lcd.begin(8, 2);
  lcd.clear();
  lcd.setCursor(0,0); lcd.print("WARMING ");
  lcd.setCursor(0,1); lcd.print("UP ...");
  digitalWrite(CLR, HIGH);
  // When tied permanently low, output is updated on the rising SYNC.
  digitalWrite(LDAC, LOW);
  delay(2000);
  
  // WRITE THE CONTROL REGISTER
  Spannung = 0b001000000000000000010000 ;
  UpdateOutput();
  first = false ;
  delay(2000);
}

void loop() 
{

  // ---------------------------------------------------------
  LastValuePoti[3] = ActualValuePoti[3];
  ActualValuePoti[3] = analogRead(PotiD) / 113 ; 
  if (ActualValuePoti[3] != LastValuePoti[3]) change = true ;
  // ---------------------------------------------------------
  LastValuePoti[2] = ActualValuePoti[2];
  ActualValuePoti[2] = analogRead(PotiC) / 113 ; 
  if (ActualValuePoti[2] != LastValuePoti[2]) change = true ;  
  // ---------------------------------------------------------
  LastValuePoti[1] = ActualValuePoti[1];
  ActualValuePoti[1] = analogRead(PotiB) / 113 ;
  if (ActualValuePoti[1] != LastValuePoti[1]) change = true ;
  // ---------------------------------------------------------
  LastValuePoti[0] = ActualValuePoti[0];
  ActualValuePoti[0] = analogRead(PotiA) / 102 ;
  // SPECIAL CASE 10.000 V 
  if (ActualValuePoti[0] == 10) 
  {
    ActualValuePoti[1] = 0 ;
    ActualValuePoti[2] = 0 ;
    ActualValuePoti[3] = 0 ;
  }
  if (ActualValuePoti[0] != LastValuePoti[0]) change = true ;

  // -------------------------------------------------------

  if ( change == true )
  {
  // UPDATE LCD
  lcd.clear(); 
  lcd.setCursor(0,0); lcd.print("OUTPUT: ");
  lcd.setCursor(0,1); 
  LCDSpannung = (float)ActualValuePoti[0] 
	  + (float)ActualValuePoti[1] * 0.1 
	  + (float)ActualValuePoti[2] * 0.01 
	  + (float)ActualValuePoti[3] * 0.001 ;
  lcd.print(LCDSpannung,3);
  lcd.print(" V");

  // UPDATE DAC
  // Vout = 10V * Spannung / 1'048'575
  Spannung = LCDSpannung * 104857.5 ;
  UpdateOutput();
  change = false ; 
  }  
  
  delay(321);    
}

✈ How not to do it - failed approach :-(

The first approach used four DACs (MCP4725) which have been buffered. They where summed up with an resistive adder. Mathematically, this works beautiful, but the DACs had such a huge nonlinear behavior, that a large look-up table would have been necessary (10k entries) to make this thing work.

The Javascript below lets you "experience" how the Voltage Summer Circuit works. You may also want to have a look at www.allaboutcircuits.com.

✈ How to improve it - Use multiple REF102

It is a well known trick to use multiple transistors in parallel to reduce the noise. This approach may also be used here. The noise of the REF102 mayst be reduced by using multiple devices. In that case the outputs have to be summed up with a resistive network. Using two devices reduces the noise by 3 dB, four devices by 6 dB and so on ...

✈ Share your thoughts

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t₁ = 6767 d

t₂ = 260 ms

Reference Voltage		V
Resolution		Bit
DAC 1, R = 1 kΩ		V
DAC 2, R = 10 kΩ		V
DAC 3, R = 100 kΩ		V
DAC 4, R = 1 MΩ		V
GAIN		V/V
OUTPUT		V
ERROR		ppm

Arduino® Reference Voltage Source : REFDUINO

Shield "RENÉ"

✈ Circuit Description

✈ Downloads

✈ Arduino Sketch - The Code

Double click on code to select ...

✈ How not to do it - failed approach :-(

Reference Voltage

V

Resolution

Bit

DAC 1, R = 1 kΩ

V

DAC 2, R = 10 kΩ

V

DAC 3, R = 100 kΩ

V

DAC 4, R = 1 MΩ

V

GAIN

V/V

OUTPUT

V

ERROR

ppm

✈ How to improve it - Use multiple REF102

✈ Share your thoughts

Arduino^® Reference Voltage Source : REFDUINO