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Friday, 30 September 2016

An isolated analog output for Arduino Uno

This project uses the Arduino PWM Uno or other systems to realize a fully isolated analog output with a range of 0-5 volts or more, changing only the reference voltage.

Introduction
This project completes the series of my articles about the Arduino analog I/O with the aim to use it as a controller of small automation systems.
In control systems of the industrial plants it is always advisable to isolate both the inputs and the outputs coming from the field. This prevents disturbances caused by power surges, lightning strikes or other EMI sources and also by ground potential differences.
Arduino Uno, or systems based on the ATmega328 chip has no a true analog output, but it may be realized using a PWM output averaged with a low-pass filter.
The use of an averaged PWM signal with 8-bit setting is not comparable with a real DAC, but in the insulation case presents undoubted advantages of simplicity since it is sufficient to use an optocoupler for isolating the PWM digital signal. Recently I designed another circuit to generate a 4-20 mA current with Arduino, that experience gave me the idea for this new project.

The Arduino PWM
Arduino Uno has several pins (3, 5, 6, 9, 10, and 11) that can be configured for PWM output. For this project I used pin 9 because the others were used by various devices (LCD, SD and RTC) in my Arduino system.
The PWM signal on pins D9 and D10 is generated by Timer# 1 of ATmega328. It has a prescaler which divides by 1, 8, 64, 256, 1024, controlled by the three least significant bits of the register TCCR1B. The default value of the prescaler set by the Arduino IDE is equal to Np= 64 (TCCR1B, bits 2-0= 110), which provides an output frequency:

PWM frequency = CPUClock/(2´Np´TOP) = 16000000/(2´64´255)= 490.196 Hz

Where the TOP value is he maximum Timer/Counter value.
The following table shows the frequencies generated by Timer# 1 of an Arduino Uno (Atmega 328) on pins 9 and 10,  with a 16 MHz clock and in “phase­correct PWM” mode. In this mode, the timer counts from 0 to 255 and then back down to 0. 

Prescaler divider (Np)
Prescaler code
PWM frequency
1
B001
31372.549
8
B010
3921.569
64
B011
490.196
256
B100
122.549
1024
B101
30.637

The prescaler code must be put in the three least significant bits of the register TCCR1B – Timer/Counter1 Control Register B. For example, to generate a PWM of 3921 Hz, the following instruction must be inserted in the setup function:

TCCR1B = TCCR1B & B11111000 | B00000010;// set timer 1 prescaler to 8

Using a common optocoupler with a phototransistor, as 4N25, the frequency is limited because of the high transition times, so I used a faster optocoupler with photodiode and with an open collector output, such as the 6N136.
To eliminate the output noise I utilized a second order active low-pass filter, Sallen-key type, with a cut-off frequency of about 11.2 Hz. The isolation is achieved with an optocoupler, of course you must use for this circuit a power supply different from the one used for Arduino. If the insulation is not required, things become even simpler and connect the filter to the PWM output, in this case not even need the reference source U2.

The circuit diagram, shown in Figure 1, is quite simple. I recommend using for U1 a double operational amplifier suitable for single-rail power supply, such as LM358. 
The LM358 chip must be powered with a voltage higher than 7 V (and lower than 32) to have in output a maximum voltage of 5V and also the regulator has a 2 V dropout.
The advantage of the open collector of the optocoupler is that you can easily obtain a different output range, for example, using a 10V reference voltage and R2=10 kohm the output range became 0-10V. In this case the LM78L05 must be replaced with a LM317 with an appropriate circuitry.
In figure 2 you can see the arrangement of the components of my prototype.

Hardware components
1x Arduino board,
Components list
R1= 330 ohm ±5%
R2= 5.1 kohm ±5%
R3= 100kohm ±5%
R4= 100 kohm ±1% metal film
C1= 100nF ceramic
C2 = 10 MF,50V Electrolytic
C3= 200 nF Mylar ±2%
C4 = 100 nF Mylar ±2%

U1= LM358 dual op amp
U2= LM78L05 regulator
OPT1= 6N136
The capacitors used for the filter must be measured with a capacimeter, for my prototype I selected for C3 some 220 nF capacitors to search for a value that approached 200nF and C4 have selected a value half of C3. 

The test on the circuit
The Figure 3 shows the results of the linear regression on the 14 measurements points made on my prototype. The test conditions are:
·        PWM frequency = 490.196 Hz;
·        Vin = 12V;
·        Vref = 5.00 V
The standard error is about 6.1 mV, so the results are very good at the default PWM frequency.

I also tested the system with a frequency of 3921.569 Hz, but with a standard error of 39 mV. The largest errors are found for high duty cycle values, in this area the pulses are narrow and the rise time is high and this phenomenon creates non-linearity. The period is: T = 1/3921.569 = 255 µs. The more narrow pulse has a duration of about 1 µs, approximately the same value as the rise time of the pulses, the cause of non-linearity is due just to this phenomenon. Using the default frequency of 490.196 Hz, the minimum pulse has a duration eight times larger, so it greatly improves the linearity.

The program list
To test the system I used an Arduino Uno with a LCD display and the analog input A0 connected to a potentiometer to vary the duty cycle of the PWM.

// program to test Arduino Uno PWM at 3.9 kHz
// G. Carrera 30 sett 2016

#include <LiquidCrystal.h>

int PWMpin = 9;      // PWM out on digital pin 9
int analogPin = 0;   // potentiometer connected to A0
int val = 0;         // variable to store the read value
char spacestring[17] ="                ";

// initialize the library with the numbers of the interface pins
LiquidCrystal lcd(7, 6, 5, 4, 3, 2);

void setup() {
  pinMode(PWMpin, OUTPUT); // sets the pin as output
  lcd.begin(16, 2);// set up number of columns and rows
  lcd.setCursor(0, 0);// set the cursor to column 0, line 0
  lcd.print("Stalker PWM");// Print a message to the LCD
  // set timer 1 prescaler to 8 for PWM frequency of 3921.57 Hz
  //TCCR1B = TCCR1B & B11111000 | B00000010;
}

void loop() {
  val = analogRead(analogPin) >> 2;// read the potentiometer as 8 bit

  analogWrite(PWMpin, val);
  val = 255-val;// complement
  lcd.setCursor(0, 1);
  lcd.print(spacestring);
  lcd.setCursor(0, 1);
  lcd.print(val);
  delay(500);
}

References
1.      “Secrets of Arduino PWM”, Ken Shirriff, https://www.arduino.cc/en/Tutorial/SecretsOfArduinoPWM
2.      “Atmel 8-bit Microcontroller with 4/8/16/32KBytes In-System Programmable Flash”, 8271G–AVR–02/2013


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