How to generate the analog output from a in-built PWM of Atmega 32AVR Microcontrloller?

How to generate an analog output
From a in-built PWM of Atmega 32AVR Microcontrloller?

You need a resistor, a capacitor and an opamp.

Opamp is isn't really necessary when you are driving a mosfets, but would make life a bit easier.

Just be aware, the this DAC would be quite noisy or/and slow.

The bigger cap/resistor values, quiter and slower the output.

Another thing to being aware is so you better use 16bit PWM, as 8 bit PWM would give you only discrete values-

That could is enough or too coarse, depends on your application, of course.

Just google "PWM DAC" for more info and calculations.

One more thing-you could put several resistor/capacitor stages in series to improve on the noise side:

Create a low-pass RC filter that would filter out the frequency of your PWM.

The cutoff frequency should is at least times lower than the frequency of your PWM.

A lower cutoff frequency would reduce the voltage ripple in the output signal.

Cutoff frequency is determined by:

f = 1/(2ΠRC)

Further reading:

Arduino ' s analogwrite–converting PWM to a Voltage

When I first started working with the Arduino platform (it is also my first experience with microcontrollers),

I was a little surprised so analogwrite didn ' t actually output a voltage, but a PWM (pulse-width modulated) sig Nal.

After all, the ATmega had a a-d (analog to digital) converter along with Arduino ' sanalogread.

The complementaryanalogwrite function is there, but no d-a (digital to analog) converter on the AVR chip itself.

Fortunately, there is an easy-to-convert a PWM signal to a analog voltage.

Need to implement a simple single-pole low pass filter. Does it sound complicated?

It isn ' t. There is some great online tools to help.

Once you learn what to do one, you can quickly and easily output analog voltages from not only the Arduino,

But the PICs as well as the any other microcontroller this has PWM output.

PWM Primer

Pulse width modulation (or PWM as it is most commonly known), is a-a-encoding a voltage onto a fixed frequency Carrie R Wave.

Commonly used for radio controlled devices, it's similar to FM (frequency modulation) or AM (amplitude modulation) in WHA T it accomplishes.

Each of the type of modulation scheme has its own advantages and disadvantages.

AM modulation is the first type of modulation used for radio transmissions.

It's the most simple modulation scheme to implement, requiring only a single transistor or vacuum tube amplifier as was D One in the early days of radio.

However, it suffers from excessive noise and therefore, FM modulation is invented.

In this modulation technique, the voltage signal are no longer related to the strength of the signal.

That's why FM Radio have superior noise and fidelity qualities over AM radio, though it's not as simple to implement in C Ircuitry.

With the need for digital communication, a new modulation technique is INVENTED–PWM.

This technique shares the same noise immunity as FM, to which it is very similar.

The biggest difference is the simplicity and digital nature of the modulation.

Instead of varying the modulation frequency with voltage, an output was merely switched on and off at a fixed frequency.

The percentage of the on-time is in proportion to the signal voltage.

To see better what's this means, let's examine what a PWM signal looks like for various levels.

In the following image, the duty cycle is the output value from the PWM pin of a Arduino divided by 255:

PWM outputs (Curtesy arduino.cc)

For the Arduino, you write a value from 0 to 255 on a PWM pin, and the Arduino library would cause the PIN

To output a PWM signal whose on time was in proportion to the value written.

When it comes time for us to actually write an output voltage, the 0-255 value lacks meaning.

What we want are many cases is a voltage. For our purposes, we'll assume the Arduino is running at Vcc = 5 volts.

In the case, a value of 255 would also be 5 volts.

We can then easily convert the desired voltage to the digital value needed using simple division.

We first divide the voltage we want by the 5 volts maximum. That's gives us the percentage of our PWM signal.

We then multiply the percentage by 255 to give us pin value. Here is the formula:

Pin Value (0-255) = 255 * (ANALOGVOLTS/5);

Modulating a Signal

In addition to just setting the output voltage, your may need to actually modulate a signal.

To modulate a signal, we simply call Analogwrite with the value corresponding to our signal voltage.

One-to-do this would is to read the voltage at a analog pin, and then write it back out.

For example:

intPwmpin =9;//output pin supporting PWMintInpin =3;//voltage connected to analog pin 3, e.g. a potentiometerintval =0;//variable to store the Read valuefloatVolt =0;//Variable to hold the voltage readvoidSetup () {Pinmode (Pwmpin, OUTPUT);//sets the pin as output}voidLoop () {Val= Analogread (Inpin);//read the input pinVolt = (5.0* val)/1023; Val=255* (Volt/5); Analogwrite (Pwmpin, Val);}

Now on this example, we obviously won ' is need to convert our output voltage back to a voltage, but would instead transmit Our modulated signal as it is. If you had an oscilloscope, you can attach it to the output, and a potentiometer to the input and watch your PWM signal C Hange with the input value. There is many applications for PWM modulation, the most commonly being control of servos–either directly by wire or by Radio-control. The Arduino has a nice library of that handles creating the correct PWM signal for servos. For more information, see Thearduino Servo Library.

Changing the modulation Frequency

Most microprocessors permit the modulation frequency for PWM pins. The Arduino have its own set default values. For pins 3,9,10,11 It is approximately 488 Hz. For pins 5 and 6, it's about 977 Hz. These values is for a stock Arduino running at 16MHz. You can change these frequencies easily by writing new values to the appropriate timer register. For example, to change the frequency of Timer 2, which controls pins 9 and ten, to 3,906 Hz, you would set its register lik E so:

TCCR1B = TCCR1B & 0b11111000 | 0x02;

On the Arduino website, there are a nice tutorial on setting timer frequencies and their ramifications.

Low Pass Filtering

Now so you understand what PWM works and can even change the frequency, it's time to take a look at what to implement a s Imple Low pass filter. This simple piece of circuitry would convert your PWM output into a voltage corresponding to the percentage of the PWM wave Form. You'll then has a complete d-a converter for your Arduino or other microcontroller.

If We examine the circuit on the left while a voltage is applied to the input of R, the capacitor C would begin to Charg E. When it was charged, it would cease to conduct current and the voltage at the output of this circuit would match the input (assuming a high impedance load). If you remember this capacitors block DC currents, but pass the AC currents, you can see this any DC voltage input would also b e output, but high frequency AC voltages would be shorted to ground. For anything in between, i.e lower frequency AC voltages, they would be filtered according to the R/C time constant formed By the Resistor-capacitor network.

While this circuit are very simple, choosing the appropriate values for R & C encompass some design decisions–name Ly, how much ripple can we tolerate and what fast does the filter need to respond? These parameters is mutually exclusive. In most filters, we would like to has the perfect filter–one that passes all frequencies below the cutoff frequency, WI Th no voltage ripple. While no such ideal the filter exists, we can achieve close to it by using a multiple pole filter. Such a filter would incorporate many components in a ladder configuration. While such a filter have wonderful performance characteristics, its complexity and cost are unnecessary for simple d-a conve Rsion.

In such cases, we are need a simple single pole filter as shown above. We can achieve a reasonable voltage ripple for a single price–a low cutoff frequency. A Low cutoff frequency has A ramifications. First, it limits the which we can vary our output voltage. Second, there is a response delay when changing the voltage until the steady-state voltage is reached. For many of the more common applications, this trade-off is perfectly acceptable. Let's now look at an example.

First, let's choose our maximum ripple voltage. When we filter this high frequency PWM signal, a small component of it would always make it through the filter. That's happens because our capacitor are too small to filter it out entirely. We could choose a very large capacitor/resistor combination that would get a very high proportion of it, and then it Woul D Take a long time to reach the proper output voltage as the capacitor charges. That would greatly limit how fast we signal can change and is seen at the output. Therefore, we need to choose a reasonable value for the ripple voltage. A popular application would is to change the voltage of a MOSFET. Since MOSFETs is voltage controlled devices, we can easily drive them with our microcontroller with PWM and a low-pass fi Lter. Any ripple voltage present at the input would also is present at the output. For this example, assume the MOSFET is driving a non-critical load such as a high power LED. In this instance, we merely need to stay Within reasonable limits so the peak current in the LED would not be exceeded. In the case a 0.1 volt ripple would is more than adequate.

Next we choose a capacitor value. While it would seem the next step would is choosing a cutoff frequency (and it normally would be), there is additional CO Nsiderations such as output load and capacitor cost. If we were only driving the gate of a MOSFET, there would is no output load to speak of. In such case, we could choose a cheap ceramic caps such as 0.1uF and then choose the resistor we need to achieve the cutoff Frequency desired. If, on the other hand, we need some, we have output, then we'll need a smaller resistor and a correspondingly LA Rger capacitor. For a recent circuit, I found I needed a 2.2uF capacitor to prevent my modest load from altering the output voltage too Si gnificantly. Designing this circuit for non-trivial loads are beyond the scope of this article. If you find yourself in such a need, the best approach would is to start with at least a 1uF capacitor and then test how Y Our output voltage changes with load. Increase your capacitor until the load has aLow enough effect to be acceptable. Another-on-the-circuit would is to think of it as a poorly regulated power supply. It only meant to convert digital signals to an output voltage; Not to drive a load as well. Buffer the output with an op-amp or a FET first. Then drive your load.

For we example, let ' s choose a capacitor value of 1.0uF. For driving a MOSFETs, you can use the something even smaller, but the this size would let us have a small load. Next, we need to choose a cutoff frequency or response time. These parameters is related but not the same. For simple things-like driving LEDs, we is more concerned with a response time. Our response time can is pretty generous. Let's choose a settling time (to reach 90% of the final value) of 0.1 seconds, which would require a resistor of 15K ohms.

Wondering how to calculate these values, or others of your own. Rather than delve to a lot of equations, I has found something better.

This excellent online calculator does all the hard math for you, calculating cutoff frequency, response times, voltage RIP Ple and other values.

It even draws a transient analysis graph for you–displaying your ripple and how the voltage ramps up over time. Here are the output graph for this example:

Conclusion

You is now armed with the knowledge-need for creating and using your own digital to analog circuit. Such circuits is incredibly useful. My favorite is driving MOSFETs and op-amps. by sampling A/voltage somewhere, you can then determine what voltage you need to output to create the level of Current or voltage need. By means of such a simple system, you can make your own voltage regulators, current regulators, LED drivers, etc. The possibilities is endless.

If you had any questions on this article, please drop me a note in the comments. If you had any improvements, corrections or additions, please let me hear about them as well.

How to generate the analog output from a in-built PWM of Atmega 32AVR Microcontrloller?