I used mega8 ADC to control duty cycle of pwm output but there is a little noise on pwm signal. I tried to reduce it using 100nf capacitor on inputs, there is caacitor on VCC. there is 1uf on AVref. the external crystal with its two capacitors are close to the mega. the noise increased when connect it to switch dc dc converter.I use the output voltage of the converter as analog input to the ADC of the mega, Is this the noise source?( because of ripple in dc converter) , How to reduce it?

 

Schematic? Is this on a breadboard? Have you tried outputting through an RC filter?

 

it is on pcb. i didn't try the RC filter which values should i try?

 

Explain what you nean by noise on the PWM output. This is a digital signal , noise should not matter much unless it is so large that the switch you are controlling goes on and off when it shouldn’t.

Or do you mean there is jitter in the duty cycle?

Bob

 

there is chattering in pwm signal, it isn't constant at 5v level there is spikes

 

You need to supply a circuit diagram which shows how everything is connected and what it’s connected to. Help us help you, otherwise we will play 50 questions or guess at possible issues. How did you determine you have spikes? Do you have an oscilloscope screenshot?

 

there is chattering in pwm signal, it isn't constant at 5v level there is spikes

How many PWM outputs are you using?

The Mega has several PWM outputs. But having several outputs does not mean they should all be used. Or used without compensating for multiple outputs.

The biggest advantage of multiple outputs is the ability to have options in a complicated system; where there may be pin conflicts.

Even on a smaller ATMega processor, there are limits. It often shows up when driving multiple RC servos without a coprocessor board. You may be able to drive more than 2 servos, but you shouldn’t. They won’t operate reliably.

Multiple PWM outputs share the same clock. When there are more uses of the clock, reliable calculations are not possible.

When I need more than 2 PWM outputs, in any form, I add the servo shield to the Arduino. This gives you 16 reliable PWM outputs using a coprocessor.

 

I use two pwm and the noise is small when I measure at the output. But when I connect dc converter the noise increased. The attached photo is measured at the output of the mosfet driver

 

I will post schematic when I get access to my laptop

 

Does the "noise" also increase when you attach a passive load like a resistor?

That level of noise probably doesn't matter.

 

I see a much higher frequency on the signal at both the high and low stated. Probably you can get a much clearer view of it with a much higher sweep speed. That signal appears to have a constant amplitude as well. That may even be due to the ground reference connection point of your scope probe.SO I suggest probing the supply voltages because it does not seem to be random noise at all. It is a signal from some part of your circuit. OR it could be an outside broadcast radio station. That was a problem at one place where the lab area was about 500 feet from a 20KW AM radio broadcast tower.

 

Power supply noise from a switch mode supply?

Bob

 

Does the "noise" also increase when you attach a passive load like a resistor?

That level of noise probably doesn't matter.

Yes it increase with passive loads and the noise disappear when I connect both converters from single pwm pin. Clear pwm photo attached

 

You may be seeing noise from an oscillator introduced into the power rails (Does the PWM use the same power source?). What sort of decoupling capacitors do you have on the MCU?

 

In the first photo that is about 300 millivolts of noise, in the recent photo the display scale makes it very hard to see the noise, but it is still obvious on the low part of that square wave. If we see the waveform with a half a vlot per division we will much better be able to see how much noise voltage remains.

 

You may be seeing noise from an oscillator introduced into the power rails (Does the PWM use the same power source?). What sort of decoupling capacitors do you have on the MCU?

Yes there is 10uf and 100nf on mcu vcc. The noise came when I connected to the two pwm why?

 

Higher current drain.

 

Higher current drain.

Quite certainly a reasonable possibility.OR it may still be a scope connection problem. Those issues with the circuit common can be both sneaky and subtle.

 

everyone is still guessing without a clear circuit diagram and exactly what’s being switched..

 

Isn't that par for the course?