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I want to increase the effective resolution of the ADC by oversampling and decimation. Unfortunately the signal I'm realing is too clean, so I would like to add a bit of artificial noise (1LSB peak to peak) to a signal.

I would like to use MCU timer to output a square wave, convert it to triangle-like wave and add it to the signal.

Below is my attempt, but it does not work as expected - the amount of noise added varies with the level of the analog signal.

Can someone enlighten me on this topic? How do I do this properly?

schematic

simulate this circuit – Schematic created using CircuitLab

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Just tying the PWM pin to ground through a capacitor didn't add enough noise? –  Ignacio Vazquez-Abrams Apr 26 at 18:25
    
" the amount of noise added varies with the level of the analog signal." Is it possible you are running into a floor effect? What's the relationship between the A/D range and the range of V2? –  gwideman Apr 26 at 18:59
1  
@gwideman It was due to unbuffered analog signal, I've used a pot when breadboarding the circuit, when I've added an opamp buffer, everythig started to work. –  miceuz Apr 26 at 20:07

1 Answer 1

up vote 4 down vote accepted

It won't vary with input signal level the way you've shown it, however it will vary with impedance of the source.

I suggest adding the noise with an op-amp to isolate the input from the noise source. You should probably have an anti-alias filter on the input signal before adding (unless it's naturally band limited) and make sure that the input is not correlated with the triangle wave.

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Looks like your source impedance suggestion was on the money! –  gwideman Apr 26 at 20:36

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