Electronics > Beginners
LM324 Noise Source
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Dabbot:
Here is something I came up with while searching for a circuit to feed random into an ADC.

It works by using the LM324 / LM2902 to amplify its own garbage.

The schematic is attached. The "ground" is offset low to account for the +V headroom required when running at its minimum supply spec.

Pros:
- Works down to 3 volts, drawing the better part of a milliamp.
- Decent signal. About 1.2v to 1.4v. YMMV across variants.

Cons:
- The spectrum of the noise is horrid, because it's an LM324. But I don't have access to anything beyond an entry level scope with FFT to comment any further than that.
- More components than your typical BJT zener noise source.

There could be oscillations that I'm not seeing, among other problems. If anyone has the time and gear to give this circuit a try and comment on it, much appreciated!

Edit: Version 2 schematic and scope shot attached.
Edit: Version 3 schematic and scope shot attached.
ogden:

--- Quote from: Dabbot on September 17, 2019, 12:03:33 pm ---If anyone has the time and gear to give this circuit a try and comment on it, much appreciated!

--- End quote ---

Why don't you show FFT screen so we can comment w/o building circuit? ;)
Dabbot:

--- Quote from: ogden on September 17, 2019, 12:12:00 pm ---
--- Quote from: Dabbot on September 17, 2019, 12:03:33 pm ---If anyone has the time and gear to give this circuit a try and comment on it, much appreciated!

--- End quote ---

Why don't you show FFT screen so we can comment w/o building circuit? ;)

--- End quote ---

Attached.  ;D
Kleinstein:
My first thought was that the coupling caps would give a rather high lower frequency limit. However this can be a good idea as it would suppress 50 Hz hum and 1/f noise.
The LM324 cross over distortion together with low frequency noise / hum could give some noise modulation.

I would consider a different, more modern OP, maybe MCP6004 or some LMV324 or similar.
ogden:
Thanx for FFT. Yes, it looks like opamp noise spectrum, so circuit indeed works as expected. BTW lm324 is awfully "noisy" opamp.

[edit] Further reading: https://www.analog.com/en/analog-dialogue/articles/understanding-and-eliminating-1-f-noise.html

ADA4622-2 noise plot:

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