Electronics > Beginners
LM324 Noise Source
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ogden:

--- Quote from: Zero999 on September 18, 2019, 11:04:28 am ---I'm surprised the output is that much lower.

--- End quote ---

We see that 1/f transition in this case is at much lower frequency. We can conclude that particular opamp have lower noise level, thus lower noise amplitude (at the same gain).


--- Quote from: Dabbot on September 18, 2019, 08:59:35 am ---...and the results are in!

--- End quote ---

Well done! Any chance to increase output capacitor value? At least to 1uF or even 10uF.
Dabbot:

--- Quote from: ogden on September 18, 2019, 11:34:24 am ---Well done! Any chance to increase output capacitor value? At least to 1uF or even 10uF.

--- End quote ---

Whoops. I forgot to include the output cap. You're looking at the scope directly connected to the output. The scope's channel was AC coupled. :palm:

Edit: I just tried various output cap values. There's no observable difference.
Zero999:

--- Quote from: Dabbot on September 18, 2019, 11:25:29 am ---Taking out R7 and R8 causes the output to diminish substantially.

Don't these chips share biasing circuitry? It might explain the smaller amount of noise, given there's less circuitry to generate it.

--- End quote ---
That makes sense. Removing R7 & R7 will reduce the noise because the bias currents of the LM358 are inherently noisy. Unfortunately the input current not specified, so it can't be calculated. For your information it's just Ohm's law INOISE*RIN.

I think using more stages will also increase the noise, because each amplifier adds noise.
Dabbot:
Update time!

I took Zero999's approach of upping the value of the gain resistors, as well as placing resistors on the +ve inputs.

This resulted in more overall noise, and allowed me to decrease the gain and get higher frequencies through. Not that it really matters for my application. I just wanted to experiment.

I also tried to use a completely passive ground so I could use four stages, but this introduced additional instability, even with a decoupling cap.

Updated schematic and scope shot attached.
Dabbot:
Another update!

Recently I got the chance to experiment with this a bit more and managed to make further improvements:

First, I removed the 100K resistors from the latter stages' non-inverting inputs, as these resistors were not contributing in any significant sense.

Second, and the most interesting to me, is that I changed the second amplifier stage to non-inverting. This further flattened out the noise spectrum. I'm unsure exactly why this is.

Third, I AC-coupled the input of the first stage. This, along with the first stage's matched input resistances let me DC-couple its output to the non-inverting second stage, as the DC offset is negligible.

Version 3 schematic and scope shot attached.
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