A cure for PedalShield Due noise!

4 years 1 month ago - 4 years 1 month ago #1932 by quarterturn
With a stock, official Arduino Due and Pedalshield Due, I was hearing a lot of what sounded like CPU-dependent noise in the output when using the Due's ADCs. Running just the DACs produces a very noise-free output.
I took a look at the AREF pin on the SAM3X8E on my oscilloscope, and sure enough, there's at least 10mVpp of noise. The Due gets AREF from the 3.3V rail and uses some basic filtering - not enough apparently. It needs a better AREF source!

I chose an MCP1501-33. It only needs a few external parts and has the advantage over a shunt regulator of lower noise and a buffered output. It is rated for 27uV noise, which is a huge improvement. There's a 0R chip resistor used as a jumper on the Due which can be moved to route AREF on the MCU to the AREF pin, It's a very tiny part, probably 0402. I did not feel like re-soldering that, even with my hot air rework station, due to all the other small parts nearby, so I just removed it and soldered the output of the MCP1501-33 to the center pad. It's probably lower-noise that way, since the trace to the AREF pin may pick up noise along the way.

How does it sound??? Awesome! What was before an obvious whirling background of CPU sounds and heterodyning whistles is now dead quiet!

MCP1501 is a SOIC-8 part. It is surface mount, but pretty easy to solder. I used a breakout board and tacked on the caps and resistor to make the circuit as shown in figure 5-4 of the datasheet. I only had 75R chip resistors, not 50R, but it seems to make no audible difference to me.

This could be incorporated into a future Due Pedalshield. All the user would have to do is move the AREF jumper on the Due. You could just let it stick to the soldering iron tip to remove it, then blob some solder to bridge the pads to connect the external AREF. Or sell the Due pre-modded for users.

Anyhow, this mod turns the Pedalshield Due into a pro-sounding effect. Try it!

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4 years 1 month ago #1935 by Ray
Hi quarterturn, thanks for your fantastic post and the image.
I was aware of the "noisy nature" of the Arduino DUE. During the development of the pedalshield DUE, I try all the tricks on the book to remove the ADC and DAC noise from the signal, but no matter how hard you filter the signal, the residual hiss was always there.

I also dig into the Arduino forums and there are some interesting post showing the "default" noise that its always there:

Anyway, I never dare to "touch" the DUE board itself, I want to keep it as it is in case the player want to use the board for other projects, but I love your mod.
I think that adding this extra feature can make a huge improvement on the pedal.

Could you confirm my diagram below, to make sure that we are on the same page?

Thanks again!

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4 years 1 month ago - 4 years 4 weeks ago #1937 by quarterturn
That's the right connection on the Due, but I used the circuit in the attachment.


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4 years 4 weeks ago - 4 years 4 weeks ago #1941 by Ray
Ok, that's fantastic, I will get some 3V3 linear regulators (I am thinking on the smaller, cheaper and easier, like the LP2950CZ-3.3G (50-100uV noise)or any 3 leg similar) on my next Mouser order and experiment with it.
Thumbs up again for your feedback, I will link this post from the main topics.

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4 years 4 weeks ago #1943 by quarterturn
It's worth a try with a standard regulator, anything will be better than the 3.3V rail. Not all regulators have good noise rejection though. Let me know how it turns out.
The following user(s) said Thank You: Ray

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