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I'm prototyping a circuit that uses the Maxim MAX7219 LED driver to power some 7-segment displays and a Microchip MCP4901 DAC to output some audio. When I listen to the audio output from the DAC while the MAX7219 is on, there is an unwanted high frequency noise. When I turn the MAX7219 off, the high frequency noise disappears. I've searched the forums for some information without finding anything. I think it might be related to the power. Any idea as to how to fix the problem?
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The MAX's multiplex at 800 Hz, you are  likely picking that up on the audio.
Along with perhaps control line switching.
Add ferrite beads on the power lines for the MAX7219 and the DAC.
Shield your audio lines, and ground them at one end. Especially from the DAC to whatever audio amp you have.
Seperate the digital and analog lines as much as you can.
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The frequency that I'm hearing is ~1750Hz. How do I know what type of ferrite bead to use? The ones that I've seen on Digikey have highest impedances around the 200Mhz range. Would these be suitable to use still?
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I'm sure there's some math that goes into picking a value for a specific frequency.
This app note gives some specific parts to use.
I

* Atmel9108 Microcontroller in a Harsh Environment.pdf (139.12 KB - downloaded 27 times.)
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I was able to get rid of the noise by using a separate voltage regulator for the audio IC's in the circuit. Not sure why that works the way it does, but it definitely works.
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Quote
Not sure why that works the way it does
This is because you had a total lack of power supply decoupling originally.
http://www.thebox.myzen.co.uk/Tutorial/De-coupling.html
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Not sure why that works the way it does
This is because you had a total lack of power supply decoupling originally.
http://www.thebox.myzen.co.uk/Tutorial/De-coupling.html

I had tried to use a pi filter before with a couple 0.1uF and 47uF caps along with a 1mH inductor, but still had a lot of noise.  So far, using separate voltage regulators for analog and digital has been the only working solution.
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There is nothing that seprate supplies will not do that can not be done by adequate decoupling. If a soloution did not work then it didn't go far enough.
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so would you recommend more pi filters in series?
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It is hard to recommend anything without seeing a schematic and a photo of your layout. Normally bigger large capacitors and a star ground should work providing of course, your power supply can cope with the current draw you are asking of it.
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