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I have an audio (mic, speaker) circuit that I am currently powering off of a 12V car adapter that is fed into a 10V switching DC regulator to power the rest of the circuit. I have noticed a horrendous amount of noise on the mics and speakers when running off of this circuit, even with the appropriate bypass capacitors from Vcc to Gnd in place, carefully tuning the DC-DC switching converter (buck-boost), and so on.

After breaking out my oscilloscope, it seems there is a lot of noise on the Vcc and Gnd rails that affects the entire circuit, and the noise is not present when I swap out the 12V car power source for an external 12V battery solution.

What would be the best approach to eliminating this noise source entirely? I am already providing a regulated power output, but it seems that the noise on Gnd is impacting everything else in the circuit, even with a regulated power source. I could create a separate Gnd for the rest of the circuit, but am not certain as to how this would work with my switching power supply if it doesn't share the same Gnd as the circuit it powers.

I have also considered using a flyback regulator, as it isolates the circuit it supplies via a transformer, but I'm not 100% sure if this will resolve the case of a noisy external Gnd.

Thank you.

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  • \$\begingroup\$ Show a circuit. Best guess is that noise is getting to the mike because this is the most sensitive area. \$\endgroup\$ – Andy aka Sep 29 '14 at 22:35
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Automotive power is one of the more notoriously annoying sources of noise there is.

You need to imagine a lot of high voltage, high frequency spikes happening on the power, this couples and radiates into your set-up, where up-flanks can be so fast and strong they couple into your VCC through your converter, as the converter has only a certain amount of noise rejection. The down flanks can push so hard they will show up in the ground you see at your device. And even more complex annoyances to do with coupling and propagation of noise sources.

The only way to get rid of those and know exactly why you do what you do is to look for a tutorial about automotive power noise filtering and seeing which of the explained steps you have not yet taken yourself.

But a good start would be to include a common-mode input filter coil on the entirety of your power that's rated for automotive filtering. Many inductor factories make them, I like Würth Electronic, but that's also because they have a very permissive sampling regimen. After that, obviously you put a nice strong capacitor: Capacitance able to provide the current needed to your circuit for about 1ms of full drop-out and low ESR to get out as much noise as possible. You may also put an input diode onto the system to protect even better against full "disappearance" of power for about 1ms, so that you don't power other stuff from "your personal capacitor".

After that capacitor you can add another inductance on the positive rail, followed by the input capacitance required for your regulator.


Edit1: Schematic coming up to clarify... Edit2: Sorry, forgot to save the edit once I was done calling :-S

schematic

simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ (+1) I like it, though I've never done auto filtering. \$\endgroup\$ – George Herold Sep 30 '14 at 1:18
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    \$\begingroup\$ Thanks. It's an annoying business. You need to think of everything you need to think about in other places, but twice as hard :-). In some vehicles you have a power system going between 5A and 500A going from A to B to D to C back to B through K. :-) \$\endgroup\$ – Asmyldof Sep 30 '14 at 3:26
  • \$\begingroup\$ One question on the L1 and L2 inductors: are they just independent discrete components, or are they part of a single common mode choke device? Additionally, to provide more information on the circuit, V+ in can be either a 12V noisy automotive source, or a 9V battery. The isolated V+ out is fed into a DC buck-boost regulator that is meant to stay at a steady +8V DC, even if V+ in varies between 7.5-12V DC. Finally, the 8V regulator output is supplying up to 100mA to an array of mics. \$\endgroup\$ – Cloud Sep 30 '14 at 13:20
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    \$\begingroup\$ As noted in the schematic L1 and 2 are coupled as a common mode choke. At the currents you are using the capacitors should be in a decent range, since you want the DC voltage to be as smooth as possible, you can increase the inductances until you reach the crossing of cost and current capability. I'd go for a 300mA DC or 350mA Saturation lower limit in the case of periods of peak use or low input voltage. If the filtering helps but not enough you can double the common mode filter-capacitor stage. Use as low Rdc / ESR components as you can afford. \$\endgroup\$ – Asmyldof Sep 30 '14 at 21:46

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