I've built an audio adapter for car stereo system. It contains microcontroller, a connector to stereo system (a few data pins and line-level audio output), Bluetooth audio module (RN52) and power connector.

I've successfully tested it with a separate, floating power supply, and audio noise falls within my expectations.

However, when I power the adapter with a power supply which is also referenced to audio amplifier/speaker system, I am hearing significant noise. This happens in car, but I also hear the same issue on my desk, if I power the PCB from PC's USB port, and speaker system is also referenced to my PC.

I kind of suspect grounding issues on my PCB. When designing it, I did not pay a lot of attention to routing GND pins of the modules/ICs used- I relied on having ground fills on both sides of the PCB.

This is a high-level overview of the PCB I made:

Option A

I recall having heard about "star ground" concept, where all the ground points should connect in a single place, power input connector.

enter image description here

The questions are:

1) How applicable the star grounding is if there are multiple connectors (each with a separate ground wire), which may, or may not be referencing a "common" ground somewhere further.

2) Is it possible to improve noise with a different/proper grounding on my PCB, or should I isolate either the audio output or power input?

Images of the actual PCB layout (warning, hairy stuff):

  • how are you grounding your circuit in your car? Try a very short inch or two wire connected right to the chasis. – Some Hardware Guy Nov 2 '14 at 2:07
  • Ground wire comes from unknown source- originally it powered navigation unit, which I duly replaced with this adapter. Will try short and thick ground wire to chassis though. – fest Nov 3 '14 at 23:22
up vote 2 down vote accepted

The noise is caused by ground loops. Wikipedia has a nice article about them:

https://en.wikipedia.org/wiki/Ground_loop_%28electricity%29

A floating supply avoids creating a ground loop, but when you power your board from a supply referenced to audio ground, it creates a ground loop through the head unit wiring and power connector wiring.

1) How applicable the star grounding is if there are multiple connectors (each with a separate ground wire), which may, or may not be referencing a "common" ground somewhere further.

It won't help since the ground loops that are causing problems are external to your board.

2) Is it possible to improve noise with a different/proper grounding on my PCB, or should I isolate either the audio output or power input?

Isolating the audio output or the power input would solve the noise issues by eliminating the ground loop. There are also several other ways to solve or minimize ground loop problems.

Here are a couple of ideas for your situation:

  1. Bundle your power wiring together with your head unit wiring and connect your power GND to the chassis as close to the head unit as possible. This will minimize the differences in magnetic fields that your power GND and audio GND are exposed to.
  2. Assuming that the head unit wiring has a good GND and your board only takes a few milliamps (about 200mA or less), connect its power as follows:

schematic

simulate this circuit – Schematic created using CircuitLab

The values shown for R1, L1, and C1 are just a starting point and should be tailored to your board. R1 can be included in L1 (DC resistance of L1). R1 should be as large as possible without causing too much voltage drop or power dissipation, so its value depends on how much current your board requires. Increasing C1 will decrease the audio noise coupled from the board's power. Increasing L1 will decrease noise coupled from the car's +12V and from the board's power, but larger inductors cost more for a given current rating.

If you go this route, check to be sure that at least one of the following conditions are met:

  1. The head connector's GND makes a connection before the signal lines
  2. +12 is never connected when the head unit is being connected or disconnected
  3. Neither the head unit or your board would be damaged by +/- 12V on the signal lines
  • Assuming that the head unit wiring has a good GND and your board only takes a few milliamps Why just a few milliamps? The power rail for the car audio is normally sized to 10A or more. My car for example (99 camry) has two. A 20Amp for Speaker/Amp power and a 7.5A for Accessory/Player power. You can pull more than a "Few" milliamps. – Passerby Nov 2 '14 at 9:02
  • If the board takes a lot of current, the voltage dropped across the head unit's GND wire(s) will be large. The amplitude of noise coupled to the audio in this way would also be large (unless the current demand is very steady), so this "solution" wouldn't help. – ken Nov 2 '14 at 9:21
  • Well, what do you consider to be an acceptable max limit on current in this situation? TBH the board probably has less than 100mA max draw. – Passerby Nov 2 '14 at 9:32
  • It depends on how noisy the board's power is (how much its current demand varies with respect to time), how much noise you can tolerate in your audio, and how large a capacitor you can afford. Beyond a point, it would be cheaper and better to use an audio ground loop isolator or an isolated power supply. – ken Nov 2 '14 at 9:42
  • 1
    I edited the schematic to add R1 and L1 (to improve filtering and prevent noise from coupling from the car's +12V). Smaller capacitors may now work, but the cost of the inductor would increase at high currents. – ken Nov 2 '14 at 11:15

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