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Old 20th June 2015, 03:53 AM   #1
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Default USB DAC - need some help with grounding

Hi, I'm developing a device - it's actually not a USB DAC, but a digital synthesizer with USB input / power. But hopefully the same principles will apply as to USB DAC development.

The circuit consists of a lot of digital circuitry, which I've given a "digital ground" plane, and then the analog circuitry (including a DAC IC), which has an "analog ground" plane. The two grounds are connected together with a thin trace - I've seen this recommended.

My main question is about how the USB shield should be connected to ground. I began by leaving it disconnected. The circuit behaves wonderfully, with no noise or hum, under a lot of scenarios (e.g. device powered by USB from laptop, and output going to an active monitor speaker).

However, under some scenarios, a grounding issue makes itself apparent as a loud hum. This happens if I have my laptop additionally connected to my old VGA monitor-screen. Clearly there's then some sort of ground loop problem happening.

Thankfully, I've noticed that this problem can be almost resolved by connecting the USB shield to analog ground. The noise almost goes dead. Connecting it to digital ground, however, does not have the same effect (it quietens the noise just slightly).

So - is that what one would normally do - connect the USB shield to analog ground? Should I be connecting it through a ferrite or anything (I've seen these recommended in some places)? And, while we're at it, am I doing the right thing by having my separate analog and digital grounds directly connected via a thin trace?

My device's metal chassis is connected to digital ground.

Many thanks in advance!
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Old 20th June 2015, 04:21 AM   #2
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Assuming that the USB screen's connected to 0V at the computer end then the ideal grounding would be the configuration which ensures common-mode currents aren't carried anywhere near to sensitive (analog) audio circuits. More accurately that common-mode currents don't share any impedance with tracks defining your analog signal ground.

You don't make mention of 'earth' but for safety reasons your metal chassis should be bonded to mains earth*. Mains earth will be the 'sink' for the common-mode currents emanating from a laptop's PSU brick.

* assuming you've not adopted classII insulation practices.
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Last edited by abraxalito; 20th June 2015 at 04:23 AM.
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Old 20th June 2015, 05:26 AM   #3
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Thanks for the reply.

My device I'm building is USB-powered only, so there is no earth. The chassis is connected to digital ground, which is connected to USB ground.

I'm afraid I don't quite understand your point regarding common-mode currents. Is this a concern around radio-frequency interference between nearby circuit traces? I assumed that this was not the problem, since the humming only occurs when connected to certain peripheral equipment as noted in my first post. Or am I missing something?
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Old 20th June 2015, 08:57 AM   #4
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I was making an assumption that you had a mains-fed power supply. OK now I'm clear that's not the case. However there's still a possibility of there being a mains earth in the system somewhere - not on the laptop but perhaps you're feeding the output of the USB DAC into an amplifier which is earthed?

Common-mode currents will flow (because you've a source of common-mode noise in the system, the laptop's power supply) back to earth if there is an earth. If the only earth is the amplifier then you'll get those currents flowing down your audio interconnects (between DAC and amp). But perhaps your amp is just a pair of active speakers (normally classII and hence no mains earth) in which case common-mode currents are only a major issue when your old VGA monitor's connected - presumably that must have a mains earth. With no earth in the system the next route for the noise to flow is across the capacitance between primary and secondary of your transformers.

No, the concern's not about RFI between circuit traces, just about common ground impedance coupling.
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Last edited by abraxalito; 20th June 2015 at 09:01 AM.
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Old 12th December 2016, 06:32 PM   #5
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Hello, I see that it is old thread, but I don't want to create new one. So now I am building dac and have problems how to ground all power supply pcb's and dac board with controlers. Do I need to connect all boards to one point and then to chassis or leave 0V floating like in the drawing? My amplifier chassis is grounded, but all after secondaries is floating, but I will connect chassis to power supply star ground with series thermistor.
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