White Noise from LDO

[hgg]

Hmmm, sounds like a good idea.  I will try it.

[Rupunzell]

 

Put a current probe on the USB ground wire and conductors that power the circuit. 


Bernice

Then the noise isn't from +5V, it's from USB_GND.

Tim

[T3sl4co1l]

Then the noise isn't from +5V, it's from USB_GND.

Tim

Can you please explain the reasoning behind that conclusion?

(The USB ground is the same as the mains ground and I can measure 33.5mVDC potential
from mains earth wall socket nearest to the PC)

Thank you.

"GND" is just a wire, it can have any potential on it.  Just because you measure a DC potential (a frequency of 0Hz, obviously not the frequency that is interfering with your circuit!) near zero, doesn't mean it's "ground" in some cosmic absolute sense!  Indeed, you seem to have proven that it is not: 33mV is hardly zero, even as DC goes!

Indeed, it's most likely picking up voltage drops across internal circuitry, before leaving the computer via the USB connector.  All that switching noise and stuff then loops back through your circuit, and you've got noise.

Tim

[hgg]

I see.  So there is definitely a ground loop problem.

What are the best solutions for that?

Will an isolated DC-DC converter solve the problem ? 
http://www.mouser.com/ds/2/281/kdc_meu1-267600.pdf
It might introduce some switching noise but maybe that will be more easily filtered(?)

Thank you.

[T3sl4co1l]

I posted this earlier:

One way to break the ground loop would be, to add a small resistor (perhaps 10-100 ohms) in series with the output connector ground.  This will drop some voltage when a load is connected (for example, if this circuit were used to drive headphones instead of driving another audio input), but you can compensate for this, and for the difference in ground loop voltage, by ground-referencing the mic input and op-amp feedback nodes to this ground, rather than power ground.

Tim

[hgg]

You are right, ok I will try that.
Thanks again!

[hgg]

It seems that there is a solution to the USB power noise problem.

 iUSBPower  http://ifi-audio.com/portfolio-view/micro-iusbpower/


A 9V battery has 0.6uV noise and they claim 0.1uV !

I have not succeeded to lower USB noise any further and may be this is why... :
  "To create a USB power supply that is ultra-quiet is an engineering feat in its own right."

Below is the PCB, nicely labeled:



All PCBs should be labeled like that I think.  Neat.

I don't see how they solve the ground isolation they claim. 
There is a switch  for the ground management. 
Maybe they are swapping the ground from the USB to the DC regulated 9V ground (?)

I think they are cheating and have not actually managed to filter the 5V USB power,
since they are using an external regulated clean 9V power supply.  Maybe they are
using the 9V to power only  the "Ultra Low Noise Super Regulator" chip.

Anyway, nice design but there is a small problem.  It costs $230...   

I think I will stick with the 9V battery.
0.6uV of noise, for free.   

George.

[wraper]

Anyway, nice design but there is a small problem.  It costs $230...   

Problems I see on this pcb are bullshit claims on the silkscreen and marketing wank. Also they made "impedance matched high speed data path"... on what seems to be 2 layer PCB considering track width.
Also:

Our engineers developed the advanced IsoEarth technology specifically for the iUSBPower. By breaking the noisy DC ground connection between the computer and your USB audio device, this further reduces the ground noise by a factor of 10. Your USB audio device can now operate in the cleanest environment possible; allowing your music to flow.

Hell yeah, BULLSHIT. And what with USB signal integrity may I ask?

[hgg]

I also thought that something was not exactly right...   

[wraper]

They do have even more ridiculous product in it's claims http://ifi-audio.com/portfolio-view/accessory-ipurifier/ 

The iPurifier does one job and one job only; it cleans and filters audio + power at the end of the digital chain; just prior to the digital signal entering the DAC .

The specially-designed circuit restores the ‘signal waveform’ and puts the ‘analogue’ back into USB audio for a true, life-like presentation.

Future-proofed, it is able to handle high-resolution PCM/DSD and DXD at all levels. A: all DACs gain a sonic uplift.

Like if there is something else other than USB data packets in the USB cable.

[T3sl4co1l]

Looks to be audiophool wank... pay it no mind.

Nothing wrong with the impedance matched traces, though I have to wonder: are they too close together?  USB spec includes both common mode and differential impedances.

Tim