USB DAC isolation ?

[jkeny]

wwenze, I think most/all of the noise that originates in the PC comes through on the V+ & Ground lines. Can the differential uSB signals D+ & D- be left as straight through connections & the Ground & V+ be fed from an external source? Does this not effect the differential signal level not having the reference ground from the PC any more?

Soundcheck, you use an Opticics, have you looked inside? All signals go through opt-couplers?

[wwenze]

You already said it's a differential signal, so it does not need a common reference ground.

However i think this is pointless though, since any DAC with its own power supply will be using that to power the USB anyway instead of through the computer. I made the cable because something was causing my USB-powered DAC to crash so I decided to isolate its power from the computer. It still crashed though.

[jkeny]

wwenze, I called D+ & D- a differential signal but I'm not even sure that it truly is differential, are you? There is another mechanism at play in PCM27XX based DACs that needs to sense the Vbus as a connection signal & this would be effected by what I've suggested. I don't know if there are similar mechanisms in other DAC chips.

The point is that if the ground is still connected it acts as a conduit for the noisy PC ground to bleed through to the DAC

Have you done this? - D+ & D- connected directly to PC but not ground & VBus (taken these from external PS)?

[wwenze]

Yes I have done that - D+ D- connected to PC, VBus and ground taken from a power adapter. And that is a PCM270x based DAC.

[jkeny]

I'm back to this again - I'm afraid wwenze situation seems different from mine as he seemed to say that his DAC was powered by it's own supply & he only isolated the Vdd & gnd because of noise but this wasn't powering his DAC anyway.

So here's what I've done - I cut into the USB cable & pulled out the +5V & gnd wires (red & black), cut them. Left the PC side ones floating (& insulated) & connected the DAC side red to pos of a 5V battery & black to neg of this battery. Now my question - I connected the shield wire & foil straight through but didn't connect the battery ground to this. Before I connect to the DAC, I wanted to check it first.

Effectively I've isolated the Vdd & Gnd on the PC from the DAC - the only issue is the shield wire. Should I also cut the shield wire & connect it to the battery gnd or is it OK?

[berni8k]

Do NOT disconnect the gnd from the PC ! the D+ and D- signals are referenced to GND. Without a gnd it could float up to a high voltage along with the data lines and fry the USB port. If you are scared of noise on the gnd line at least connect the PCs GND trough a 10Ohm or so resistor.

Shield is supposed to be connected to ground at the PC side.

[jkeny]

Thanks Berni8k, I suspected that - but as wwenze says are D+ & - floating anyway or is there a compliance issue on these lines? So I should connect my battery ground to the ground from the PC & also the shield?

I asked this early in the thread about the Opticics & how it worked - is the PC ground connected straight through? - I thought it galvanically isolated the PC?

[glt]

Check this out: http://pinouts.ru/Slots/USB_pinout.shtml

[wwenze]

What you did is almost the same as what I did, except instead of connecting to a 5V battery I connected to an adapter. My DAC is USB-powered, think I mentioned that in my second post.

[jkeny]

Thanks glt, but does it answer the ground question? I'm still not sure whether differential signals need a reference - my logic says they don't but I don't want to risk a DAC or PC testing it.

Here's a quote which seems to answer it: http://www.beyondlogic.org/usbnutshell/usb2.htm

As we have discussed, USB uses a differential transmission pair for data. This is encoded using NRZI and is bit stuffed to ensure adequate transitions in the data stream. On low and full speed devices, a differential ‘1’ is transmitted by pulling D+ over 2.8V with a 15K ohm resistor pulled to ground and D- under 0.3V with a 1.5K ohm resistor pulled to 3.6V. A differential ‘0’ on the other hand is a D- greater than 2.8V and a D+ less than 0.3V with the same appropriate pull down/up resistors.

The receiver defines a differential ‘1’ as D+ 200mV greater than D- and a differential ‘0’ as D+ 200mV less than D-. The polarity of the signal is inverted depending on the speed of the bus. Therefore the terms ‘J’ and ‘K’ states are used in signifying the logic levels. In low speed a ‘J’ state is a differential 0. In high speed a ‘J’ state is a differential 1.

USB transceivers will have both differential and single ended outputs. Certain bus states are indicated by single ended signals on D+, D- or both. For example a single ended zero or SE0 can be used to signify a device reset if held for more than 10mS. A SE0 is generated by holding both D- and D+ low (< 0.3V). Single ended and differential outputs are important to note if you are using a transceiver and FPGA as your USB device. You cannot get away with sampling just the differential output.

This seems to say that both differential & SINGLE-ENDED signals are passed over D+ & D- so a gnd is needed in the SE case, I guess?

I was hoping somebody had trodden this path already.