[sdiy] USB isolator chip from Analog Devices; rugged USB panel-mount socket

[Robin Whittle]

In the thread "LED current source needed for 'digital' outputs?" Achim
mentioned the desirability of isolating a USB interface to avoid ground
loops.

I think this is necessary for two reasons:

  1 - To avoid a ground connection between the audio device (or any
      audio device in the entire system) and a PC or laptop.   These
      may have a strong (low impedance) ground noise with respect to
      the ground pin of their mains plug and/or they may have
      AC coupled noise from their ungrounded power adaptor, due to
      both the switching nature of the adaptor and the capacitors which
      are installed between active and neutral mains pins and the DC
      output, for the purposes (I think) of meeting EMI emission
      regulations.

  2 - To avoid excessive voltages (static electricity and/or the just
      mentioned capacitive coupling from the mains) damaging the
      USB interface.  This will likely destroy the microcontroller
      chip.

I have been using this remarkable device from Analog Devices:


http://www.analog.com/en/interface-isolation/digital-isolators/adum4160/products/product.html

I follow Figure 5 almost exactly, including with the 22 ohm resistors
(they specify 24 ohms).  The left side is to a socket which goes to the
PC, which powers it with +5 volts, which goes to VBUS1.  The right side
goes to the USB interface of the audio instrument, and must get +5 volts
from there as well for VBUS2, which is not present on the normal socket
of that device.

As shown in the diagram, pin 4 (PDEN) is connected to the regulated 3.3
volts at pin 3.  Not shown are:

   So is pin 4 (SPU).

   The grey vertical track shown between pin 16 (VBUS2) and
   pin 14 (VDD2 = regulated 3.3 volts) is erroneous - don't connect
   these, assuming you are powering it from 5 volts.

   Connect pin 13 (SPD) and 12 (PIN) to pin 14

I made a little SMD board for this and have used them with great success
in Devil Fish modified TB-303s with the Quicksilver 303 CPU replacement
system, which has a USB interface.  (Likewise for the Quicksilver 606
for the TR-606.)

The chips are available from Farnell/element14/Newark.  It supports USB
2.0 with 1.5Mbps (low speed) and 12Mbps (full speed).  It does not use
optical isolation - instead it used five very small transformers
integrated onto the two chips, which transact ~1ns pulses.

There is an excellent rugged solid metal, sealed against liquid, Mini-B
USB socket from Amphenol Canada, the MUSB-B551-04:

  http://www.firstpr.com.au/rwi/parts/panel-mount-usb-sockets/

  - Robin