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As a rule medical equipment in contact with the patient should be galvanically isolated from mains. With a notebook PC one can feel light dizzling when simultaneously touching some connector and water pipe, because notebook power supply output is not grounded and there is normally some small leakage currents. One possibility is of course to disconnect notebook from mains and run on batteries.
- Some years ago in photonics lab we were measuring weak light levels and noticed a slight value shift every time voltmeter data were being saved on a PC. This was due to a ground loop caused by hard drive. Optoisolated USB hub helped to solve the ground loop problem, but such hub was somewhat expensive.
- GPS receiver placed on the roof of metrology lab to receive precise timing can potentially be stroken by lightning that could destroy the data logging PC or in worst scenario also other computers connected via Ethernet. So galvanic insulation is desired.
- Audiophiles report that USB sound card galvanically isolated from PC makes better sound with less noise. I haven’t tried this out myself as I think my ears might not be trained well enough. (Came into mind that Hi-Fi DVD players and receivers have optical fiber output to prevent ground loops).
Analog Devices ADuM4160 USB isolator
Analog Devices are producing a chip named ADuM4160 for USB galvanic isolation. It is working bidirectionally at low and full data rate 1.5 Mbps and 12Mbps isolating against up to 25 kV transients. Isolation is done by radio frequency transmission coupling.
This adapter chip ADuM4160 is transparent to PC and does not need any drivers. One can assemble adapter from a kit:
https://www.circuitsathome.com/adum4160-usb-isolator-assembly-guide
I am grateful to AD for sending me test samples. Below you can see my version of the home made USB isolation adapter assembled with transformer wire on a prototyping board.
USB isolator needs a 5 V power for supplying the isolated side. I tried to supply this power from the wireless power kit. Of course, one can correctly argue, that for creating isolated 5 V power one does not need to employ the wireless power kit, but a DC/DC converter would be enough. I just had the wireless power kit around to play with.
Dental camera
I tried to galvanically isolate from a desktop PC a dental USB camera that I recently bought on Ebay. It allows not only doctors but also personally to follow the teeth condition. One puts this camera into the mouth and wouldn’t like to become electrocuted. Test showed that camera connected via the galvanically isolated USB is recognized by PC. Message comes up: “This USB device can perform faster if you connect it to a high speed USB 2.0 port”. Unfortunately, when would like to take a picture, the screen is blank. Apparently the Analog Devices ADuM4160 USB isolator is not transmitting data fast enough.
Next I tried to connect Logitech C270 webcam. It was recognized by the PC, but there was no picture coming.
So these my efforts to galvanically isolate USB cameras from PC were not successful.
GPS receiver
USB isolator is desirable for lightning protection between the Global Navigation System (GPS/GLONAS) receiver board placed on the roof and sending NMEA messages to a metrology lab downstairs via serial/USB communication.
Below is a photo of a zero-budget GNSS receiver module with antenna model GNSS GLONASS GPS Engine Module GNS701 TTL MTK-3333 Navilock is providing NMEA messages via serial port and 1 pulse per second output. I placed it on the roof inside a plastic pipe. Metal sieve shields it from multipass reflections. It is in operation for half a year now. The system needs to run unatended around the clock and reliability is an important issue.
During 1 day the measured coordinates of antenna wander around by ca 20 m.
Live timing and coordinate data are stored online:
http://85.254.232.222/webcamcapture1/
Below is a typical 1 day long plot of 1 pulse per second signal used to calibrate a Rb clock.
GPS receiver allows to calibrate rubidium (Rb) clock frequency drift within ca 100 ns. GPS timing on long term is the most precise as it is synchronized to worlds best clocks, but GPS signal can have bad days when solar activity causes larger deviations. And not only that. For example, on April 2 one could observe large jumps when GLONAS system was malfunctioning for a whole day due to uploaded wrong corrections to satellites. This topic was covered also in the news. Rb clock is operating all the time with less short-term noise than GPS, but is suffering from aging drift.
Picture above shows that it is possible to galvanically isolate the GNSS receiver and Qi Wireless Power Kit is producing enough power to supply the GNSS receiver with 5V. This modification did not influence GNSS data noise and scatter.
Next blog entry: 06 Ordering parts from Farnell / Element 14