Our two previous columns discussed USB-DAQ software, timing and triggering. This column wraps up the USB topic with a few aspects of USB connections engineers might not think about.
Most USB connectors provide two signal contacts, a ground and a +5V connection. That means your DAQ equipment shares a common ground with a host PC and other USB-based equipment, such as other DAQ or control devices, a printer, scanner and so on. Unfortunately, in most experiments or tests, the last thing you want is this common-ground connection, which can lead to ground loops. Past columns have covered grounding and ground loops, so there's no need to provide the details here. Search Google for USB and “ground loop” and you'll find many cries for help.
You cannot simply snip the ground signal in a USB cable to “break” a ground loop because the USB data signals need a ground reference. Thus, isolation must occur within your DAQ equipment between its I/O lines and an internal USB transceiver. (Data sheets should include isolation spec). If you suspect ground-loop problems in installed equipment, you can buy USB isolators and some USB hubs also offer electrical isolation. In addition to preventing ground-loop problems, isolated DAQ equipment protects your computer and other devices from accidental short circuits and high voltages at a USB-DAQ module. Suppose a 120V ac power line touches a sensor input. Isolation devices keep that ruinous potential from reaching your PC.
If you plan to use USB-DAQ modules, consider buying units that isolate your computer's USB ground from the local ground at your DAQ equipment. That means you should not try to power DAQ modules from the USB's +5V supply and you — or your DAQ-equipment vendor — must provide a separate power supply. Ensure that supply does not create its own ground loop!
In some cases, you must use +5V power from a USB port. In theory, you can draw up to 500 mA from a USB port, but you shouldn't try to draw more than 100 mA from a battery-powered host laptop. You may have seen instructions on USB equipment that caution against connecting it to USB-powered hubs. That's because USB cables and devices experience voltage drops and hubs use power. The official USB 2.0 specifications include a voltage-drop budget (see figure, above). The farther you go from your actual power source — a host PC or powered hub — the greater the possible voltage drop. In some cases, you can measure just over 4V at the end of a long USB run through a hub. Voltage drops can exceed those in the USB spec and affect DAQ measurements. Transients also can affect remote voltages and ground potentials can vary, too. When possible, use local power with USB-DAQ devices.
|Jon Titus, a former designer and chief editor of EDN and Test & Measurement World magazines, remembers when “fast” signals operated at 10 MHz and programs came on paper tape.