Ultra-Accurate... By Design
This type of isolation offers the best means of protecting the equipment and also preserving the accuracy of measurement. It provides a physical space, i.e. no connection, between the sensor source and the actual measurement equipment. USB offers many advantages in ease of measuring signals, but has a common ground between the sensor and the PC. The best approach is to isolate these grounds through galvanic isolation. The DT9800 series offers galvanic isolation as a standard design approach. Additionally, these modules offer the rigorous certification to CE standards. CE tests modules to 4kV contact voltages with no disruption to the applications program.
Much care must be exercised in using non-isolated USB modules. The environment must be such that no spikes or ground loops are possible in the system. Voltage spikes above the common mode voltage of the module can lock-up the PC through the ground connection almost instantaneously. Additionally even minor differences in voltage potential of as little as several millivolts between signal sensor and the USB module can cause inaccuracies that make 16-bit data acquisition modules exhibit accuracies of only 8 to 10-bit systems. This is due to the ground loop of the USB connection (USB connections allow 5 meter wire length). Without isolation potential users must be aware of these inaccuracies. For example, only 0.1 ohm cable impedance and 100 mV of ground error, will result in 1 ampere of current flow. This can cause the application to produce gross errors and lock-up. Suitable embedded systems and isolated housings must accommodate a non-isolated module.