Chapter 11 Isolation and Digital Isolators

The isolated ground is an input to the USB-6215/6218 device. The user must connect this ground to the ground of system being measured or controlled. Refer to Chapter 5, Connecting AI Signals on the USB-6210/6211 Devices, Chapter 6, Connecting AI Signals on the USB-6215/6218 Devices, the Connecting Analog Output Signals section of Chapter 7, Analog Output, the Connecting Digital I/O Signals section of Chapter 8, Digital I/O, and the Connecting PFI Input Signals section of Chapter 10, PFI, for more information.

Digital Isolation

The USB-6215/6218 uses digital isolators. Unlike analog isolators, digital isolators do not introduce any analog error in the measurements taken by the device. The A/D converter, used for analog input, is on the isolated side of the device. The analog inputs are digitized before they are sent across the isolation barrier. Similarly, the D/A converters, used for analog output, are on the isolated side of the device.

Benefits of an Isolated DAQ Device

With isolation, engineers can safely measure a small signal in the presence of a large common-mode voltage signal. Some advantages of isolation are as follows:

•Improved rejection—Isolation increases the ability of the measurement system to reject common-mode voltages. Common-mode voltage is the signal that is present or “common” to both the positive and negative input of a measurement device, but is not part of the signal to be measured.

•Improved accuracy—Isolation improves measurement accuracy by physically preventing ground loops. Ground loops, a common source of error and noise, are the result of a measurement system having multiple grounds at different potentials.

•Improved safety—Isolation creates an insulation barrier so you can make floating measurements while protecting the USB host computer against large transient voltage spikes.

Reducing Common-Mode Noise

Isolated products require an isolated power supply to deliver power to the isolated side from the non-isolated side. Isolated power supplies work by switching voltages through a transformer with high-speed transistors.