Digital isolation, Benefits of an isolated daq device, Reducing common-mode noise – National Instruments DAQ M Series User Manual
Page 108
Chapter 9
Isolation and Digital Isolators
9-2
ni.com
Connecting Digital I/O Signals
section of Chapter 8,
for more information.
Digital Isolation
The NI 6236 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 current 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 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.
Switching voltages through the transformer causes charging and
discharging of the parasitic capacitances and inductances in the switching
power supplies that occur on every switch cycle, resulting in high-speed
currents flowing through the isolated side and returning to the non-isolated
side, which is earth ground.