The Industrial Feature Set: Optical Isolation

This document is part of a comprehensive tutorial on industrial Digital I/O and Counter/Timer hardware. Learn about National Instruments product offerings for digital and timing I/O, the Industrial Feature Set including watchdog timers and isolation, complementary devices like relays, solenoids and encoders, concepts like sinking and sourcing, and see how these devices can be used in your industrial application.

For more information return to the Complete Industrial Digital I/O and Counter/Timer Tutorial

What is Optical Isolation?

Optical isolation is a form of built-in signal conditioning that provides several advantages. Isolation provides an extended voltage range and allows direct connection to industrial sensors and actuators. Isolation also improves signal quality and protects computer circuitry.

Recommended Products that Feature Optical Isolation

NI 6514
NI 6515
NI 6528

How Optical Isolation Works

Isolation physically and electrically separates two parts of a circuit. This breaks ground loops, improves common-mode voltage and noise rejection, and permits the two parts of the circuit to be at different voltage levels. Many industrial applications require isolation to protect the electronics from transient voltage spikes and provide greater common mode noise rejection in electrically noisy environments containing machinery and inductive loads.

In the case of channel-to-channel isolated I/O, each channel has its own ground terminal. In bank-to-bank isolated devices, each bank (or group) of 8 channels share the same ground but are isolated from other banks.

Consider a pump, an emergency stop switch and a control valve connected to a single digital I/O module. In the absence of isolation (see figure 1), all three devices must be directly connected to one another at the ground reference, causing a ground loop due to voltage differences.


Figure 1: Non-isolated connection


This flow of current can cause damage to pump and valve circuitry. In addition, incorrect readings can occur due to the increased load on the signal outputs of the devices. With a bank-isolated digital module (figure 2), the devices can operate with their own ground reference, even with the large differences in voltage.



Figure 2: Bank-isolated connection

Reader Comments | Submit a comment »

information is incomplete
What kinds of signals are optically isolated? Digital only?
- Craig Landefeld. LandefeldC@comcast.net - Jan 18, 2007