Case Studies > Signal Conditioning Technical Note
There are four very broad applications of signal conditioning:
Elimination of Ground Loops and AC Electrical Noise
Ground loops can occur where there are multiple current return paths or multiple connections to ‘earth ground’. Ground loops cause problems by adding or subtracting a noise current or voltage from the process signal. The measuring system only sees the effected signal and so returns an inaccurate or unstable reading.
Putting a signal isolator between the earthed devices breaks the galvanic path (dc continuity) between the grounds but allows the analogue signal through. In addition common mode voltages (ac continuity) generated by ac noise can also be rejected leading to an electrically ‘clean’, accurate signal being sent to the measuring instrument.
Earth loops can occur where more than ‘earth ground’ exists, causing inaccurate signals
As well as providing isolation between input and output signal conditioners are used to change an incoming signal into the form required by the control or monitoring system. For example a PLC may require 0-10Vdc input from a field instrument which generates a 4-20mA signal. An isolating signal converter can be used to both maintain and integrity of the 4-20mA signal whilst also providing an isolated 0-10Vdc output for the PLC. As well as changing the signal type some conditioners can linearise the incoming signal from, say a thermocouple and provide an output which is proportional to temperature. Other linearising functions available are square-root extraction for flow measurements using a pressure drop and linearisation for tank contents where the tank content is not linear with tank level.
Transmitter Sharing and Signal Boosting
It is quite common for one process transmitter, say a fluid temperature transmitter, to be connected to several different instruments, such as a temperature controller a chart recorder and a DCS. Signal isolators can be used to generate extra drive capability for an existing loop or to generate an extra loop, which can be adjusted using zero, and span potentiometers without affecting the existing loop. Our Dualcon unit creates two isolated outputs from a single input, both with independent zero and span adjustment makiing it ideal for this application.
Bucking Power Supplies
This occurs when an existing loop and the measuring instrument are both attempting to power the same loop. A suitable isolator will accept power on both input and output and provide isolation between the two. A typical example would be where a PLC with 24Vdc transmitter excitation needs to be connected to a transmitter, which is line powered.
We manufacture a wide range of signal converters and signal isolators including the popular Isocon-6 unit which has a universal input configured using DIP switches. For our complete range see the Signal Conditioning section of the website.
Here's a PDF of some of the typical applications for Industrial Interface's range of Signal Conditioners together with diagrams of how our products can be used in these applications.
I hope you find this useful and would welcome any comments you may have.