Analog inputs, Isolation – Nematron OptiLogic Series User Manual
Page 11
Optimation, Inc.
(256)883-3050
11
OptiLogic Series
Analog Inputs
Analog inputs are used to monitor the
value
of
some
continuously
variable
measurement. Typical analog inputs are
measurements of temperature, pressure, weight,
liquid level, pH, flow rate and many other
“real
world
” parameters.
The purpose of an analog input module is
to convert the measurement into a format that is
usable by the data acquisition or control system.
To be usable by an computer-based system, the
analog measurement must be converted to
digital format. Doing so accurately and, in some
cases, quickly, is the goal of the analog to digital
converter module.
A good understanding of analog input
modules includes an understanding of isolation,
accuracy, single and differential inputs,
multiplexing, resolution and range. The
following paragraphs provide an overview of
these subjects.
Isolation
In many applications there is a good deal
of benefit to be derived from isolating the analog
measurement source from the RTU
’s power
supply. In some cases, signal inputs may contain
voltages or noise signals which could adversely
affect the main processor
’s operation. Likewise,
noise on the main power bus can degrade the
accuracy of the analog value measurement.
Both potential problems can be solved by
isolating the analog inputs from the main power
supply.
Isolation involves totally isolating the
analog to digital (A/D) converter from the main
power bus. This can be accomplished in two
ways. The A/D input module can use a separate
power source input, which is isolated from the
power input to the base. The A/D module can
also use the main power supply and isolated
power via a switching power converter and a
transformer. There are OptiLogic analog input
modules in both categories. Neither is
functionally superior to the other. The on-board
power generation may save the cost of an
additional external power supply.
The other aspect of isolation is the fact
that the measured value must be transmitted
from the analog to digital converter, operating
on one power supply, to the main system, which
is operating on another power supply. This is
commonly
accomplished
through
optical
isolators.
Analog
inputs
Converter Module
A/D
Converter
Bus
Interface