Specifications

6 Maxim Industrial Solutions
Programmable logic controllers (PLCs)
Analog input function
Overview
The analog input portion of a PLC
accepts analog signals from a variety
of sensors and factory or field wiring.
These sensors are used to convert
physical phenomena such as light,
temperature, sound, gas, or vibration
from mechanical signals into
electrical representations. In the
analog-input signal path, signals are
conditioned for maximum integrity,
range, and resolution before being
sampled by the analog-to-digital
converters (ADCs). In the industrial
environment common to PLCs, there
is a wide variety of signal levels,
signal bandwidths, and noise
sources. It is, therefore, essential to
reject as much of the irrelevant
information as possible. Equally
important, the maximum amount
of relevant information must be
retained when the signals are
converted from the analog to the
digital domain.
The PLC’s analog input accepts
voltage and current inputs from
remote sensors. Voltage inputs can
have different amplitudes, the most
common of which are either 0 to
10V, or 0 to 5V, ±10V, or ±5V. The
most popular current-input standard
is 4–20mA, although ±20mA is
sometimes used. Despite its name,
the 4–20mA standard accepts
0–24mA both to detect an open
input (< 3.6mA) and overrange
(> 20mA), and to allow headroom
for calibration. To guarantee that
the current loop is never broken, the
current input is typically terminated
into a relatively low-value resistor
(e.g., 50Ω to 250Ω) prior to the signal-
conditioning analog chain.
The signal chain
Various implementations of the
signal chain are possible, with
simultaneous-sampling ADCs and
independent conditioning amplifiers,
or with a multiplexer as the first stage
followed by a common amplifying
signal path into an ADC, or with
individual amplifying channels and
a multiplexer prior to the ADC. The
input stage is commonly required
to cope with both positive and
negative high voltages (e.g., ±30V
or higher). This protects the PLC’s
analog-input card from external fault
conditions and lets the input module
accommodate variable common-
mode voltages on the long lines
that connect to the remote sensors.
Low-temperature drift and low noise
are also critical requirements of the
analog signal path. The errors at
+25°C are typically calibrated out in
software. The drift over temperature
can also be removed, although it is
not removed in many systems and
thus becomes a critical specification.
Analog-to-digital conversion
Standard PLC designs typically
require a high-accuracy ADC. The
bandwidth of the input signal
Analog input function
ADC
ADC
AC VOLTAGE/CURRENT
TRANSFORMERS (TIMES 6)
ANALOG FRONT-END (AFE) FILTERS & RESISTORS
ESD/SIGNAL
PROTECTION
SENSOR SIGNAL
CONDITIONER
SWITCHED
C FILTERS
VOLTAGE
REFERENCE
THERMAL
MANAGEMENT
ISOLATED POWER SUPPLY
VOLTAGE
MONITORS
DIGITAL
POTENTIOMETER
PRECISION
RESISTORS
RFI/EMI
FILTERS
BUFFER
TO CPU
MODULE
ISOLATION
SIMULTANEOUS
SAMPLING*
ACTIVE FILTERS
= MAXIM SOLUTION
OP AMP OR
INSTRUMENTATION AMP
FROM ANALOG
SENSORS AND
FIELD WIRING
MUX
*Designers can choose among multiple ADCs for this function.
Maxim’s extensive product offerings are found throughout this block diagram of PLC analog-input functions.
For a list of Maxim's recommended PLC solutions, please go to: www.maxim-ic.com/plc.