Specifications

6 Control and Automation Solutions Guide
Overview
The analog input portion of a PLC
accepts analog signals from a variety of
sensors via factory or eld wiring. These
sensors are used to convert physical
phenomena (such as light, temperature,
pressure, proximity, sound, gas, or
vibration) 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). The analog input
module receives many dierent signals
in the tough industrial environment. It is,
therefore, essential to lter out as much
of the noise and retain as much of the
crucial information as possible when the
signals are converted analog to digital.
The PLC’s analog input accepts voltage
and current inputs from remote sensors.
Voltage inputs can have dierent
amplitudes, the most common of which
are either 0 to 10V, 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 typically terminates
into a relatively low-value resistor
(typically from 25 to 250) 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 ampliers, or with a mux
at the rst stage followed by a common
amplifying signal path into an ADC, or
with individual amplifying channels and
a mux 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.
Errors at +25°C are typically calibrated
in software. Drift over temperature
can also be removed, although it is
not removed in many systems and
thus becomes a critical specication.
Analog-to-Digital
Conversion
Standard PLC designs typically require
a high-accuracy ADC. The bandwidth
of the input signal dictates the ADC’s
maximum sampling rate. The signal-
to-noise ratio (SNR) and spurious-free
dynamic range (SFDR) specications
dictate the ADC’s resolution, ltering
requirements, and gain stages.
It is also important to determine how the
ADC will interface to the microcontroller
or CPU. For example, high-bandwidth
applications perform better using a
parallel or fast serial interface. SPI with
unidirectional signaling oers easy
galvanic isolation to reduce ground
loops in the plant. Galvanic isolation can
be accomplished via optical, capacitor,
or transformer coupling. I
2
C’s two-line
digital interface can also be used, but it is
best for slower systems where the PLC’s
signal is used within a small area and
galvanic isolation is not needed since it
is dicult to isolate bidirectional lines.
Analog Input Functions
ADC
ADC
AC VOLTAGE/CURRENT
TRANSFORMERS (TIMES 6)
ANALOG FRONT-END (AFE) FILTERS & RESISTORS
ESD/SIGNAL
PROTECTION
CALIBRATION
SWITCHED
C FILTERS
VOLTAGE
REFERENCE
THERMAL
MANAGEMENT
POWER SUPPLY
VOLTAGE
MONITORS
DIGITAL
POTENTIOMETER
PRECISION
RESISTORS
EMI/RFI
FILTERS
BUFFER
TO CPU
MODULE
ISOLATION
SIMULTANEOUS
SAMPLING
ACTIVE FILTERS
= MAXIM SOLUTION
OP AMP OR
INSTRUMENTATION AMP
FROM ANALOG
SENSORS AND
FIELD WIRING
MUX
Maxim’s extensive product offerings are found throughout this block diagram of PLC analog input functions.