Specifications

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Programmable logic controllers (PLCs)
Analog input function
dictates the ADC’s maximum
sampling rate. The signal-to-noise
ratio (SNR) and spurious-free
dynamic range (SFDR) specifications
dictate the ADC’s resolution, filtering
requirements, and gain stages. It is
also important to determine how
the ADC will interface to the micro-
controller or CPU. For example,
high-bandwidth applications
perform better using a parallel or
fast serial interface. With its two-line
digital interface, however, I
2
C is ideal
for slower systems. When the results
from the analog-input measurement
are transferred through a 4–20mA
loop, PLC designers can choose
between an ADC with a separate
digital-to-analog converter (DAC),
an integrated DAC that can drive
the 4–20mA lines directly, or a high-
voltage op amp configured as a
precision current source. Applications
that require extraction of phase
information between channels are
well suited for multiple ADCs or
simultaneous-sampling ADCs.
Although PLCs are used in distinct
ways, many PLC designs share some
common factors. For example, the
most used ADCs and DACs are 16
bits. Maxim offers over a hundred
16-bit ADCs and DACs for a wide
range of input and output voltages,
and this broad product offering
is a distinct advantage for the
PLC designer. Consider a situation
where using sensors with varying
accuracies could dictate the need
for three ADCs with 12-, 14-, and
16-bit resolution. But to reduce cost
and complexity, it may be best to
discard bits for some sensors and
utilize the higher resolution only
where it is needed. In this case, a
designer may choose to multiplex
the analog signals to a differential
input amplifier or programmable
gain amplifier (PGA) into a single
16-bit ADC.
When choosing a multiplexer, sensor
reaction speed must be considered.
This means that a designer needs
to determine the input bandwidth
and how quickly the switches will be
opening and closing. Slow-response
sensors measuring signals such as
temperature and humidity can be
sensed every few seconds. Faster
changes like speed, position, and
torque typically need to be sensed at
least thousands of times per second.
Similarly, on the output side DACs
can be multiplexed depending on
how often the outputs must be
serviced to maintain control.
Signal conditioning and
calibration
There are many design challenges
when selecting the analog-input
signal-path components. The inputs
to the multiplexer and the ADC
require analog signal conditioning
such as filtering; converting currents
to voltages; and changing gain, offset,
impedance, and bias. Caution must be
taken both to anticipate the expected
voltage amplitude and signal polarity,
and to understand the unexpected
like unwanted voltage or current
transients. Maxim provides a wide
selection of operational amplifiers,
instrumentation amps, PGAs, precision
resistors, filters, references, ADCs, and
multiplexers to aid the PLC design.
Calibration improves system per-
formance and increases accuracy
(see chapter titled "Trim, calibrate,
and adjust" on page 143. The MAX9939,
a PGA with an SPI™ interface, is
ideally suited for a thermocouple
application as it provides the needed
level-shifting circuitry to signal
condition both negative and positive
sensor signals. The MAX9939’s inputs
provide ±16V transient protection to
prevent damage to the PLC system.
Multiplexers (muxes) are useful for
switching multiple input channels. A
mux that meets high-voltage-supply
requirements (up to ±35kV) or is
fault protected against overvoltage
conditions, can help eliminate
expensive external circuitry such as
voltage-dividers and opto relays.
A low, matching on-resistance
(R
ON
) is essential for low distortion
to improve circuit reliability, and
low-leakage currents are critical for
minimizing voltage-measurement
errors. Maxim‘s product portfolio
includes more than 15 fault-
protected/high-voltage, low-
leakage, and low-R
ON
muxes
ideal for PLC applications.
The designer will choose the physical
position for the signal-conditioning
circuits. That placement may require
the sensor signal to be conditioned
before it is transmitted to the
input ADC.
The sensor’s output can be very small
or very large, which would require gain
or attenuation stages (respectively) to
maximize the ADC’s dynamic input
range. These conditioning stages are
usually implemented with PGAs or
discrete op amps and precision
resistor-dividers. The ADC and
amplifier work in tandem to achieve
the best signal-to-noise ratio (SNR)
within the cost, power, and size
budgets. Another alternative is to use
an ADC with the conditioning stages
integrated. Regardless of how the signal-
conditioning stages are implemented,
the voltage range, low-temperature
drift, and low noise are among the
most critical specifications when
determining the best architecture.
The industrial environment presents
numerous noise sources, such as
50HZ/60Hz power-line mains which
get coupled into the signal. These
unwanted noise signals put an arti-
ficial limit on the gain stages and