Specifications

7PLCs
Applications that require extraction of
phase information between channels
are well suited for multiple ADCs or
simultaneous-sampling multichannel ADCs.
Although PLCs are used in distinct ways,
many PLC designs share some common
factors. For example, ADCs and DACs
used in process-control systems range
from 10 to 16 bits with 16 bits being
one of the most common resolutions.
Maxim oers many choices from 10-bit
to 24-bit ADCs for a wide range of input
voltages. This broad product oering is a
distinct advantage for the PLC designer.
For precise systems, ADCs (and DACs)
require an accurate voltage reference.
The voltage reference can be internal
or external to the data converter. In
addition to many ADCs and DACs
with internal references, Maxim
has stand-alone voltage references
with temperature coecients as
low as 1ppm/°C, output voltage as
accurate as ±0.02%, and output noise
as low as 1.3µV
P-P
that can be used
external to the data converter for
ultimate precision and accuracy.
The number of data converters can
often be kept low by using muxes to
choose from several signal sources
and programmable gain amps (PGAs)
to maximize the ADC dynamic range
for each signal. Of course this method
does not preserve phase information
between the signal sources. When
choosing a mux, the sensor’s output
bandwidth must be considered. The
designer needs to determine how
often the sensor must be measured.
Sensors responding to slowly changing
parameters such as temperature and
humidity can usually be read every
few seconds to capture the needed
information, but sensors detecting
quickly changing properties like pressure
or proximity may need to be measured
thousands of times per second.
Signal Conditioning
There are many design challenges
when selecting analog-input signal-
path components. Input analog signal
conditioning could require sensor
biasing, converting currents to voltages,
selecting from multiple signal sources
signal amplication at potentially
adjustable gains, and anti-alias ltering.
Maxim provides a wide selection
of muxes, operational ampliers,
instrumentation amps, PGAs, precision
resistor dividers, lters, references, and
ADCs to solve these design challenges.
Maxim provides high-precision high-
voltage analog front-end operational
ampliers that increases system accuracy
and performance. The MAX44251, for
example, oers dual-channel precision
signal conditioning. Operating from
split supplies of either ±5V or ±10V,
the MAX44251 oers low-noise
performance, 5.9nV/√Hz at 0.1Hz,
making it ideal for RTD applications.
Selecting a mux can be an involved
process since it is very close to the
harsh industrial environment. A mux
that meets high ESD ratings or is fault
protected against overvoltage can help
eliminate expensive external circuitry
such as voltage-dividers and opto relays.
Low matching on-resistance (R
ON
) and
low-leakage currents are essential for low
distortion to improve circuit accuracy
and precision by minimizing voltage-
measurement errors. Maxim’s product
portfolio includes ESD-protected, high-
voltage-protected, low-leakage, and low-
R
ON
muxes ideal for PLC applications.
The designer will choose the physical
position for the signal-conditioning
circuitry. 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 ampliers
work in tandem to achieve the best
SNR within budgets for cost, power,
and size. An 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 specications when
determining the best architecture.
The industrial environment presents
numerous noise sources, such as
50Hz/60Hz powerline mains, which
get coupled into the signal. These
unwanted noise signals put an articial
limit on the gain stages and should
be rejected beforehand. This is best
accomplished using Maxim’s PGAs
or dierential ampliers with a high
common-mode rejection ratio (CMRR).
Maxim has a variety of laser-trimmed,
matching resistor-dividers for precise
gain and attenuation. There are also
trimmable calibration potentiometers
for system self-calibration, and
ADCs with dierential inputs and
PGAs integrated in a single IC.
Lowpass or bandpass ltering before
the ADC sampling network is necessary
for anti-aliasing requirements and
for rejecting noise sources at other
frequencies. PLC designers have a choice
between active lters implemented
with op amps or switched-capacitor
lters with a very sharp (up to
8-pole) roll-o and a programmable
cut-o frequency. Maxim provides
a selection of 5th- and 8th-order
switched-capacitor and continuous-
time lters ideal for anti-aliasing.