Four Channel Linearized
Thermocouple
Description
The Model 5120 Signal Conditioner is a four-channel module
that provides voltage amplification, non-linearity correction,
and cold-junction compensation for multiple thermocouple types.
Incorporating a “lookup” table approach for real
time signal correction, any transducer signal can be converted
real time into equivalent engineering units. Correction tables
for common thermocouple types and ranges are integrated into
each 5120 module. Although designed for the linearization of
thermocouple inputs with cold junction compensation, the flexible
configuration of the 5120 amplifier provides the capability
of dynamic sensor calibration, including temperature compensation,
when utilizing the cold junction temperature sensor to monitor
transducer temperature.
Design Features
The 5120 uses an ultra stable low noise instrumentation amplifier
and digital signal processing to convert the millivolt output
of non-linear thermocouples to a linear output voltage. Included
is a programmable input voltage offset feature allowing specific
ranges of interest to be offset prior to amplification for more
detailed measurements. Programmable Low-pass filtering between
0.01Hz and 10kHz can be used to reduce system noise, and a programmable
calibration voltage source is available for establishing baseline
temperature values before and after a test is run. Front-panel
LEDs indicate signal presence and warn of over-temp conditions
or module operational problems. All circuitry is housed in a
shielded enclosure for improved
Input Amplifiers
The primary thermocouple signal is first fed to a low-noise
instrumentation pre-amp and then followed by a variable gain
amplifier. A programmable offset voltage is injected after the
pre-amp stage allowing for amplified measurement of offset temperatures
without forcing a zero degree reference. For example, the 5120
output of 0 to 10 volts could be set to span 500 to 1000°C.
The gain of this amplifier is automatically set by an on-board
DSP depending on the thermocouple type and temperature measurement
range selected.
DSP - Linearization
The Digital Signal Processor is the heart of the 5120
module. Following input signal amplification and offset adjustment,
the analog signal is then digitized by a 16 bit high-speed Sigma-Delta
converter. As the string of digitized values is fed to the DSP,
an 8K lookup table is used to translate any arbitrary non-linear
input data string into an error corrected digitized output.
Processing includes real time error correction based upon internal
amplifier calibration values, digital low pass filtering and
addition of any “cold junction compensation” correction
input from the secondary A/D converter. Lookup tables are included
for common temperature and thermocouple ranges along with the
ability to download a user calibration table. This “lookup
table” approach avoids computation limitations normally
associated with “Nth order polynomial” approximations.
Output Circuit
The processed digital output is converted back to
an analog voltage by a high-speed 16-bit Digital-to-Analog Converter.
A four-pole low-pass filter/buffer-amp removes the digitizing
steps in the reconstructed signal, along with any high-frequency
noise. As with the input circuit, temperature-stable components
are used to ensure that system calibration holds over a wide
temperature range
Cold Junction Compensation
Thermocouples measure the temperature difference between
the point where the two thermocouple wires connect to each other
(the “hot” junction) and the point where they connect
to the amplifier copper wiring (the “cold”junction).The
thermocouple table of voltage values assume that the cold junction
is at 0°C. If this is not the case (and it rarely is), the
cold-junction temperature must be measured and added to the
thermocouple reading. The 5120 uses a cold-junction temperature
sensor, amplifier, and
A/D converter to determine the cold junction temperature with
the results sent to the Digital Signal Processor for inclusion
in the final result.
System Calibration
High accuracy is obtained during the conversion process
by implementing a unique end-to-end calibration scheme within
the 5120 Converter. A precision programmable voltage generator
is connected to the input, and two calibration voltages (0v
and 80% of full-scale) are fed in, amplified by the input stage,
converted by the A/D, processed by the DSP, converted back to
analog by the D/A, filtered by the output filter, and then measured
by a high-accuracy 24-bit A/D converter. The input and output
voltages are compared, and gain and offset correction values
are computed and saved in the DSP memory. When data is being
collected, these correction factors are applied to each thermocouple
data point in real time, resulting in amplifier accuracy better
than ±0.05% of full-scale.
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5120
Specifications
Schematic
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