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PRECISION 28000
Signal Conditioning
System






 
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28208 Octal Thermocouple Conditioner

The 28208 Octal Thermocouple Conditioner provides eight
channels of conditioning for thermocouples or other DC and AC voltage inputs.
The 28208 consists of a low-drift programmable differential pre-amplifier, a
3-pole programmable filter and a programmable post-amplifier. A rugged remote
isothermal block with digital temperature sensor provides the reference junction
temperature for hardware correction.
28208 APPLICATIONS
Precision 28208 Features
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8 channels per plug-in card
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128 channels per 28016
chassis, 64 channels per 28008 chassis
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Low offset temperature coefficient:
0.5 mV/°C max
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Open thermocouple detection
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Overload detection
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Programmable 3-pole Bessel
low-pass filter
(1, 10, 100 Hz and wideband)
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Precise cold junction
compensation for type
B, E, J, K, N, R, S and T thermocouples
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Automatic adjustment of DC
offset
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Programmable gain: x1, 10,
100, 1000 with
out-band reserve settings of x1, 10, or 100
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Remote isothermal block with digital temperature sensor
Click to enlarge
28208 Description
The 28208 channel consists of a
low-drift programmable differential pre-amplifier, a 3-pole programmable filter
and a programmable post-amplifier. Overall gain may be programmed to x1, 10, 100
or 1000. A programmable reserve setting of x1, 10 or 100 is provided for
protection against out of band signals. For example, a reserve setting of 100
with a gain of 1000 will program the pre-amplifier to 10 and the post amplifier
to 100, allowing the filter to reject out of band energy before all of the gain
is applied.
An open thermocouple condition or
a channel overload condition is automatically detected and reported to the
graphical user interface (GUI) software. Additionally, these fault conditions
are indicated by 28208 front panel LEDs.
A 3-pole programmable Bessel
filter is provided with cutoff frequency settings of 1, 10 and 100. The filter
may be bypassed to provide wide-band operation.
A single-ended output stage is
standard. The 28208 may be optionally fitted with a differential output.
Isothermal Block
The 28208-1-ITB Isothermal Block
provides a reference junction for a single 28208 card (8-channels). Removable
3-pole terminal connectors allow for easy wiring of the thermocouples. The
28208-1-ITB has a 4-foot cable which mates directly with the 28208 input
connector. Extension cables are available to locate the block remotely from the
28000 System. A removable cover provides a more stable environment while
protecting the block.

For accurate cold junction
compensation, the temperature of the isothermal block must be converted to the
thermocouple thermoelectric voltage that would be generated if the junction were
heated to the temperature that is measured. Thermocouple thermoelectric voltage
has a non-linear relationship with temperature that is commonly referred to as
"bowing". If a straight-line approximation to the thermoelectric
voltage versus temperature were used, reference junction errors of over 1° C
would result for a J-type thermocouple and over 2° for a T-type thermocouple.
A digital temperature sensor on
the 28208-1-ITB provides an accurate reading of the block temperature. The block
temperature is interrogated once every 10 seconds. To remove errors caused by
the bowing, a microcomputer in the 28208 sets a cold junction compensation DAC
by calculating the correction voltage for the measured temperature based on NIST
coefficients. Overall reference junction compensation accuracy is better than
0.6 degrees.
28208-2-ITB Isothermal Block Diagram
Download the
Spec Sheet
from the Reference Library
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Reference
The following Specifications
are available in Precision Filters'
On-line Library
for downloading:
28104A Quad Bridge Conditioner Specifications
28208 Octal Thermocouple Conditioner
28302B Dual
Vibration Amplifier
27304 Quad Dual Mode Charge Amplifier
28314 Quad Dynamic Charge/IEPE Conditioner
with LDTEDS
28316B 16-Channel TEDS/IEPE Accelerometer
Conditioner
28454A Quad Conditioner Specifications
28524 Quad Frequency to Voltage Converter
Modules
28608B Octal Programmable Filter Amplifier
27708A Octal
AC/DC Amplifier
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