PRECISION 28000
Signal Conditioning
System

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

Static (DC) or dynamic (AC) temperature measurement
Low-level DC or AC filter amplifier (<1 mV to 10 V inputs)

Precision 28208 Features

8 channels per plug-in card
128 channels per 28016 chassis, 64 channels per 28008 chassis
Low offset temperature coefficient: 0.5 mV/°C max
Open thermocouple detection
Overload detection
Programmable 3-pole Bessel low-pass filter
(1, 10, 100 Hz and wideband)
Precise cold junction compensation for type
B, E, J, K, N, R, S and T thermocouples
Automatic adjustment of DC offset
Programmable gain: x1, 10, 100, 1000 with
out-band reserve settings of x1, 10, or 100
Remote isothermal block with digital temperature sensor

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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.