Precision Filters, Inc. today introduced its latest product for its 28000 Series product line, the 28208A. Providing eight channels of highly accurate analog signal conditioning for all standard thermocouple types, the card is ideal for static (DC) or dynamic (AC) temperature measurement applications.
Each 28208A channel consists of a gain-distributed low-drift differential preamplifier, 4-pole Bessel low-pass filter, and post-amplifier. Gain is programmable to x1, 10, 100 or 1000, with out-band reserve settings of x1 or 10. Filter cutoff frequency is programmable to 1, 10, 100 Hz or 10 kHz wideband. An open thermocouple condition or channel overload condition is automatically detected and reported to the GUI software as well as the card front-panel LEDs. The card may also be employed as a low-level DC or AC filter amplifier to condition other DC and AC voltage inputs (<1 mV to 10 V inputs).
Precise cold (“reference”) junction compensation for type B, E, J, K, N, R, S and T thermocouples is enabled via the separate, remotely locatable 28208A-1-ITB isothermal block, which provides a reference junction for four 28208A cards (32 channels). The 1U rack-mountable block incorporates a high-accuracy digital temperature sensor for each card, which repeatedly interrogates the reference junction temperature, providing this to its associated card. In order to properly reference the thermocouple, it is necessary to add the voltage corresponding to the measured reference junction temperature to the measured thermocouple voltage. However, the Seebeck coefficient relating temperature and voltage differentials is inherently non-linear. To accommodate this non-linearity, a microcomputer in the 28208A calculates—by means of a high-order polynomial approximation based on NIST coefficients—the proper reference junction correction voltage for the measured reference junction temperature such that the resultant thermocouple voltage is properly referenced to the 0 °C standard.
Given the inherent low-voltage character of thermocouple measurement, it is critical that the applied conditioning minimizes errors, whether of offset or gain, to the conditioned signal. With respect to reference junction compensation, overall accuracy is better than 0.15 °C. The DC offset and gain are automatically adjusted and corrected at any ambient temperature. The careful minimization and management of these various sources of potential error yields an overall signal conditioning temperature accuracy of better than 0.5 °C for all thermocouple types and temperatures and better than 0.25 °C for many thermocouple types and temperatures of interest.
For more information on the 28208A thermocouple conditioning card and the 28208A-1-ITB isothermal block, please visit our Thermocouple Conditioner page.