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Offset Seen Using Open TC Detection

Hardware: PXI/CompactPCI>>Controllers

Problem:
When I use open TC detection with TC-2095 and SCXI 1100, I see a offset of 2.5 VDC. How can I eliminate this?

Solution:
Open-thermocouple detection circuitry can cause two types of measurement errors:

Common-mode voltage at the input of the SCXI module
Current leakage into the signal leads

Common-Mode Voltage at the Input of the SCXI Module

With 10 MΩ pull-up and bias resistors, a common-mode voltage of 2.5 VDC develops if the thermocouple is floating. At a gain of 100, the common-mode rejection of the SCXI-1102/B/C module is sufficiently high that the resulting offset error is negligible.

If the application demands extremely high accuracy, you can eliminate this offset error by:

Calibrating the system with both the pull-up and ground-referencing resistor switches ON
Turn off the pull-up resistor switch, which eliminates the open-thermocouple detection feature
Use the 10Ω ground-referencing resistor networks, which brings the common-mode voltage down to nearly 0 VDC

Current Leakage

The open-thermocouple detection circuitry results in a small current leakage into the thermocouple.

With the 10 MΩ bias and pull-up resistor networks, the current leakage results in a negligible error
With the 10 Ω bias resistor, the 10 MΩ pull-up resistor connected to 5 VDC causes a current leakage of approximately 0.5 μA (5 VDC/10 MΩ) to flow into the unbroken thermocouple

For more information p.16 of the manual :
Product Manuals: BNC/TC-2095 Rack-Mount Adapter Installation Guide and Specifications

Related Links:
KnowledgeBase 2Y57ST7Y: Open-Thermocouple Detection
BNC/TC-2095 Rack-Mount Adapter Installation Guide and Specifications

Attachments:

Report Date: 01/12/2009
Last Updated: 06/03/2009
Document ID: 4TB5UUHV

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