National Instruments 9211 - Manual (Page 10)

USB-9211/9211A User Guide and Specifications 10 ni.com
Figure 8. Input Circuitry for One Channel
If thermocouples are connected to the USB-9211/9211A, the gain and
offset errors resulting from the source impedance of the thermocouples are
negligible for most applications. Other voltage sources with a higher source
impedance can introduce more significant errors. For more information
about errors resulting from source impedance, refer to the
Specifications
section.
Determining Temperature Measurement Accuracy and
Minimizing Errors
Temperature measurement errors depend in part on the thermocouple type,
the temperature being measured, the accuracy of the thermocouple, and the
cold-junction temperature.
Using the Autozero Channel
The USB-9211/9211A has an internal autozero channel for measuring the
offset error. If the ambient temperature of the USB-9211/9211A is less than
15 °C or more than 35 °C, use this channel to read the offset error. The
USB-9211 always has this channel enabled. Use NI-DAQmx to configure
the behavior of the autozero channel for the USB-9211A. Refer to the
NI-DAQmx Help, accessible from Start»All Programs»National
Instruments»NI-DAQ, for more information.
TC+
TC–
COM
USB-9211/9211A
Input
Impedance
Open
Thermocouple
Detection
Current
Filtered
Differential
Amplifier
Isolated
ADC
10 M Ω
10 M Ω