NI USB-9211/9211A User Guide and Specifications
14
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NI USB-9211/9211A Circuitry
The NI USB-9211/9211A channels share a common ground that is isolated
from the chassis and the host computer. Each channel has an impedance
between the TC
+
and COM terminals and between the TC– and COM
terminals. Each channel is filtered and then sampled by a 24-bit
analog-to-digital converter (ADC). There is a current source between the
TC+ and TC– terminals. If an open thermocouple is connected to the
channel, the current source forces a full-scale voltage across the terminals.
Effects of Source Impedance on Voltage Measurement Accuracy
The resistors shown in Figure 9 produce an input impedance at the
terminals of the NI USB-9211/9211A.
Figure 9.
Input Circuitry for One Channel
If thermocouples are connected to the NI 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.
TC+
TC–
COM
NI USB-9211/9211A
Open
Thermocouple
Detection
Current
Filtered
Differential
Amplifier
Input
Impedance
Isolated
ADC
