National Instruments NI 9234 - Manual (Page 11)

© National Instruments Corp. 11 NI 9234 Operating Instructions and Specifications
Figure 4. NI 9234 Input Circuitry for One Channel
The NI 9234 uses common-mode bias current to bias the
current-limiting diodes when IEPE current is turned off. When the
NI 9234 is using grounded signal sources, this current causes an
error that is dependent on the AI– lead impedance. This error is
approximately 50 ppm of range and 15 ppm of reading per
Ω of
AI– impedance. The common-mode bias current causes an error
only with grounded sources and is not an issue with floating signal
sources. For best accuracy, use a floating connection or use
low-impedance leads when connecting grounded signal sources.
2 mA IEPE on/off
AI+
AI–
Amplifier
and
Prefilter
ADC
+
–
AC/DC Coupling
NI 9234
Common
Mode
Bias
Current Current
Limiting
Diodes
50 Ω