National Instruments NI USB-9234 User Manual page 13

NI USB-9234 Circuitry
© National Instruments Corporation
The NI USB-9234 analog input channels are referenced to chassis ground
through a 50 Ω resistor. To minimize ground noise, make sure that the
chassis ground is connected to earth ground. Each channel is protected
from overvoltages. You can configure the NI USB-9234 for AC or DC
coupling for each channel. You can turn the IEPE excitation current on or
off in AC-coupled mode (in software). IEPE current is not available in
DC-coupled mode. The input signal on each channel is buffered,
conditioned, and then sampled by a 24-bit, Delta-Sigma ADC.
AI+
AI–
Figure 9. Input Circuitry for One Channel
The NI USB-9234 uses common-mode bias current to bias the
current-limiting diodes when IEPE current is turned off. When 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.
Signal
Source
+
–
Figure 10. Measurement Error Introduced by Common-Mode Bias Current
2 mA IEPE On/Off
AC/DC Coupling
Current-
Common-
Limiting
Mode
Diodes
Bias
50 Ω
Current
AI+
AI– Lead
Impedance
AI–
13 NI USB-9234 User Guide and Specifications
+
ADC
–
Amplifier
and
Prefilter
NI USB-9234
NI USB-9234
Common-
Mode
Bias
Current