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Measurement Considerations
© National Instruments Corporation
sampling and requires higher acquisition rates than are possible with SS/H
enabled. Refer to the
Configuring Channel Properties
Developing Your
Application, for more information about
programmatically enabling and disabling SS/H in LabVIEW.
Maximum Acquisition Rate
Table 4-3 shows the maximum acquisition rates for the NI PXI-4220 when
scanning one or two channels, and with SS/H enabled or disabled.
Table 4-3. Maximum Sampling Rates
Number of
Channels
1
2
This section provides more information on the type of signal connection
made to the NI PXI-4220 and important factors that can affect your
measurement.
Differential Signals
Both of the analog inputs of the NI PXI-4220 are differential. In general,
a differential measurement system is preferable because it rejects not only
common-mode voltages and ground loop-induced errors, but also the noise
picked up in the environment to a certain degree.
Common-Mode Rejection Ratio
The ability of a measurement device to reject voltages that are common to
both input pins is referred to as the common-mode rejection ratio (CMRR)
and is usually stated in decibels at a given frequency or over a given
frequency band of interest. Common-mode signals can arise from a variety
of sources and can be induced through conductive or radiated means. One
of the most common sources of common-mode interference is due to 50 or
60 Hz powerline noise.
The minimum NI PXI-4220 CMRR is 85 dB at gains ≥20. This results in
0.006% of the CMV introduced as error on the measured signal.
SS/H Enabled
100.0 kS/s
66 kS/s/ch
4-31
Chapter 4
Theory of Operation
section of Chapter 5,
SS/H Disabled
333 kS/s
100.0 kS/s/ch
NI PXI-4220 User Manual
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