Chapter 4
Theory of Operation
©
National Instruments Corporation
4-31
NI PXI-4220 User Manual
Effective CMR
When the frequency of a common-mode signal is known and outside of the
measurement frequency band of interest, you can use an analog or digital
filter, or both, to further reduce the residual error left from the finite CMRR
of the instrument. The combined CMR of the instrument and the filter
attenuation results in an effective CMR. When expressed in decibels, the
effective CMR is equal to the sum of the CMRR and the attenuation due to
the filter at a specified frequency.
Timing and Control Functional Overview
The NI PXI-4220 is similar to the NI E Series DAQ device architecture in
many ways. This architecture uses the NI data acquisition system timing
controller (DAQ-STC) for time-related functions. The DAQ-STC consists
of two timing groups that control AI and general-purpose counter/timer
functions. These groups include a total of seven 24-bit and three 16-bit
counters and a maximum timing resolution of 50 ns. The DAQ-STC makes
possible applications such as internal hardware timing or multiple-point
acquisitions, equivalent time sampling, and seamless changing of the
sampling rate.
The NI PXI-4220 uses the PXI trigger bus to easily synchronize several
measurement functions between multiple NI PXI-4220 devices to a
common trigger or timing event. The PXI trigger bus is connected through
the rear signal connector to the PXI chassis backplane. The DAQ-STC
provides a flexible interface for connecting timing signals to other devices
or external circuitry. The NI PXI-4220 can also use the PXI trigger bus
to interconnect timing signals between PXI devices and the programmable
function input (PFI) pin on the front SMB connector to connect the device
to external circuitry. These connections are designed to enable the device
to both control and be controlled by other devices and circuits.
The DAQ-STC has internal timing signals you can control by an external
source. These timing signals can also be controlled by signals internally
generated by the DAQ-STC, and these selections are software configurable.
Figure 4-18 shows an example of the signal routing multiplexer controlling
the AI CONV CLK signal.
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