Chapter 4 Theory of Operation© National Instruments Corporation 4-5 NI PXI-4224 User ManualEffective CMRWhen the frequency of a common-mode signal is known and outside of themeasurement frequency band of interest, you can use an analog or digitalfilter, or both, to further reduce the residual error left from the finite CMRRof the instrument. The combined CMR of the instrument and the filterattenuation results in an effective CMR. When expressed in decibels, theeffective CMR is equal to the sum of the CMRR and the attenuation due tothe filter at a specified frequency.Timing and Control Functional OverviewThe NI PXI-4224 is based on the NI E Series DAQ device architecture.This architecture uses the NI data acquisition system timing controller(DAQ-STC) for time-related functions. The DAQ-STC consists oftwo timing groups that control AI and general-purpose counter/timerfunctions. These groups include a total of seven 24-bit and three 16-bitcounters and a maximum timing resolution of 50 ns. The DAQ-STC makespossible applications such as equivalent time sampling, and seamlesschanging of the sampling rate.The NI PXI-4224 uses the PXI trigger bus to easily synchronize severalmeasurement functions to a common trigger or timing event. The PXItrigger bus is connected through the rear signal connector to thePXI chassis backplane. The DAQ-STC provides a flexible interface forconnecting timing signals to other devices or external circuitry. TheNI PXI-4224 uses the PXI trigger bus to interconnect timing signalsbetween PXI devices, and the programmable function input (PFI) pin onthe front SMB connector to connect the device to external circuitry. Theseconnections are designed to enable the device to both control and becontrolled by other devices and circuits.The DAQ-STC has internal timing signals you can control by an externalsource. These timing signals also can be controlled by signals internallygenerated to the DAQ-STC, and these signals are software configurable.Figure 4-3 shows an example of the signal routing multiplexer controllingthe AI CONVERT CLOCK signal.
