National Instruments NI PXI-562X User Manual

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National Instruments NI PXI-562X User Manual Manual pdf 17 page image

Chapter 2 Hardware Overview© National Instruments Corporation 2-3 NI PXI-562x User ManualConditioning the Signal—Impedance, Dither, Gain, and AC CouplingTo minimize distortion, signals receive a minimal amount of conditioning.Gain and coupling are nonadjustable. The NI PXI-5620 is AC coupled,meaning it rejects any DC signal components. The NI PXI-5621 is DCcoupled, meaning its wider passband acquires DC signal components also.Both versions of the NI 562x digitizer module have a set input impedanceof 50 Ω and may apply dither to the input signal.Input ImpedanceThe input impedance of the NI 562x and the output impedance of the sourceconnected to the NI 562x form an impedance divider, which attenuates theinput signal according to the following formula:where Vm is the measured voltageV s is the unloaded source voltageR in is the input impedance of the NI 562xR s is the output impedance of the external deviceIf the signal you are measuring has an output impedance other than 50 Ω,your measurements are affected by this impedance divider. For example,if the device has 75 Ω output impedance, your measured signal has 80%of the voltage it would have at 50 Ω.DitherDither is random noise added to the input signal between 0 and 5 MHz.Dither lowers the amount of distortion caused by differential nonlinearityin the ADC when a signal is digitized. When an FFT is applied to the signal,this random noise cancels out most of the distortion created by differentialnonlinearity. Dither is not automatically applied, but you can enable it insoftware.Digitizing the Signal—The ADCRegardless of your requested sample rate, the NI 562x ADC is alwaysrunning at 64 MS/s. If you request a rate less than 64 MS/s, the timingengine of the NI 562x stores only one sample in a group of n samples,effectively reducing the sample rate to 64/n MS/s.V m V sR inR in R s+-------------------  ×=