Chapter2 HardwareOverview
©NationalInstruments Corporation 2-3 NIPXI-562x User Manual
Conditioning the Signal—Impedance, Dither,Gain, and AC Coupling
Tominimize 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 DC
coupled,m eaning its wider passband acquires DC signal components also.
Both versions of the NI 562xdigitizer module have a set input impedance
of 50 Ωand may apply dither to the input signal.
Input Impedance
Theinput impedance of the NI 562xand the output impedance of the source
connectedto the NI 562xform an impedance divider, which attenuates the
inputsignal according to the following formula:
where Vmis the measured voltage
Vsist he unloaded source voltage
Rin is the input impedance of the NI 562x
Rsisthe output impedance of the external device
Ift he signal you are measuring has an output impedance other than 50 Ω,
yourmeasurements are affected by this impedance divider. For example,
ifthe device has 75 Ωoutput impedance, your measured signal has 80%
of the voltageit would have at 50 Ω.
Dither
Dither is random noise added to the input signal between 0 and 5 MHz.
Dither lowers the amount of distortion caused by differential nonlinearity
inthe ADC when a signal is digitized. When an FFT is applied t o the signal,
thisrandom noise cancels out most of the distortion created by differential
nonlinearity. Dither is not automatically applied, but you can enable it in
software.
Digitizing the Signal—The ADC
Regardlessof your requested sample rate, the NI 562xADC is always
running at 64 MS/s. If you request a rate less than 64 MS/s, the timing
engine of the NI 562xstores only one sample in a group of nsamples,
effectivelyreducing the sample rate to 64/nMS/s.
VmVs
Rin
Rin Rs
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