Chapter2 HardwareOverview
©NationalInstruments Corporation 2-3 NI5620 User Manual
Conditioning the Signal—Impedance, Dither,Gain, and AC Coupling
Tominimize distortion, signals receive a minimal amount of conditioning.
There is one set gain, and all signals are AC coupled, meaning that the
NI5 620 rejects any DC portion of a signal. The NI 5620 also has a set input
impedance of 50 Ωand applies dither to the configurable signal.
Input Impedance
Theinput impedance of the N I 5620 is 50 Ω. The output impedance of the
sourceco nnected to the NI 5620 and the input impedance of the NI 5620
forman impedance divider, which attenuatesthe input signal according to
the following formula:
where Vmis the measured voltage
Vsist he unloaded source voltage
Rsisthe output impedance of the external device
Rin is the input impedance of the NI 5620
Ifthe source whose output you arem easuring has an output impedance
other than 50 Ω,your measurements will be affectedby this impedance
divider.For example, if the device has 75 Ωoutput impedance, your
measured signal will be 80% of the valueit would have been at 50 Ω.
Dither
Ditheri srando m 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 5620 ADC is always
running at 64 MS/s. If you request a rate less than 64 MS/s, the timing
engine of the NI 5620 stores only 1 sample in a group of nsamples,
effectivelyreducing the sample rate to 64/nMS/s.
VmVs
Rin
Rin Rs
+
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