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Conditioning the Signal—Impedance, Dither, Gain, and AC Coupling
Digitizing the Signal—The ADC
© National Instruments Corporation
To 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 DC
coupled, meaning its wider passband acquires DC signal components also.
Both versions of the NI 562x digitizer module have a set input impedance
of 50 Ω and may apply dither to the input signal.
Input Impedance
The input impedance of the NI 562x and the output impedance of the source
connected to the NI 562x form an impedance divider, which attenuates the
input signal according to the following formula:
where V
is the measured voltage
m
V
is the unloaded source voltage
s
R
is the input impedance of the NI 562x
in
R
is the output impedance of the external device
s
If 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.
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
in 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 differential
nonlinearity. Dither is not automatically applied, but you can enable it in
software.
Regardless of your requested sample rate, the NI 562x ADC is always
running at 64 MS/s. If you request a rate less than 64 MS/s, the timing
engine of the NI 562x stores only one sample in a group of n samples,
effectively reducing the sample rate to 64/n MS/s.
R
in
×
V
=
V
------------------ -
m
s
R
+
in
2-3
Chapter 2
Hardware Overview
R
s
NI PXI-562x User Manual
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