AppendixB DigitizerBasics
NI5911 User Manual B-2 ni.com
Analog BandwidthAnalog bandwidth describesthe frequency range (in Hertz) in which
asignal can be digitized accurately. This limitation is determined by
the inherent frequencyrespon se of the input path which causes loss of
amplitude and phase information. Analog bandwidth is the frequency at
which the measured amplitude is 3 dB belowthe actual ampli tude of the
signal. This amplitude loss occurs at very low frequencies if the signal is
ACcoupled and at very high frequencies regardless of coupling. When the
signalis DC coupled, the bandwidth of the amplifier will extend all the way
tothe DC voltage. Figure B -2 illustrates the effectof analog bandwid thon
ahigh-frequency signal. The result is a loss of high-frequency components
and amplitude in the original signal as the signal passes through the
instrument.
FigureB-2. AnalogBandwidth
Sample RateSample rate is the rate at which a signal is sampled and digitized byan
ADC. According tothe Nyquist theorem, a higher sample rate produces
accuratemeasurement of higher frequency signals if the analog bandwidth
is wide enough to let the signal to pass through without attenuation. A
highersample rate also captures more waveform details. Figure B-3
illustrates a 1 MHz sine wavesampled by a 2 MS/s ADC and a 20 MS/s
ADC. The faster ADC digitizes 20 points per cycle of the input signal
compared with 2 points percycle with the slower ADC. In this example,
the higher sample rate more accurately captures the waveformshape as
wellas frequency.
Bandwidth
+2V
+1V
0V 0V
+1/2V
–1/2V
–1V
–2V
InputSignal Instrument MeasuredSignal
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