Nexys4 DDR™ FPGA Board Reference Manual
Copyright Digilent, Inc. All rights reserved.
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Figure 29. Sallen-Key Butterworth Low-Pass 4
th
Order Filter.
The frequency response of SK Butterworth Low-Pass Filter is shown in Figure 30. The AC analysis of the circuit is
done using NI Multisim 12.0.
Figure 30. SK Butterworth Low-Pass Filter frequency response.
16.1 Pulse-Width Modulation
A pulse-width modulated (PWM) signal is a chain of pulses at some fixed frequency, with each pulse potentially
having a different width. This digital signal can be passed through a simple low-pass filter that integrates the digital
waveform to produce an analog voltage proportional to the average pulse-width over some interval (the interval is
determined by the 3dB cut-off frequency of the low-pass filter and the pulse frequency). For example, if the pulses
are high for an average of 10% of the available pulse period, then an integrator will produce an analog value that is
10% of the Vdd voltage. Figure 31 shows a waveform represented as a PWM signal.
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TUD
E (
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FREQUENCY (HZ)
Stage II
Stage I
Overall
