Lowpass Filter - NEC mPD7755 User Manual

4.2 Lowpass Filter
Signals that have been digitally analyzed and synthesized
by a sampling method can only reproduce frequency ranges
up to one half the sampling frequency. Frequencies above
this range appear as unwanted noise and if the D/A output
is directly amplified, the result would be a poor quality
sound with an extremely unfavorable SiN ratio. It is
therefore necessary to use a lowpass filter that will
only allow signal frequencies of less than half the
sampling frequency to pass.
A filter with sharp bandpass characteristics is desirable
and ideally a filter with damping characterics of
48dB/oct or better should be used. The complexity of such
a filter, however, is prohibitive and we have opted, in
the interest of simplicity and operability, for a filter
with characterisics of 24dB/oct for use in the ~PD7755 /
7756 . Fig. 4-4 shows a
~PC358C
analog filter with
Butterworth damping characteristics (24dB/oct) that can
operate on a +5V single power supply.
~PD7755.
~PD775G
RL
Rl
Fig. 4-4
Vee V~
Lowpass Filter Circuit Example
FILTER
OUT
When the ~PC358C operates on +5V, input signals in a
range of 0 to 3.5V may be used. Resistors Rl and R2
should therefore be used to set the operating point to
2.5V.
Table 4-1 shows the appropriate constants when sampling
frequencies (f SMPL ) of 4, 5, 6, and 8kHz are used.
4-5