Photo 1. Photo 3 is actually a pretty
good sine wave. The measured distortion is only a few percent. Not too bad
for four resistors and four capacitors.
Note that the amplitude of the
final filtered square wave is about 0.5
volts. This is only 10% of the original
amplitude of 5.0 volts. It becomes
clear that filtering with passive components reduces the amplitude.
Another problem with this
approach is that it is frequency
specific (about 1,500 Hz here). A
higher input frequency will result in
smaller final amplitude. A lower input
frequency will not be filtered as well
and will not be as good a sine wave.
However, if all you need is a fixed
frequency sine wave, this method can
be very effective.
A S Fiwct ltii etvce rsheFdiltCearipnagcaitnodr
PHOTO 3. After four stages, (output
C) a pretty good sine wave is created.
Distortion is only a few percent.
between the filter cutoff frequency
and the clock/switching frequency.
This is usually 50 or 100. What this
means is that the clock frequency is
generally exactly 50 or 100 times
the cutoff frequency. (For example,
for a filter with a cutoff frequency of
1 kHz, the clock would have to be
50 kHz or 100 kHz.) So, you can
tune or sweep your filter over a
wide range of frequencies very
easily. The number of additional
components needed to implement
this approach can be very small (see
However, probably the strongest
feature of these filters is that they can
have extremely sharp cutoffs. This
means that they can remove the
higher harmonics of a square wave
very effectively which leaves a pure
sine wave. The “order” of the
complete filter shown in Figure 1 is
four. That is, with four capacitors it can
reduce the higher frequencies at a rate
that is four times a single capacitor
Figure 2 incorporates an eighth
order filter, and filters up to the 10th
order are available from Linear
Technology. (Maxim is another manufacturer and National Semiconductor
You can certainly use active filters PHOTO 4. A switched capacitor filter
(filters incorporating an op-amp) to produces a stepped waveform because
of the sampled nature of the circuit.
convert a square wave to a sine
wave. They have the advantage of
maintaining the amplitude of
the signal but are fairly complicated
and require a number of additional
components. They are also limited to
a fixed frequency response. This
restricts their usefulness.
Switched capacitor filters are a
form of an active filter. However, by
switching capacitors on and off, the
parameters of the filter can be modified by changing the switching rate.
This makes the filter extremely useful
for digital applications for a number of
reasons. The first (already mentioned)
is that the filter can be modified by
changing the switching frequency.
Another is that the switching signal is
usually a digital clock with typical
logic levels. This is easy to generate
and control from a digital circuit.
There is also a direct relationship
PHOTO 5. A simple RC filter after the
switched capacitor filter creates a very
good sine wave with very low distortion.
FIGURE 2. A switched capacitor filter
may only need an input anti-aliasing
filter and an output smoothing filter to
July 2007 81