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Maxim > Design Support > Technical Documents > Application Notes > Signal Generation Circuits > APP 21
Keywords: Three ICs Produce Pure Sine Waves
APPLICATION NOTE 21
Three ICs Produce Pure Sine Waves
Jul 09, 1998
A TTL counter, an 8-channel analog multiplexer, and a 4th-order lowpass filter can generate 1kHz to
25kHz sine waves with a THD better than -80dB (Figure 1). The circuit cascades the two 2nd-order,
continuous-time Sallen-Key filters within IC
3
to implement the 4th-order lowpass filter. Two resistive
dividers connected from ground to V
DD
and ground to V
SS
provide bipolar dc input to the multiplexer.
Figure 1. This circuit produces a pure, -80dB THD sine wave whose frequency equals fC of the IC3 filter.
To operate the circuit, you must first choose the filter's cutoff frequency, f
C
, by tying IC
3
's D
0
through D
6
inputs to 5V or ground. The cutoff frequency can be at 128 possible levels between 1kHz and 25kHz
depending on those seven digital input levels. Because Figure 1 ties D
0
through D
6
to ground, f
C
equals
1kHz. The 100kΩ potentiometer adjusts the output leve between V
DD
-1.5V and V
SS
+1.5V.
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