This is a circuit that can generate 1kHz to 25kHz sine waves with a THD better than -80dB. This circuit consist of a 4th-order low-pass filter, a TTL counter and 8-channel analog multiplexer. Here is the circuit :
The bipolar dc input of multiplexer is provided by two resistive dividers connected from ground to VDD and ground to VSS. The multiplexer is used to produces the clock’s input frequency. To implement the 4th-order lowpass filter, this cricuit cascades the two 2nd-order, continuous-time Sallen-Key filters within IC3.
The filter’s cutoff frequency, fC, can be obtained by tying IC3’s D0 through D6 inputs to 5V or ground. The filter’s cut off of this circuit is 1 KHz. The output level is adjusted by the 100k potentiometer between VSS +1.5V and VDD -1.5V. The filter’s fC must be eight times lower than the clock’s input frequency. It is greatly simplifies the smoothing requirements of the low-pass filter by pushing the first significant harmonic out to seven times the fundamental. IC3 is used to remove all higher-order harmonics. [Source: maxim-ic.com]