DAC8408

–14–

REV. A

Figure 11. A Digitally Programmable, Low-Distortion Sinewave Oscillator

A LOW-DISTORTION, PROGRAMMABLE

SINEWAVE OSCILLATOR

By varying the previous state-variable filter topology slightly,

one can obtain a very low distortion sinewave oscillator with

programmable frequency feature as shown in Figure 11. Again,

DAC B and DAC D in tandem control the oscillating frequency

πRC. Positive feedback is

accomplished via the 82.5 k

Ω and the 20 kΩ potentiometer.

The Q of the oscillator is determined by the ratio of 10 k

Ω and

475

Ω in series with the FET transistor, which acts as an auto-

matic gain control variable resistor. The AGC action maintains

a very stable sinewave amplitude at any frequency. Again, only

two ICs accomplish a very useful function.

At the highest frequency setting, the harmonic distortion level

measures 0.016%. As the frequencies drop, distortion also drops

to a low of 0.006%. At the lowest frequency setting, distortion

came back up to a worst case of 0.035%.