Datasheet

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
based on the relationship f
C
= 1/2πRC. Positive feedback is
accomplished via the 82.5 kΩ and the 20 kΩ potentiometer.
The Q of the oscillator is determined by the ratio of 10 kΩ and
475Ω 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%.