8408 Datasheet, PDF (17/20 Pages) Maxwell Technologies – Quad 8-Bit Multiplying CMOS D/A Converter with Memory

Quad 8-Bit Multiplying CMOS
D/A Converter with Memory
8408
FIGURE 13. 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 13. Again, DAC B and DAC D in tandem control the oscillat-
ing frequency based on the relationship fC = 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 transis-
tor, which acts as an automatic gain control variable resistor. The AGC action maintains a very stable sinewave ampli-
tude 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%
08.20.02 REV 1
All data sheets are subject to change without notice 17
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