Sometimes a design need more DACs than the mixed-signal microcontroller has available. An easy solution is to put in a pulse-width modulator (PWM) circuit and filter it output with an RC filter. An 8-bit digital/analog converter (DAC) can have 20-mV resolution from a PWM operating from a 5.0-V supply, and it's reasonable to require the AC noise on the RC filter's output to be less than a single bit...
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