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Using PWM to Generate an Analog Output Pulse Width Modulation outputs produce digital waveforms that can be used as low-cost digital-to-analog converters with only a few external components. To convert the PWM signal to an analog voltage, a low-pass filter is used. Concerns when selecting the components for the filter are noise components inherent in digital waveforms. PWM signals contain strong noise components at the PWM frequency and at odd harmonics of that frequency. PWM modules in all ADR products can operate at 9.76 Khz thus the strong noise components are at 9.76Khz and higher. An RC filter with a bandwith many magnitudes lower than the PWM frequency will keep noise to a minimum. We recommend a filter bandwidth of 30Hz since the ADR devices are serial based and producing higher bandwidth outputs is not practical. The load on the filter should be kept as low as possible and use of a voltage follower buffer amplifier may be required in some applications. The Bottom Line... To convert a PWM output to an analog voltage use a low-pass filter with an R = 4.7K and C = 1.0uF. This gives a bandwidth of 30Hz. Ensure the PWM frequency for the ADR product is set to 9.76Khz. ( The ADR2100 default is 610Hz ).
Note:
The ADR2100 Ref. Application Note AN538 , Microchip Technology Inc. |
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has space on the PCB to install the RC components. For PWMA use
position R6 and C6. For PWMB, use positions R7 and C7. If it is desired to keep the
RC filter components off the PCB, remove the pull-down resistors shipped with the
ADR2100 in positions C6 and C7 before connecting RC filter.