Which method is used to convert a PWM signal to an analog voltage using op-amps and passive components?

Which method is used to convert a PWM signal to an analog voltage using op-amps and passive components?

• A. Use a low-pass RC filter to average the duty cycle; an op-amp buffer may be added.
• B. Use a high-pass filter to emphasize edges.
• C. Use a comparator to convert to digital.
• D. Use a differentiator to convert to velocity.

Answer: (A)

Converting a PWM signal to an analog voltage relies on the fact that the average value of the PWM waveform is proportional to its duty cycle. To recover that average, you filter out the high-frequency switching components while leaving the slow-changing DC component that represents the duty cycle. A low-pass RC filter does exactly that: it attenuates the high-frequency carrier of the PWM and passes the DC level that corresponds to the duty cycle. An op-amp buffer can be added to provide a low-impedance output and, if needed, to scale the voltage to the desired range without loading the filter. The other options don’t fit the goal. A high-pass filter would remove the DC content and emphasize edges, giving a non-stable, changing signal. A comparator would convert the PWM to a digital pulse train rather than an analog voltage. A differentiator would respond to changes, producing spikes rather than a steady voltage proportional to duty cycle. So, using a low-pass filter to average the duty cycle, optionally followed by an op-amp buffer, is the standard method.

Converting a PWM signal to an analog voltage relies on the fact that the average value of the PWM waveform is proportional to its duty cycle. To recover that average, you filter out the high-frequency switching components while leaving the slow-changing DC component that represents the duty cycle.

A low-pass RC filter does exactly that: it attenuates the high-frequency carrier of the PWM and passes the DC level that corresponds to the duty cycle. An op-amp buffer can be added to provide a low-impedance output and, if needed, to scale the voltage to the desired range without loading the filter.

The other options don’t fit the goal. A high-pass filter would remove the DC content and emphasize edges, giving a non-stable, changing signal. A comparator would convert the PWM to a digital pulse train rather than an analog voltage. A differentiator would respond to changes, producing spikes rather than a steady voltage proportional to duty cycle.

So, using a low-pass filter to average the duty cycle, optionally followed by an op-amp buffer, is the standard method.