Dual DC Voltage Follower

[BlakeM]

Thanks audioguru, your comments are appreciated!

Edit:
Do you mean that using pwm at 500Hz is to low?

 

[BlakeM]

This creates another question I have been wondering about, is it actually better to have the PWM run at higher frequencies, I can get the arduino to output pwm smoothly at 31,250 Hz if it is best to do so?

Thanks

 

[MikeMl]

The choice of PWM rate depends entirely on how long you want to wait for the filter to settle to a new value when the duty-cycle changes. If settling to a new value in 1/10 sec is ok, then 500Hz is fine. OTOH, if you want more rapid step changes, then up-ing the rate might be useful.

 

[audioguru]

You never told us what the PWM is used for. If it drives an actuator or motor (in a wheelchair?) then you want its frequency above 20kHz so it will not make a loud noise. Your 500Hz will make the light from an LED flicker (like on new cars) if you move your eyes.

 

[ronsimpson]

I can get the arduino to output pwm smoothly at 31,250 Hz if it is best to do so?

The 100uF cap from post #1 is too large. Any time you can increase the PWM frequency you can reduce the size of the caps. I want to use caps like 0.1uF or smaller.
If you move the corner frequency of the filter up the response time of the filter is faster.
You need to ask, do I want the response time faster?

 

[MikeMl]

.... If it drives an actuator or motor (in a wheelchair?) then you want its frequency above 20kHz so it will not make a loud noise. Your 500Hz will make the light from an LED flicker (like on new cars) if you move your eyes.

Not relevant because the TS wants to filter the PWM to where it has only a tiny bit of ripple. It is essentially a DC level by the time he does something with it.

As to what the PWM rate should be, notice that the second-order low-pass filter takes about 40 cycles of the PWM period to settle to a new value. 40 cycles at 32kHz is much shorter than 40 cycles at 500Hz. You could also use a Chebyshev filter which will settle quicker but with more initial overshoot. You can also go to higher-order filters such as the third-order you showed, or cascade two second-order filters using two opamp stages.

 

[willeng]

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