Hi Erichi Frank,
What are the details of your PWM signal.?
E
Please confirm the actual values of the two capacitors.
Hi Bob.The filter must suppress the sampling frequency and pass the highest signal frequency of interest. You cannot pick the values without knowing those two things.
And the order of the filter depends also on the two frequencies. The smaller the ratio, the higher order you will need. For example, if your sampling frequency is 100 times the signal frequency, a first order filter is fine. If the ratio is two, you might need an eighth order filter.
Of course, it also depends on the quality of the output you need.
In the case if DC output, you choose RC based on the desired settling time when the output changes.
Bob
If the duty cycke is constant, why are you using PWM and then smoothing it to a DC value when a voltage divider can give you a better signal?Hi Bob.
Thanks for your reply.
what do you mean by ratio?
Say the frequency of PWM is 20 kHz(Vp-p = 3.3V), duty is 50%.
what should I do?
The ratio of the PMW fundamental (clock) frequency to the highest PWM modulating frequency.what do you mean by ratio?
Hi Zapper.The ratio of the PMW fundamental (clock) frequency to the highest PWM modulating frequency.
You also need to know how much clock ripple in the recovered DC your application can tolerate, and the accuracy needed in recovering the highest modulation frequency.
From that info you can determine the filter order and corner frequency you need.
Hi IanHow fast do you need your output voltage to change when you change the PWM duty cycle?
Nothing I know of that doesn't involve some complex math including Laplace transforms.is there a formulation can be used to calculate the orders and the RC values?
Hi zapper.Nothing I know of that doesn't involve some complex math including Laplace transforms.
Since I (or Wally) never do any more math then we have to (hurts my head), I took the easy way.so how did you come up with the circuit?
The reduction in ripple is roughly proportional to the ratio of the PWM frequency and the output frequency.thanks guys.
say the frequency f, duty cycle is D which is adjustable from 20% to 80%, the output ripple voltage is less than Vripple mV
is there a formulation can be used to calculate the orders and the RC values?
by Jake Hertz
by Jake Hertz
by Duane Benson