hi, I want to use the pwm output of the microcontroller as digital potentiometer because this is a cheaper solution (respect to buy digital pot) ...I think about an integrator and a sample and hold circuit,so i should have a stable dc output varying with the characteristics of pwm,but do you know a more cheaper solution? thaks
If you use high values for the RC element, the integrated signal is very clean, so you don't need a sample and hold circuit. But you need an Op Amp, if you want to drive a low resistance input with it. The integrator and amplifier could be something like R3, C1 and U3 in this schematic:
-- Frank Buss, fb@frank-buss.de http://www.frank-buss.de, http://www.it4-systems.de
When I've done this - from the first time, back in 1974 - I've always used a Sallen and Keys low-pass filter, mostly second order, optimised for fastest settling time. Now that op amps have gotten really cheap, the one-op-amp-per-pole variant lets you adjust the various RC products by varying only the resistors, which saves you from the problem of getting precise capacitor ratios.
If you wanted to change the potentiometer output very frequently you might run into trouble getting enough attentuation out of your low pass filter while still maintaining a fast-enough settling time.
Up to a point, raising the order of the low pass filter can help, as can playing with the way you do the pulse-width modulation - sigma- delta D/A converter theory could be worth looking into - but digital potentiometers aren't usually twidled all that fast.
-- Bill Sloman, Nijmegen
I assume you want a DC output that tracks duty cycle.
pwm--------R--------+----------out | | | C | | | gnd
That's it. Make the RC time constant a lot longer than the pwm period, and be aware that loads on "out" will drag the output down, how much depending on the size of R.
If those constraints aren't OK, fancier stuff can be done.
Lately, we've been doing delta-sigma, rather than pwm, using one pin from an fpga. That works better at the 16-bit level, where a pwm may need a very slow r-c to get the ripple down.
John
Le Fri, 26 Oct 2007 08:51:30 -0700, John Larkin a écrit:
In case of stable DC level needed, that one can buy you better response time. Of course it's of no use when generating AC signals, unless you can tolerate a good deal of distorsion...
VCC | |/ .------| | |>
___ | ___ | PWM >-|___|--+--|___|-+-----+----- 10K | 100K | | | |< | +------| | | |\\ | --- | --- C --- | --- C | | | GND GND GND
-- Thanks, Fred.
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