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Date: Thu, 13 May 2004 23:03:58 +0200
From: Alberto di Bene <dibene@usa.net>
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Subject: LF: Re: VDC source with microvolt resolution
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Johan Bodin wrote:

>I can't see that as a problem. Timer1 overflow flag works in PWM mode too
>so it is possible to write an interrupt routine that is synchronized to the PWM
>cycle. Every time the ISR is fired, it reads the 20-bit value from a global and
>updates the 10-bit PWM control register with either N or N+1, according to
>the "sub-PWM" algorithm.
>  
>
Hi Johan,
  you spared me compulsing the '8535 data sheet. It looks like it is 
quite feasible and with a minimal
consumption of CPU time, thanks.

>I like Stewart's idea of maximizing the dither frequency by spreading the N+1
>corrections all over the main PWM cycles but I don't know enough math for
>optimizing it. What if the value of the 10 LSBits is 1 for example? It cannot
>be spread out and there will be some some Fpwm/1024 frequency component
>(although very small).
>  
>
The DDA algorithm mentioned by Stewart is what sometimes is used to draw 
lines on a display
with a finite number of pixels. If the line is quite close to be 
horizontal (or vertical), you cannot
avoid some more pronounced jagginess. The example you give, a 1 in the 
10 LSBs, is conceptually
similar to an almost horizontal line. It is a worst case situation and 
it must be accepted as such
(unless Stewart has some more clever suggestions...)

>Assuming 8MHz clock on the '8535 (Yes, I know it :-), main PWM frequency
>is 3910 Hz max in 10-bit mode. Adding a 10-bit sub-PWM brings the lowest
>frequency component down to 3910/1024 = 3.8 Hz. A 3:rd order LPF with
>0.3 Hz cut-off, or so, should be about 60 dB down at 4 Hz and still have a
>reasonably fast response. With more filter poles you should be able to put the
>cutoff "knee" closer to 3.8 Hz and get faster response without sacrificing
>"sub-PWM" ripple rejection. Maybe a Bessel response is preferable to
>avoid overshoot although Bessel has a very "lazy" roll-off compared to
>Butterworth et. al.
>  
>
Good. You analysis shows that what remains as a concern is only the 
noise. Anybody knows of
an op-amp (readily available) better than the OP27, as far as noise is 
concerned ? TNX.

73  Alberto  I2PHD