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LF: Re: VDC source with microvolt resolution

To: [email protected]
Subject: LF: Re: VDC source with microvolt resolution
From: Alberto di Bene <[email protected]>
Date: Thu, 13 May 2004 23:03:58 +0200
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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





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