Return-Path: Received: (qmail 22564 invoked from network); 13 May 2004 21:04:41 -0000 Received: from unknown (HELO ptb-mxscan01.plus.net) (212.159.14.235) by ptb-mailstore03.plus.net with SMTP; 13 May 2004 21:04:41 -0000 Received: (qmail 48434 invoked from network); 13 May 2004 21:04:41 -0000 X-Filtered-by: Plusnet (hmail v1.01) X-Spam-detection-level: 11 Received: from ptb-mxcore01.plus.net (212.159.14.215) by ptb-mxscan01.plus.net with SMTP; 13 May 2004 21:04:36 -0000 Received: from post.thorcom.com ([193.82.116.20]) by ptb-mxcore01.plus.net with esmtp (Exim 4.30; FreeBSD) id 1BONNc-000CLQ-2w for dave@picks.force9.co.uk; Thu, 13 May 2004 21:04:36 +0000 X-Fake-Domain: majordom Received: from majordom by post.thorcom.com with local (Exim 4.14) id 1BONNA-0004iG-J8 for rs_out_1@blacksheep.org; Thu, 13 May 2004 22:04:08 +0100 Received: from [213.232.95.59] (helo=relay.salmark.net) by post.thorcom.com with esmtp (Exim 4.14) id 1BONN9-0004i7-SW for rsgb_lf_group@blacksheep.org; Thu, 13 May 2004 22:04:07 +0100 Received: from cmsrelay02.mx.net ([165.212.11.111]) by relay.salmark.net with smtp (Exim 4.24) id 1BOTvX-0007Rn-M9 for rsgb_lf_group@blacksheep.org; Fri, 14 May 2004 05:04:03 +0100 Received: from uadvg128.cms.usa.net (165.212.11.128) by cmsoutbound.mx.net with SMTP; 13 May 2004 21:03:55 -0000 X-Fake-Domain: usa.net Received: from usa.net [151.41.143.11] by uadvg128.cms.usa.net (ASMTP/dibene@usa.net) via mtad (C8.MAIN.3.13N) with ESMTP id 305iemVD10306M28; Thu, 13 May 2004 21:03:52 GMT X-USANET-Auth: 151.41.143.11 AUTH dibene@usa.net usa.net Message-ID: <40A3E2BE.1010806@usa.net> Date: Thu, 13 May 2004 23:03:58 +0200 From: Alberto di Bene User-Agent: Mozilla/5.0 (Windows; U; Win98; en-US; rv:1.4) Gecko/20030624 Netscape/7.1 (ax) X-Accept-Language: en-us, en MIME-Version: 1.0 To: rsgb_lf_group@blacksheep.org References: <40A2578C.2080600@usa.net> <003301c438f8$97eaaa40$511297d4@oemcomputer> <40A3913E.9000406@usa.net> <003d01c4392a$d0b5c6c0$ac1397d4@oemcomputer> In-Reply-To: <003d01c4392a$d0b5c6c0$ac1397d4@oemcomputer> X-Spam-Score: 2.8 (++) X-Spam-Report: autolearn=no,FAKE_HELO_USA_NET=2.8 Subject: LF: Re: VDC source with microvolt resolution Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 8bit X-Spam-Checker-Version: SpamAssassin 2.63 (2004-01-11) on post.thorcom.com X-Spam-Level: ** X-Spam-Status: No, hits=2.8 required=5.0 tests=FAKE_HELO_USA_NET autolearn=no version=2.63 X-SA-Exim-Scanned: Yes Sender: owner-rsgb_lf_group@blacksheep.org Precedence: bulk Reply-To: rsgb_lf_group@blacksheep.org X-Listname: rsgb_lf_group X-SA-Exim-Rcpt-To: rs_out_1@blacksheep.org X-SA-Exim-Scanned: No; SAEximRunCond expanded to false X-PN-SPAMFiltered: yes X-Spam-Rating: 1 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